American National Standard part 1
1. Scope
This American National Standard provides the mechanical, electrical, and
functional requirements for a small computer input/output bus and command sets
for peripheral device types commonly used with small computers.
The small computer system interface, described in this standard, is a local
I/O bus that can be operated at data rates up to 4 megabytes per second
depending upon circuit implementation choices. The primary objective of the
interface is to provide host computers with device independence within a
class of devices. Thus, different disk drives, tape drives, printers, and
even communication devices can be added to the host computer(s) without
requiring modifications to generic system hardware or software. Provision
is made for the addition of nongeneric features and functions through vendor
unique fields and codes.
The interface uses logical rather than physical addressing for all data
blocks. For direct access devices, each logical unit may be interrogated to
determine how many blocks it contains. A logical unit may coincide with all
or part of a peripheral device.
Provision is made for cable lengths up to 25 meters using differential
drivers and receivers. A single-ended driver and receiver configuration is
defined for cable lengths of up to 6 meters and is primarily intended for
applications within a cabinet.
The interface protocol includes provision for the connection of multiple
initiators (SCSI devices capable of initiating an operation) and multiple
targets (SCSI devices capable of responding to a request to perform an
operation). Optional distributed arbitration (i.e., bus-contention logic) is
built into the architecture of SCSI. A priority system awards interface
control to the highest priority SCSI device that is contending for use of the
bus. The time to complete arbitration is independent of the number of devices
that are contending and can be completed in less than 10 microseconds.
The physical characteristics are described in Section 4. There are two
electrical alternatives: single-ended and differential. Single-ended and
differential devices are electrically different and shall not be mixed on the
same bus. In addition, there are several options: shielded or unshielded
connectors may be used and parity may or may not be implemented.
Section 5 describes the logical characteristics of the interface. An
arbitration option is defined to permit multiple initiators and to permit
concurrent I/O operations. All SCSI devices are required to be capable of
operating with the defined asynchronous transfer protocol. In addition, an
optional synchronous transfer protocol is defined. Section 5 also specifies a
message protocol for control of the interface. In most cases, messages are
not directly apparent to the host computer software. Only one message,
COMMAND COMPLETE, is mandatory; all others are optional and are not
necessarily implemented. Note that some options (e.g., synchronous transfer)
require the implementation of certain messages.
The SCSI command structure is specified in Section 6. Commands are
classified as mandatory (M), extended (E), optional (O), or vendor unique (V).
SCSI devices shall implement all mandatory commands defined for the
appropriate device type and may implement other commands as well. Extended
SCSI devices shall implement all extended plus all mandatory commands and may
implement other commands as well. Extended SCSI devices contain commands that
facilitate the writing of self-configuring software drivers that can
"discover" all necessary attributes without prior knowledge of specific
peripheral characteristics (such as storage capacity). Extended commands for
direct access devices also implement a very large logical block address space
(232 blocks), although mandatory commands for direct access devices implement
a somewhat smaller logical block address space (221 blocks).
Section 7 specifies those commands that have a consistent meaning for all
device types.
Sections 8 through 13 contain commands for direct-access (e.g., magnetic
disk), sequential-access (e.g., magnetic tape), printer, processor, write-
once-read-multiple (e.g., optical disk), and read-only direct-access devices,
respectively. The commands in each of these sections are unique to the device
type, or they have interpretations, fields, or features that are specific for
the device type. Thus, for example, although the WRITE command is used for
several device types, it has a somewhat different form for each type, with
different parameters and meanings. Therefore, it is specified separately for
each device type.
Section 14 describes the status byte for all device types. Status is
returned by targets at the end of each command.
Appendixes A through C provide examples of SCSI signal sequences, timing,
and phase sequences. Appendix D contains information on recommended shielded
connectors. Appendix E contains information on conformance statements.
Appendix F contains information on other standards related to medium types and
density codes for flexible disks and magnetic tapes. Appendix G contains
information on future extensions to SCSI that are being considered by X3T9.2
However, the appendixes are not part of this standard.
2. Referenced Standard
This standard is intended for use in conjunction with EIA RS-485-1983,
Standard for Electrical Characteristics of Generators and Receivers for use in
Balanced Digital Multipoint Systems.1
3. Glossary and Conventions
3.1 Glossary
byte. In this standard, this term indicates an 8-bit (octet) byte.
command descriptor block (CDB). The structure used to communicate requests
from an initiator to a target.
connect. The function that occurs when an initiator selects a target to start
an operation.
____________
1 Available from the Electronic Industries Association, 2001 Eye Street NW,
Washington, D.C. 20006.
disconnect. The function that occurs when a target releases control of the
SCSI bus, allowing it to go to the BUS FREE phase.
initiator. An SCSI device (usually a host system) that requests an operation
to be performed by another SCSI device.
INTERMEDIATE status. A status code sent from a target to an initiator upon
completion of each command in a set of linked commands except the last command
in the set.
logical unit. A physical or virtual device addressable through a target.
logical unit number. An encoded three-bit identifier for the logical unit.
LSB. Least significant byte.
LUN. Logical unit number.
mm. Millimeter.
ms. Millisecond.
MSB. Most significant byte.
ns. Nanosecond.
one. A true signal value.
peripheral device. A peripheral that can be attached to an SCSI device
(e.g., magnetic-disk, printer, optical-disk, or magnetic-tape).
reconnect. The function that occurs when a target selects an initiator to
continue an operation after a disconnect.
reserved. The term used for bits, bytes, fields, and code values that are set
aside for future standardization.
SCSI address. The octal representation of the unique address (0-7) assigned
to an SCSI device. This address would normally be assigned and set in the
SCSI device during system installation.
SCSI ID. The bit-significant representation of the SCSI address referring to
one of the signal lines DB(7-0).
SCSI device. A host computer adapter or a peripheral controller or an
intelligent peripheral that can be attached to the SCSI bus.
signal assertion. The act of driving a signal to the true state.
signal negation. The act of driving a signal to the false state or allowing
the cable terminators to bias the signal to the false state (by placing the
driver in the high impedance condition).
signal release. The act of allowing the cable terminators to bias the signal
to the false state (by placing the driver in the high impedance condition).
status. One byte of information sent from a target to an initiator upon
completion of each command.
target. An SCSI device that performs an operation requested by an initiator.
us. Microsecond.
vendor unique. In this standard, this term indicates bits, fields, or code
values that are vendor specific.
xxH. Numbers followed by capital H subscript are hexadecimal values. All
other numbers are decimal values.
zero. A false signal value.
3.2 Editorial Conventions. Certain words and terms used in this standard have
a specific meaning beyond the normal English meaning. These words and terms
are defined either in the glossary (see 3.1) or in the text where they first
appear (e.g., initiator). Names of signals, phases, conditions, messages,
commands, statuses, and sense keys are in all uppercase (e.g., REQUEST SENSE).
Lowercase is used for words having the normal English meaning.
4. Physical Characteristics
This section contains the physical definition of the SCSI. The connectors,
cables, signals, terminators, and bus timing needed to implement SCSI are
covered.
4.1 Physical Description. SCSI devices are daisy-chained together using a
common cable. Both ends of the cable are terminated. All signals are common
between all SCSI devices. Two driver/receiver alternatives are available:
(1) Single-ended drivers and receivers, which allow a maximum cable length
of six meters (primarily for connection within a cabinet)
(2) Differential drivers and receivers, which allow a maximum cable length
of 25 meters (primarily for connection outside of a cabinet)
4.2 Cable Requirements. An ideal impedance match with cable terminators
implies a cable characteristic impedance of 132 ohms (singled-ended option) or
122 ohms (differential option). In general, cables with this high of a
characteristic impedance are not available; however, impedances that are
somewhat lower are satisfactory. A characteristic impedance of 100 ohms + 10%
is recommended for unshielded flat or twisted pair ribbon cable. A
characteristic impedance greater than 90 ohms is preferred for shielded
cables; however, most available cables have a somewhat lower characteristic
impedance. To minimize discontinuities and signal reflections, cables of
different impedances should not be used in the same bus. Implementations may
require trade-offs in shielding effectiveness, cable length, the number of
loads, transfer rates, and cost to achieve satisfactory system operation.
A minimum conductor size of 28 AWG shall be employed to minimize noise
effects and ensure proper distribution of optional terminator power.
4.2.1 Single-Ended Cable. A 50-conductor flat cable or 25-signal twisted-
pair cable shall be used. The maximum cable length shall be 6.0 meters.
A stub length of no more than 0.1 meters is allowed off the mainline
interconnection within any connected equipment.
SCSI bus termination may be internal to the SCSI devices that are at the
ends of the cable.
4.2.2 Differential Cable. A 50-conductor cable or 25-signal twisted-pair
cable shall be used. The maximum cable length shall be 25 meters.
A stub length of no more than 0.2 meters is allowed off the mainline
interconnection within any connected equipment.
SCSI bus termination may be internal to the SCSI devices that are at the
ends of the cable.
4.3 Connector Requirements. Nonshielded connectors are specified. The
nonshielded connectors are typically used for in-cabinet applications.
Appendix D defines recommended shielded connectors and their pin assignments.
These connectors are typically used for external applications where
electromagnetic compatibility (EMC) and electrostatic discharge (ESD)
protection may be required. Either type of connector may be used with the
single-ended or differential drivers.
The nonshielded SCSI device connector (Figure 4-1) shall be a 50-conductor
connector consisting of two rows of 25 male pins with adjacent pins 2.54 mm
(0.1 in) apart. A shroud and header body should be used. The nonmating
portion of the connector is shown for reference only.
The nonshielded cable connector (Figure 4-2) shall be a 50-conductor
connector consisting of two rows of 25 female contacts with adjacent contacts
2.54 mm (0.1 in) apart. It is recommended that keyed connectors be used.
The unshielded connector pin assignments shall be as shown in Table 4-1 for
single-ended drivers and as shown in Table 4-2 for differential drivers.
Figure 4-1a. Nonshielded SCSI Device Connector
==============================================================================
Dimensions Millimeters Inches
------------------------------------------------------------------------------
D1 2.54 0.100
D2* 82.80 3.260
D3 2.54 0.100
D4 4.83 0.190
D5* 8.51 0.335
D6* 72.64 2.860
D7* 78.74 3.100
D8* 13.94 0.549
D9 4.19 0.165
D10 6.09 0.240
D11 6.60 0.260
==============================================================================
NOTES:
(1) Fifty Contacts on 2.54-mm (0.100-inch) spacing = 60.96 mm (2.40 inch).
(2) Tolerances + 0.127 mm (0.005 inch) noncumulative.
(3) Dimensions listed with asterisks (*) are shown for reference only.
Figure 4-1b. Nonshielded SCSI Device Connector
(Editors note: Figures 4-1a and 4-1b are to be combined into a single figure
during the final editing.)
Figure 4-2a. Nonshielded Cable Connector
==============================================================================
Dimensions Millimeters Inches
------------------------------------------------------------------------------
C1 2.5400 0.100
C2 60.9600 2.400
C3 2.5400 0.100
C4 8.3570 0.329
C5 3.3025 0.130
C6 68.0720 2.680
C7 6.0960 0.240
C8* 8.1530 0.321
C9* 13.4870 0.531
C10* 3.8100 0.150
C11* 1.2700 0.050
C12* 6.0960 0.240
C13 32.3850 1.275
C14 3.3020 0.130
C15 7.4930 0.295
C16 2.6670 0.105
C17 1.6250 0.064
==============================================================================
NOTES:
(1) Fifty contacts on 1.27-mm (0.05-inch)* staggered spacing = 62.23 mm
(2.450 inch)*.
(2) Tolerances + 0.127 mm (0.005 inch) noncumulative.
(3) Dimensions listed with asterisks (*) are shown for reference only.
Figure 4-2b. Nonshielded Cable Connector
(Editors note: Figures 4-2a and 4-2b are to be combined into a single figure
during the final editing.)
(Pages 17-19 and 19.1-19.4 are deleted. The information previously contained
on these pages has been moved to Appendix D.)
Table 4-1
Single-Ended Pin Assignments
==============================================================================
Signal Pin Number
------------------------------------------------------------------------------
-DB(0) 2
-DB(1) 4
-DB(2) 6
-DB(3) 8
-DB(4) 10
-DB(5) 12
-DB(6) 14
-DB(7) 16
-DB(P) 18
GROUND 20
GROUND 22
GROUND 24
TERMPWR 26
GROUND 28
GROUND 30
-ATN 32
GROUND 34
-BSY 36
-ACK 38
-RST 40
-MSG 42
-SEL 44
-C/D 46
-REQ 48
-I/O 50
==============================================================================
NOTES:
(1) All odd pins except pin 25 shall be connected to ground. Pin 25 should
be left open. Some products designed prior to the generation of this standard
connected this pin to ground.
(2) The minus sign next to the signals indicates active low.
Table 4-2
Differential Pin Assignments
==============================================================================
Signal Name Pin Number Signal Name
------------------------------------------------------------------------------
SHIELD GROUND 1 2 GROUND
+DB(0) 3 4 -DB(0)
+DB(1) 5 6 -DB(1)
+DB(2) 7 8 -DB(2)
+DB(3) 9 10 -DB(3)
+DB(4) 11 12 -DB(4)
+DB(5) 13 14 -DB(5)
+DB(6) 15 16 -DB(6)
+DB(7) 17 18 -DB(7)
+DB(P) 19 20 -DB(P)
DIFFSENS 21 22 GROUND
GROUND 23 24 GROUND
TERMPWR 25 26 TERMPWR
GROUND 27 28 GROUND
+ATN 29 30 -ATN
GROUND 31 32 GROUND
+BSY 33 34 -BSY
+ACK 35 36 -ACK
+RST 37 38 -RST
+MSG 39 40 -MSG
+SEL 41 42 -SEL
+C/D 43 44 -C/D
+REQ 45 46 -REQ
+I/O 47 48 -I/O
GROUND 49 50 GROUND
==============================================================================
NOTE:
(1) SHIELD GROUND is optional on some cables. (Implementors note: Some
shielded flat ribbon cables use pin 1 as a connection to the shield.)
4.4 Electrical Description
NOTE: For these measurements, SCSI bus termination is assumed to be external
to the SCSI device. An SCSI device may have the provision for allowing
optional internal termination.
4.4.1 Single-Ended Alternative. All assigned signals shall be terminated
with 220 ohms to +5 volts (nominal) and 330 ohms to ground at each end of the
cable. (See Figure 4-5.) All signals shall use open-collector or three-state
drivers.
4.4.1.1 Output Characteristics. Each signal driven by an SCSI device
shall have the following output characteristics when measured at the SCSI
device's connector:
Signal assertion = 0.0 volts dc to 0.4 volts dc
Minimum driver output capability = 48 milliamps (sinking) at 0.5 volts dc
Signal negation = 2.5 volts dc to 5.25 volts dc
4.4.1.2 Input Characteristics. Each signal received by an SCSI device
shall have the following input characteristics when measured at the SCSI
device's connector:
Signal true = 0.0 volts dc to 0.8 volts dc
Maximum total input load = -0.4 milliamps at 0.4 volts dc
Signal false = 2.0 volts dc to 5.25 volts dc
Minimum input hysteresis = 0.2 volts dc
4.4.2 Differential Alternative. All signals consist of two lines denoted
+SIGNAL and -SIGNAL. A signal is true when +SIGNAL is more positive than
-SIGNAL, and a signal is false when -SIGNAL is more positive than +SIGNAL.
All assigned signals shall be terminated at each end of the cable as shown in
Figure 4-6.
NOTE: As an option, the DIFFSENS signal of the connector is reserved for an
active high enable for the differential drivers. If a single-ended device or
terminator is inadvertently connected, this signal is grounded, disabling the
drivers. (See Figure 4-7.)
4.4.2.1 Output Characteristics. Each signal driven by an SCSI device
shall have the following output characteristics when measured at the SCSI
device's connector:
VOL (Low-level output voltage) = 2.0 V maximum at IOL (Low-level output
current) = 55 milliamps.
VOH (High-level output voltage) = 3.0 V minimum at IOH (High-level output
current) = -55 milliamps.
VOD (Differential voltage) = 1.0 V minimum with common-mode voltage ranges
from -7 volts dc to +12 volts dc.
VOL and VOH shall be as measured between the output terminal and the SCSI
device's logic ground reference.
The output characteristics shall additionally conform to EIA RS-485-1983.
4.4.2.2 Input Characteristics. Each signal received by an SCSI device
shall have the following input characteristics when measured at the SCSI
device's connector:
II (Input current on either input) = + 2.0 milliamps maximum.
NOTE: These characteristics include both receivers and passive drivers.
This requirement shall be met with the input voltage varying between
-7 volts dc and +12 volts dc, with power on or off, and with the hysteresis
equaling 35 millivolts, minimum.
The input characteristics shall additionally conform to EIA RS-485-1983.
4.4.3 Terminator Power (Optional). Single-ended SCSI devices providing
terminator power (TERMPWR) shall have the following characteristics:
VTerm = 4.0 volts dc to 5.25 volts dc
800 milliamps minimum source drive capability
1.0 milliamp maximum sink capability (except for the purposes of
providing power to an internal terminator) with 1.0 amp
recommended current limiting (e.g., a fuse).
Differential SCSI devices providing terminator power (TERMPWR) shall have
the following characteristics:
VTerm = 4.0 volts dc to 5.25 volts dc
600 milliamps minimum source drive capability
1.0 milliamp maximum sink capability (except for the purposes of
providing power to an internal terminator) with 1.0 amp
recommended current limiting (e.g., a fuse).
The use of keyed connectors is recommended in SCSI devices that provide
terminator power to prevent accidental grounding or misconnection of
terminator power.
SCSI devices that supply terminator power shall do so through a diode or
similar semiconductor that prevents the backflow of power to the SCSI device.
Figure 4-5. Termination for Single-Ended Devices
Figure 4-6. Termination for Differential Devices
Figure 4-7. Differential Driver Protection Circuit (Optional)
4.5 SCSI Bus. Communication on the SCSI bus is allowed between only two SCSI
devices at any given time. There is a maximum of eight SCSI devices. Each
SCSI device has an SCSI ID bit assigned as shown in Figure 4-8.
When two SCSI devices communicate on the SCSI bus, one acts as an initiator
and the other acts as a target. The initiator originates an operation and the
target performs the operation. An SCSI device usually has a fixed role as an
initiator or target, but some devices may be able to assume either role.
An initiator may address up to eight peripheral devices that are connected
to a target. An option allows the addressing of up to 2,048 peripheral
devices per target using extended messages. Three sample system
configurations are shown in Figure 4-9.
DB(7) DB(6) DB(5) DB(4) DB(3) DB(2) DB(1) DB(0) <-- DATA BUS
| | | | | | | |
| | | | | | | SCSI ID = 0
| | | | | | |
| | | | | | SCSI ID = 1
| | | | | |
| | | | | SCSI ID = 2
| | | | |
| | | | SCSI ID = 3
| | | |
| | | SCSI ID = 4
| | |
| | SCSI ID = 5
| |
| SCSI ID = 6
|
SCSI ID = 7
Figure 4-8. SCSI ID Bits
Figure 4-9. Sample SCSI Configurations
Up to eight SCSI devices can be supported on the SCSI bus. They can be any
combination of initiators and targets.
Certain SCSI bus functions are assigned to the initiator and certain SCSI
bus functions are assigned to the target. The initiator may arbitrate for the
SCSI bus and select a particular target. The target may request the transfer
of COMMAND, DATA, STATUS, or other information on the DATA BUS, and in some
cases it may arbitrate for the SCSI bus and reselect an initiator for the
purpose of continuing an operation.
Information transfers on the DATA BUS are asynchronous and follow a defined
REQ/ACK handshake protocol. One byte of information may be transferred with
each handshake. An option is defined for synchronous data transfer.
4.6 SCSI Bus Signals. There are a total of eighteen signals. Nine are used
for control and nine are used for data. (Data signals include the parity
signal option). These signals are described as follows:
BSY (BUSY). An "OR-tied" signal that indicates that the bus is being used.
SEL (SELECT). A signal used by an initiator to select a target or by a target
to reselect an initiator.
C/D (CONTROL/DATA). A signal driven by a target that indicates whether
CONTROL or DATA information is on the DATA BUS. True indicates CONTROL.
I/O (INPUT/OUTPUT). A signal driven by a target that controls the direction
of data movement on the DATA BUS with respect to an initiator. True indicates
input to the initiator. This signal is also used to distinguish between
SELECTION and RESELECTION phases.
MSG (MESSAGE). A signal driven by a target during the MESSAGE phase.
REQ (REQUEST). A signal driven by a target to indicate a request for a
REQ/ACK data transfer handshake.
ACK (ACKNOWLEDGE). A signal driven by an initiator to indicate an
acknowledgment for a REQ/ACK data transfer handshake.
ATN (ATTENTION). A signal driven by an initiator to indicate the ATTENTION
condition.
RST (RESET). An "OR-tied" signal that indicates the RESET condition.
DB(7-0,P) (DATA BUS). Eight data-bit signals, plus a parity-bit signal that
form a DATA BUS. DB(7) is the most significant bit and has the highest
priority during the ARBITRATION phase. Bit number, significance, and priority
decrease downward to DB(0). A data bit is defined as one when the signal
value is true and is defined as zero when the signal value is false.
Data parity DB(P) is odd. The use of parity is a system option (i.e., a
system is configured so that all SCSI devices on a bus generate parity and
have parity detection enabled, or all SCSI devices have parity detection
disabled or not implemented). Parity is not valid during the ARBITRATION
phase.
4.6.1 Signal Values. Signals may assume true or false values. There are
two methods of driving these signals. In both cases, the signal shall be
actively driven true, or asserted. In the case of OR-tied drivers, the driver
does not drive the signal to the false state, rather the bias circuitry of the
bus terminators pulls the signal false whenever it is released by the drivers
at every SCSI device. If any driver is asserted, then the signal is true. In
the case of non-OR-tied drivers, the signal may be actively driven false, or
negated. In this standard, wherever the term negated is used, it means that
the signal may be actively driven false, or may be simply released (in which
case the bias circuitry pulls it false), at the option of the implementor.
The advantage to actively drive signals false is that the transition from true
to false occurs more quickly, and noise margins may be somewhat improved; this
may permit somewhat faster data transfer.
4.6.2 OR-Tied Signals. The BSY and RST signals shall be OR-tied only. In
the ordinary operation of the bus, these signals are simultaneously driven
true by several drivers. No signals other than BSY, RST, and DB(P) are
simultaneously driven by two or more drivers, and any signal other than BSY
and RST may employ OR-tied or non-OR-tied drivers. DB(P) shall not be driven
false during the ARBITRATION phase. There is no operational problem in mixing
OR-tied and non-OR-tied drivers on signals other than BSY and RST.
4.6.3 Signal Sources. Table 4-3 indicates which type of SCSI device is
allowed to source each signal. No attempt is made to show if the source is
driving asserted, driving negated, or is passive. All SCSI device drivers
that are not active sources shall be in the passive state. Note that the RST
signal may be sourced by any SCSI device at any time.
Table 4-3
Signal Sources
==============================================================================
Signals
----------------------------------------------------------
C/D, I/O,
Bus Phase BSY SEL MSG, REQ ACK/ATN DB(7-0,P)
------------------------------------------------------------------------------
BUS FREE None None None None None
ARBITRATION All Winner None None SCSI ID
SELECTION I&T Initiator None Initiator Initiator
RESELECTION I&T Target Target Initiator Target
COMMAND Target None Target Initiator Initiator
DATA IN Target None Target Initiator Target
DATA OUT Target None Target Initiator Initiator
STATUS Target None Target Initiator Target
MESSAGE IN Target None Target Initiator Target
MESSAGE OUT Target None Target Initiator Initiator
==============================================================================
All: The signal shall be driven by all SCSI devices that are
actively arbitrating.
SCSI ID: A unique data bit (the SCSI ID) shall be driven by each SCSI
device that is actively arbitrating; the other seven data bits
shall be released (i.e., not driven) by this SCSI device. The
parity bit (DB(P)) may be undriven or driven to the true state,
but shall never be driven to the false state during this phase.
I&T: The signal shall be driven by the initiator, target, or both, as
specified in the SELECTION phase and RESELECTION phase.
Initiator: If this signal is driven, it shall be driven only by the active
initiator.
None: The signal shall be released; that is, not be driven by any SCSI
device. The bias circuitry of the bus terminators pulls the
signal to the false state.
Winner: The signal shall be driven by the one SCSI device that wins
arbitration.
Target: If the signal is driven, it shall be driven only by the active
target.
4.7 SCSI Bus Timing. Unless otherwise indicated, the delay-time measurements
for each SCSI device, shown in 4.7.1 through 4.7.14, shall be calculated from
signal conditions existing at that SCSI device's own SCSI bus connection.
Thus, these measurements (except cable skew delay) can be made without
considering delays in the cable.
4.7.1 Arbitration Delay (2.2 microseconds). The minimum time an SCSI device
shall wait from asserting BSY for arbitration until the DATA BUS can be
examined to see if arbitration has been won. There is no maximum time.
4.7.2 Assertion Period (90 nanoseconds). The minimum time that a target
shall assert REQ while using synchronous data transfers. Also, the minimum
time that an initiator shall assert ACK while using synchronous data
transfers.
4.7.3 Bus Clear Delay (800 nanoseconds). The maximum time for an SCSI
device to stop driving all bus signals after:
(1) The BUS FREE phase is detected (BSY and SEL both false for a bus settle
delay)
(2) SEL is received from another SCSI device during the ARBITRATION phase
(3) The transition of RST to true.
NOTE: For the first condition above, the maximum time for an SCSI device to
clear the bus is 1200 nanoseconds from BSY and SEL first becoming both false.
If an SCSI device requires more than a bus settle delay to detect BUS FREE
phase, it shall clear the bus within a bus clear delay minus the excess time.
4.7.4 Bus Free Delay (800 nanoseconds). The minimum time that an SCSI
device shall wait from its detection of the BUS FREE phase (BSY and SEL both
false for a bus settle delay) until its assertion of BSY when going to the
ARBITRATION phase.
4.7.5 Bus Set Delay (1.8 microseconds). The maximum time for an SCSI device
to assert BSY and its SCSI ID bit on the DATA BUS after it detects BUS FREE
phase (BSY and SEL both false for a bus settle delay) for the purpose of
entering the ARBITRATION phase.
4.7.6 Bus Settle Delay (400 nanoseconds). The time to wait for the bus to
settle after changing certain control signals as called out in the protocol
definitions.
4.7.7 Cable Skew Delay (10 nanoseconds). The maximum difference in
propagation time allowed between any two SCSI bus signals when measured
between any two SCSI devices.
4.7.8 Data Release Delay (400 nanoseconds). The maximum time for an
initiator to release the DATA BUS signals following the transition of the I/O
signal from false to true.
4.7.9 Deskew Delay (45 nanoseconds). The minimum time required for deskew
of certain signals.
4.7.10 Hold Time (45 nanoseconds). The minimum time added between the
assertion of REQ or ACK and the changing of the data lines to provide hold
time in the initiator or target, respectively, while using synchronous data
transfers.
4.7.11 Negation Period (90 nanoseconds). The minimum time that a target
shall negate REQ while using synchronous data transfers. Also, the minimum
time that an initiator shall negate ACK while using synchronous data
transfers.
4.7.12 Reset Hold Time (25 microseconds). The minimum time for which RST is
asserted. There is no maximum time.
4.7.13 Selection Abort Time (200 microseconds). The maximum time that a
target (or initiator) shall take from its most recent detection of being
selected (or reselected) until asserting a BSY response. This timeout is
required to ensure that a target (or initiator) does not assert BSY after a
SELECTION (or RESELECTION) phase has been aborted. This is not the selection
timeout period; see Sections 5.1.3.5 and 5.1.4.2 for a complete description.
4.7.14 Selection Timeout Delay (250 milliseconds, recommended). The minimum
time that an initiator (or target) should wait for a BSY response during the
SELECTION (or RESELECTION) phase before starting the timeout procedure. Note
that this is only a recommended time period. The specifications for the
peripheral devices shall be consulted for the actual timing requirements.
4.7.15 Transfer Period (set during a MESSAGE phase). The Transfer Period
specifies the minimum time allowed between the leading edges of successive REQ
pulses and of successive ACK pulses while using synchronous data transfers.
(See Sections 5.1.5.2 and 5.5.5.)
5. Logical Characteristics
5.1 SCSI Bus Phases. The SCSI architecture includes eight distinct phases:
BUS FREE phase
ARBITRATION phase
SELECTION phase
RESELECTION phase
COMMAND phase \
DATA phase \ These phases are collectively termed the
STATUS phase / information transfer phases.
MESSAGE phase /
The SCSI bus can never be in more than one phase at any given time. Unless
otherwise noted in the following descriptions, signals that are not mentioned
shall not be asserted.
5.1.1 BUS FREE Phase. The BUS FREE phase is used to indicate that no SCSI
device is actively using the SCSI bus and that it is available for subsequent
users.
SCSI devices shall detect the BUS FREE phase after SEL and BSY are both
false for at least a bus settle delay.
SCSI devices shall release all SCSI bus signals within a bus clear delay
after BSY and SEL become continuously false for a bus settle delay. If an
SCSI device requires more than a bus settle delay to detect the BUS FREE phase
then it shall release all SCSI bus signals within a bus clear delay minus the
excess time to detect the BUS FREE phase. The total time to clear the SCSI
bus shall not exceed a bus settle delay plus a bus clear delay.
5.1.2 ARBITRATION Phase. The ARBITRATION phase allows one SCSI device to
gain control of the SCSI bus so that it can assume the role of an initiator or
target.
NOTE: Implementation of the ARBITRATION phase is a system option. Systems
that do not implement this option can have only one initiator. The
ARBITRATION phase is required for systems that use the RESELECTION phase.
The procedure for an SCSI device to obtain control of the SCSI bus is as
follows:
(1) The SCSI device shall first wait for the BUS FREE phase to occur. The
BUS FREE phase is detected whenever both BSY and SEL are simultaneously and
continuously false for a minimum of a bus settle delay. (Implementors Note:
This bus settle delay is necessary because a transmission line phenomenon
known as a "wire-OR glitch" may cause BSY to briefly appear false, even though
it is being driven true.)
(2) The SCSI device shall wait a minimum of a bus free delay after
detection of the BUS FREE phase (i.e. after BSY and SEL are both false for a
bus settle delay) before driving any signal.
(3) Following the bus free delay in Step (2), the SCSI device may arbitrate
for the SCSI bus by asserting both BSY and its own SCSI ID, however the SCSI
device shall not arbitrate (i.e. assert BSY and its SCSI ID) if more than a
bus set delay has passed since the BUS FREE phase was last observed.
(Implementors Note: There is no maximum delay before asserting BSY and the
SCSI ID following the bus free delay in Step (2) as long as the bus remains in
the BUS FREE phase. However, SCSI devices that delay longer than a bus settle
delay plus a bus set delay from the time when BSY and SEL first become false
may fail to participate in arbitration when competing with faster SCSI
devices.)
(4) After waiting at least an arbitration delay (measured from its
assertion of BSY) the SCSI device shall examine the DATA BUS. If a higher
priority SCSI ID bit is true on the DATA BUS (DB(7) is the highest), then the
SCSI device has lost the arbitration and the SCSI device may release its
signals and return to Step (1). If no higher priority SCSI ID bit is true on
the DATA BUS, then the SCSI device has won the arbitration and it shall assert
SEL. Any other SCSI device that is participating in the ARBITRATION phase has
lost the arbitration and shall release BSY and its SCSI ID bit within a bus
clear delay after SEL becomes true. An SCSI device that loses arbitration may
return to Step (1).
(5) The SCSI device that wins arbitration shall wait at least a bus clear
delay plus a bus settle delay after asserting SEL before changing any signals.
NOTE: The SCSI ID bit is a single bit on the DATA BUS that corresponds to the
SCSI device's unique SCSI address. All other seven DATA BUS bits shall be
released by the SCSI device. Parity is not valid during the ARBITRATION
phase. During the ARBITRATION phase, DB(P) may be undriven or driven to the
true state, but shall not be driven to the false state.
5.1.3 SELECTION Phase. The SELECTION phase allows an initiator to select a
target for the purpose of initiating some target function (e.g., READ or WRITE
command).
NOTE: During the SELECTION phase the I/O signal shall be negated so that this
phase can be distinguished from the RESELECTION phase.
5.1.3.1 Nonarbitrating Systems. In systems with the ARBITRATION phase
not implemented, the initiator shall first detect the BUS FREE phase and then
wait a minimum of a bus clear delay. Then, except in certain single initiator
environments with initiators employing the single initiator option (see
5.1.3.4), the initiator shall assert the desired target's SCSI ID and its own
initiator SCSI ID on the DATA BUS. After two deskew delays the initiator
shall assert SEL.
5.1.3.2 Arbitrating Systems. In systems with ARBITRATION phase
implemented, the SCSI device that won the arbitration has both BSY and SEL
asserted and has delayed at least a bus clear delay plus a bus settle delay
before ending the ARBITRATION phase. The SCSI device that won the arbitration
becomes an initiator by releasing I/O. Except in certain single initiator
environments with initiators employing the single initiator option (see
5.1.3.4), the initiator shall set the DATA BUS to a value which is the OR of
its SCSI ID bit and the target's SCSI ID bit. The initiator shall then wait
at least two deskew delays and release BSY. The initiator shall then wait at
least a bus settle delay before looking for a response from the target.
5.1.3.3 All Systems. In all systems, the target shall determine that it
is selected when SEL and its SCSI ID bit are true and BSY and I/O are false
for at least a bus settle delay. The selected target may examine the DATA BUS
in order to determine the SCSI ID of the selecting initiator unless the
initiator employed the single initiator option (see 5.1.3.4). The selected
target shall then assert BSY within a selection abort time of its most recent
detection of being selected; this is required for correct operation of the
timeout procedure. In systems with parity implemented, the target shall not
respond to a selection if bad parity is detected. Also, if more than two SCSI
ID bits are on the DATA BUS, the target shall not respond to selection.
At least two deskew delays after the initiator detects BSY is true, it shall
release SEL and may change the DATA BUS.
5.1.3.4 Single Initiator Option. Initiators that do not implement the
RESELECTION phase and do not operate in the multiple initiator environment are
allowed to set only the target's SCSI ID bit during the SELECTION phase. This
makes it impossible for the target to determine the initiator's SCSI ID.
5.1.3.5 Selection Timeout Procedure. Two optional selection timeout
procedures are specified for clearing the SCSI bus if the initiator waits a
minimum of a selection timeout delay and there has been no BSY response from
the target:
(1) Optionally, the initiator shall assert the RST signal (see 5.2.2).
(2) Optionally, the initiator shall continue asserting SEL and shall release
the DATA BUS. If the initiator has not detected BSY to be true after at least
a selection abort time plus two deskew delays, the initiator shall release SEL
allowing the SCSI bus to go to the BUS FREE phase. SCSI devices shall ensure
that when responding to selection that the selection was still valid within a
selection abort time of their assertion of BSY. Failure to comply with this
requirement could result in an improper selection (two targets connected to
the same initiator, wrong target connected to an initiator, or a target
connected to no initiator).
5.1.4 RESELECTION Phase (Optional). RESELECTION is an optional phase that
allows a target to reconnect to an initiator for the purpose of continuing
some operation that was previously started by the initiator but was suspended
by the target, (i.e., the target disconnected by allowing a BUS FREE phase to
occur before the operation was complete).
5.1.4.1 RESELECTION. RESELECTION can only be used in systems that have
ARBITRATION phase implemented.
Upon completing the ARBITRATION phase, the winning SCSI device has both BSY
and SEL asserted and has delayed at least a bus clear delay plus a bus settle
delay. The winning SCSI device becomes a target by asserting the I/O signal.
The winning SCSI device shall also set the DATA BUS to a value that is the OR
of its SCSI ID bit and the initiator's SCSI ID bit. The target shall wait at
least two deskew delays and release BSY. The target shall then wait at least
a bus settle delay before looking for a response from the initiator.
The initiator shall determine that it is reselected when SEL, I/O, and its
SCSI ID bit are true and BSY is false for at least a bus settle delay. The
reselected initiator may examine the DATA BUS in order to determine the SCSI
ID of the reselecting target. The reselected initiator shall then assert BSY
within a selection abort time of its most recent detection of being
reselected; this is required for correct operation of the timeout procedure.
In systems with parity implemented, the initiator shall not respond to a
RESELECTION if bad parity is detected. Also, the initiator shall not respond
to a RESELECTION if more than two SCSI ID bits are on the DATA BUS.
After the target detects BSY, it shall also assert BSY and wait at least two
deskew delays and then release SEL. The target may then change the I/O signal
and the DATA BUS. After the reselected initiator detects SEL false, it shall
release BSY. The target shall continue asserting BSY until the target is
ready to relinquish the SCSI bus.
NOTE: When the target is asserting BSY, a transmission line phenomenon known
as a "wire-OR glitch" may cause BSY to appear false for up to a round-trip
propagation delay following the release of BSY by the initiator. This is the
reason why the BUS FREE phase is recognized only after both BSY and SEL are
continuously false for a minimum of a bus settle delay. Cables longer than 25
meters should not be used even if the chosen driver, receiver, and cable
provide adequate noise margins, because they increase the duration of the
glitch and could cause SCSI devices to inadvertently detect the BUS FREE
phase.
5.1.4.2 RESELECTION Timeout Procedure. Two optional RESELECTION timeout
procedures are specified for clearing the SCSI bus during a RESELECTION phase
if the target waits a minimum of a selection timeout period and there has been
no BSY response from the initiator:
(1) Optionally, the target shall assert the RST signal (see 5.2.2).
(2) Optionally, the target shall continue asserting SEL and I/O and shall
release all DATA BUS signals. If the target has not detected BSY to be true
after at least a selection abort time plus two deskew delays, the target shall
release SEL and I/O allowing the SCSI bus to go to the BUS FREE phase. SCSI
devices that respond to RESELECTION shall ensure that the RESELECTION was
still valid within a selection abort time of their assertion of BSY. Failure
to comply with this requirement could result in an improper reselection (two
initiators connected to the same target or the wrong initiator connected to a
target).
5.1.5 Information Transfer Phases.
NOTE: The COMMAND, DATA, STATUS, and MESSAGE phases are all grouped together
as the information transfer phases because they are all used to transfer data
or control information via the DATA BUS. The actual contents of the
information is beyond the scope of this section.
The C/D, I/O, and MSG signals are used to distinguish between the different
information transfer phases. (See Table 5-1.) The target drives these three
signals and therefore controls all changes from one phase to another. The
initiator can request a MESSAGE OUT phase by asserting ATN, while the target
can cause the BUS FREE phase by releasing MSG, C/D, I/O, and BSY.
Table 5-1
Information Transfer Phases
==============================================================================
Signal
-----------
MSG C/D I/O Phase Name Direction Of Transfer Comment
------------------------------------------------------------------------------
0 0 0 DATA OUT Initiator to target \ Data
0 0 1 DATA IN Initiator from target / Phase
0 1 0 COMMAND Initiator to target
0 1 1 STATUS Initiator from target
1 0 0 *
1 0 1 *
1 1 0 MESSAGE OUT Initiator to target \ Message
1 1 1 MESSAGE IN Initiator from target / Phase
==============================================================================
Key: 0 = False, 1 = True, * = Reserved for future standardization.
The information transfer phases use one or more REQ/ACK handshakes to
control the information transfer. Each REQ/ACK handshake allows the transfer
of one byte of information. During the information transfer phases BSY shall
remain true and SEL shall remain false. Additionally, during the information
transfer phases, the target shall continuously envelope the REQ/ACK
handshake(s) with C/D, I/O, and MSG in such a manner that these control
signals are valid for a bus settle delay before the assertion of REQ of the
first handshake and remain valid until the negation of ACK at the end of the
last handshake.
5.1.5.1 Asynchronous Information Transfer. The target shall control the
direction of information transfer by means of the I/O signal. When I/O is
true, information shall be transferred from the target to the initiator. When
I/O is false, information shall be transferred from the initiator to the
target.
If I/O is true (transfer to the initiator), the target shall first drive
DB(7-0,P) to their desired values, delay at least one deskew delay plus a
cable skew delay, then assert REQ. DB(7-0,P) shall remain valid until ACK is
true at the target. The initiator shall read DB(7-0,P) after REQ is true,
then signal its acceptance of the data by asserting ACK. When ACK becomes
true at the target, the target may change or release DB(7-0,P) and shall
negate REQ. After REQ is false the initiator shall then negate ACK. After
ACK is false the target may continue the transfer by driving DB(7-0,P) and
asserting REQ, as described above.
If I/O is false (transfer to the target) the target shall request
information by asserting REQ. The initiator shall drive DB(7-0,P) to their
desired values, delay at least one deskew delay plus a cable skew delay and
assert ACK. The initiator shall continue to drive DB(7-0,P) until REQ is
false. When ACK becomes true at the target, the target shall read DB(7-0,P),
then negate REQ. When REQ becomes false at the initiator, the initiator may
change or release DB(7-0,P) and shall negate ACK. The target may continue the
transfer by asserting REQ, as described above.
5.1.5.2 Synchronous Data Transfer (Optional). Synchronous data transfer
is optional, and may be used only in the data phase if previously agreed to by
the initiator and target through the message system (see SYNCHRONOUS DATA
TRANSFER REQUEST message, 5.5.5). The messages determine the use of
synchronous mode by both SCSI devices and establish a REQ/ACK offset and a
transfer period.
The REQ/ACK offset specifies the maximum number of REQ pulses that can be
sent by the target in advance of the number of ACK pulses received from the
initiator, establishing a pacing mechanism. If the number of REQ pulses
exceeds the number of ACK pulses by the REQ/ACK offset, the target shall not
assert REQ until the next ACK pulse is received. A requirement for successful
completion of the data phase is that the number of ACK and REQ pulses be
equal.
The target shall assert the REQ signal for a minimum of an assertion period.
The target shall wait at least the greater of a transfer period from the last
transition of REQ to true or a minimum of a negation period from the last
transition of REQ to false before asserting the REQ signal.
The initiator shall send one pulse on the ACK signal for each REQ pulse
received. The initiator shall assert the ACK signal for a minimum of an
assertion period. The initiator shall wait at least the greater of a transfer
period from the last transition of ACK to true or for a minimum of a negation
period from the last transition of ACK to false before asserting the ACK
signal.
If I/O is true (transfer to the initiator), the target shall first drive
DB(7-0,P) to their desired values, wait at least one deskew delay plus one
cable skew delay, then assert REQ. DB(7-0,P) shall be held valid for a
minimum of one deskew delay plus one cable skew delay plus one hold time after
the assertion of REQ. The target shall assert REQ for a minimum of an
assertion period. The target may then negate REQ and change or release DB(7-
0,P). The initiator shall read the value on DB(7-0,P) within one hold time of
the transition of REQ to true. The initiator shall then respond with an ACK
pulse.
If I/O is false (transfer to the target), the initiator shall transfer one
byte for each REQ pulse received. After receiving a REQ pulse, the initiator
shall first drive DB(7-0,P) to their desired values, delay at least one deskew
delay plus one cable skew delay, then assert ACK. The initiator shall hold
DB(7-0,P) valid for at least one deskew delay plus one cable skew delay plus
one hold time after the assertion of ACK. The initiator shall assert ACK for
a minimum of an assertion period. The initiator may then negate ACK and may
change or release DB(7-0,P). The target shall read the value of DB(7-0,P)
within one hold time of the transition of ACK to true.
5.1.6 COMMAND Phase. The COMMAND phase allows the target to request command
information from the initiator.
The target shall assert the C/D signal and negate the I/O and MSG signals
during the REQ/ACK handshake(s) of this phase.
5.1.7 Data Phase. The data phase is a term that encompasses both the DATA
IN phase and the DATA OUT phase.
5.1.7.1 DATA IN Phase. The DATA IN phase allows the target to request
that data be sent to the initiator from the target.
The target shall assert the I/O signal and negate the C/D and MSG signals
during the REQ/ACK handshake(s) of this phase.
5.1.7.2 DATA OUT Phase. The DATA OUT phase allows the target to request
that data be sent from the initiator to the target.
The target shall negate the C/D, I/O, and MSG signals during the REQ/ACK
handshake(s) of this phase.
5.1.8 STATUS Phase. The STATUS phase allows the target to request that
status information be sent from the target to the initiator.
The target shall assert C/D and I/O and negate the MSG signal during the
REQ/ACK handshake of this phase.
5.1.9 Message Phase. The message phase is a term that references either a
MESSAGE IN, or a MESSAGE OUT phase. Multiple messages may be sent during
either phase. The first byte transferred in either of these phases shall be
either a single-byte message or the first byte of a multiple-byte message.
Multiple-byte messages shall be wholly contained within a single message
phase.
5.1.9.1 MESSAGE IN Phase. The MESSAGE IN phase allows the target to
request that message(s) be sent to the initiator from the target.
The target shall assert C/D, I/O, and MSG during the REQ/ACK handshake(s) of
this phase.
5.1.9.2 MESSAGE OUT Phase. The MESSAGE OUT phase allows the target to
request that message(s) be sent from the initiator to the target. The target
may invoke this phase at its convenience in response to the ATTENTION
condition (see 5.2.1) created by the initiator.
The target shall assert C/D and MSG and negate I/O during the REQ/ACK
handshake(s) of this phase. The target shall handshake byte(s) in this phase
until ATN goes false, unless an error occurs (see MESSAGE REJECT, 5.5.2).
If the target detects one or more parity error(s) on the message byte(s)
received, it may indicate its desire to retry the message(s) by asserting REQ
after detecting ATN has gone false and prior to changing to any other phase.
The initiator, upon detecting this condition, shall resend all of the previous
message byte(s) sent during this phase. When resending more than one message
byte, the initiator shall assert ATN prior to asserting ACK on the first byte
and shall maintain ATN asserted until the last byte is sent as described in
5.2.1.
If the target receives all of the message byte(s) successfully (i.e., no
parity errors), it shall indicate that it does not wish to retry by changing
to any information transfer phase other than the MESSAGE OUT phase and
transfer at least one byte. The target may also indicate that it has
successfully received the message byte(s) by changing to the BUS FREE phase
(e.g., ABORT or BUS DEVICE RESET messages).
5.1.10 Signal Restrictions Between Phases. When the SCSI bus is between two
information transfer phases, the following restrictions shall apply to the
SCSI bus signals:
(1) The BSY, SEL, REQ, and ACK signals shall not change.
(2) The C/D, I/O, MSG, and DATA BUS signals may change. When switching the
DATA BUS direction from out (initiator driving) to in (target driving), the
target shall delay driving the DATA BUS by at least a data release delay plus
a bus settle delay after asserting the I/O signal and the initiator shall
release the DATA BUS no later than a data release delay after the transition
of the I/O signal to true. When switching the DATA BUS direction from in
(target driving) to out (initiator driving), the target shall release the DATA
BUS no later than a deskew delay after negating the I/O signal.
(3) The ATN and RST signals may change as defined under the descriptions for
the ATTENTION condition (5.2.1) and RESET condition (5.2.2).
5.2 SCSI Bus Conditions. The SCSI bus has two asynchronous conditions; the
ATTENTION condition and the RESET condition. These conditions cause the SCSI
device to perform certain actions and can alter the phase sequence.
5.2.1 ATTENTION Condition. The ATTENTION condition allows an initiator to
inform a target that the initiator has a message ready. The target may get
this message at its convenience by performing a MESSAGE OUT phase.
The initiator creates the ATTENTION condition by asserting ATN at any time
except during the ARBITRATION or BUS FREE phases.
The target may respond with the MESSAGE OUT phase.
The initiator shall keep ATN asserted if more than one byte is to be
transferred. The initiator may negate the ATN signal at any time except it
shall not negate the ATN signal while the ACK signal is asserted during a
MESSAGE OUT phase. Normally, the initiator negates ATN while REQ is true and
ACK is false during the last REQ/ACK handshake of the MESSAGE OUT phase.
5.2.2 RESET Condition. The RESET condition is used to immediately clear all
SCSI devices from the bus. This condition shall take precedence over all
other phases and conditions. Any SCSI device may create the RESET condition
by asserting RST for a minimum of a reset hold time. During the RESET
condition, the state of all SCSI bus signals other than RST is not defined.
All SCSI devices shall release all SCSI bus signals (except RST) within a
bus clear delay of the transition of RST to true. The BUS FREE phase always
follows the RESET condition.
The effect of the RESET condition on uncompleted commands, SCSI device
reservations, and SCSI device operating modes is determined by whether the
SCSI device has implemented the "hard" RESET option or the "soft" RESET option
(one of which shall be implemented) as defined in 5.2.2.1 and 5.2.2.2.
5.2.2.1 "Hard" RESET Option. SCSI devices that implement the "hard"
RESET option, upon detection of the RESET condition, shall:
(1) Clear all uncompleted commands
(2) Release all SCSI device reservations
(3) Return any SCSI device operating modes (MODE SELECT, PREVENT/ALLOW
MEDIUM REMOVAL commands, etc) to their default conditions.
5.2.2.2 "Soft" RESET Option. SCSI devices that implement the "soft"
RESET option, upon detection of the RESET condition, shall:
(1) Attempt to complete any uncompleted commands that were fully identified
(2) Preserve all SCSI device reservations
(3) Preserve any SCSI device operating modes (MODE SELECT, PREVENT/ALLOW
MEDIUM REMOVAL commands, etc)
The "soft" RESET option allows a single initiator to reset the SCSI bus
without disturbing the operation of other initiators in a multiple initiator
system. To ensure proper operation the following conditions shall be met:
(1) An initiator shall not consider a command to be fully identified until
the IDENTIFY message is sent to the target and the target responds by changing
to any other information transfer phase and requests that at least one byte be
transferred.
(2) A target shall consider a command to be fully identified when it
successfully receives the IDENTIFY message.
(3) If an initiator selects a logical unit for which there already is an
active command for the same initiator, the target shall clear the original
command and perform the new command.
(4) If a target reselects an initiator to continue a command for which the
initiator has no record, the initiator shall abort that command by sending the
ABORT message.
(5) An initiator shall consider a command to be completed when it negates
ACK for a successfully received COMMAND COMPLETE message.
(6) A target shall consider a command to be completed when it detects the
false transition of ACK for the COMMAND COMPLETE message with the ATN signal
false.
(7) An initiator shall not negate ACK for the SAVE DATA POINTER message
until it has actually saved the data pointer for the operation.
(8) A target shall consider the data pointer to be saved when it detects the
false transition of ACK for the SAVE DATA POINTER message with the ATN signal
false.
(9) If the RESET condition occurs between the time that the target asserts
REQ for the SAVE DATA POINTER message and it detects the false transition of
ACK, the target shall terminate the command with a CHECK CONDITION status. If
extended sense is implemented, the target shall set the sense key to ABORTED
COMMAND. This is necessary because the target cannot determine whether the
data pointer has actually been saved.
NOTE: If the ATN signal is true in conditions (6) or (8), the target would
normally switch to MESSAGE OUT phase and attempt to transfer a message byte.
If the RESET condition occurs before it is able to successfully receive the
message byte, the target shall assume that the initiator may not have
successfully received the COMMAND COMPLETE message or the SAVE DATA POINTER
message. In the case of COMMAND COMPLETE message, the target shall reselect
the initiator and attempt to send the COMMAND COMPLETE message again. In the
case of the SAVE DATA POINTER message, the target shall reselect the initiator
and terminate the command as described in condition (9).
5.3 SCSI Bus Phase Sequences. The order in which phases are used on the SCSI
bus follows a prescribed sequence.
In all systems, the RESET condition can abort any phase and is always
followed by the BUS FREE phase. Also, any other phase can be followed by the
BUS FREE phase.
5.3.1 Nonarbitrating Systems. In systems where the ARBITRATION phase is not
implemented, the allowable sequences shall be as shown in Figure 5-1. The
normal progression is from the BUS FREE phase to SELECTION, and from SELECTION
to one or more of the information transfer phases (COMMAND, DATA, STATUS, or
MESSAGE).
5.3.2 Arbitrating Systems. In systems where the ARBITRATION phase is
implemented, the allowable sequences shall be as shown in Figure 5-2. The
normal progression is from the BUS FREE phase to ARBITRATION, from ARBITRATION
to SELECTION or RESELECTION, and from SELECTION or RESELECTION to one or more
of the information transfer phases (COMMAND, DATA, STATUS, or MESSAGE).
5.3.3 All Systems. There are no restrictions on the sequences between
information transfer phases. A phase type may even be followed by the same
phase type (e.g., a data phase may be followed by another data phase).
Figure 5-1. Phase Sequences without Arbitration
Figure 5-2. Phase Sequences with Arbitration
5.4 SCSI Pointers. Consider the system shown in Figure 5-3 in which an
initiator and target communicate on the SCSI bus in order to execute a
command.
------------------------- -------------------------
| Function | | Initiator|-----------------| Target | | Function |
| Origin | | Path | SCSI BUS | Path | | Execution|
| | | Control |-----------------| Control | | |
------------------------- -------------------------
Initiator Target
Figure 5-3. Simplified SCSI System
The SCSI architecture provides for two sets of three pointers within each
initiator. The pointers reside in the initiator path control. The first set
of pointers are known as the current (or active) pointers. These pointers are
used to represent the state of the interface and point to the next command,
data, or status byte to be transferred between the initiator's memory and the
target. There is only one set of current pointers in each initiator. The
current pointers are used by the target currently connected to the initiator.
The second set of pointers are known as the saved pointers. There is one
set of saved pointers for each command that is currently active (whether or
not it is currently connected). The saved command pointer always points to
the start of the command descriptor block (see 6.2) for the current command.
The saved status pointer always points to the start of the status area for the
current command. At the beginning of each command, the saved data pointer
points to the start of the data area. It remains at this value until the
target sends a SAVE DATA POINTER message (see 5.5.2) to the initiator. In
response to this message, the initiator stores the value of the current data
pointer into the saved data pointer. The target may restore the current
pointers to their saved values by sending a RESTORE POINTERS message (see
5.5.2) to the initiator. The initiator moves the saved value of each pointer
into the corresponding current pointer. Whenever an SCSI device disconnects
from the bus, only the saved pointer values are retained. The current pointer
values are restored from the saved values upon the next reconnection.
5.5 Message System Specification. The message system allows communication
between an initiator and target for the purpose of physical path management.
5.5.1 Message Protocol. All SCSI devices shall implement the COMMAND COMPLETE
message. A functional SCSI device can be constructed without using any of the
other messages if the logical unit number is specified in the command
descriptor block. The remainder of this section deals with the additional
requirements on SCSI devices that support messages other than COMMAND
COMPLETE.
SCSI devices indicate their ability to accommodate more than the COMMAND
COMPLETE message by asserting or responding to the ATN signal. The initiator
indicates this in the SELECTION phase by asserting ATN prior to the SCSI bus
condition of SEL true, and BSY false. The target indicates its ability to
accommodate more messages by responding to the ATTENTION condition with the
MESSAGE OUT phase after going through the SELECTION phase.
For SCSI devices that support messages other than COMMAND COMPLETE, the first
message sent by the initiator after the SELECTION phase shall be the IDENTIFY
message. This allows the establishment of the physical path for a particular
logical unit specified by the initiator. After the RESELECTION phase, the
target's first message shall be IDENTIFY. This allows the physical path to be
reestablished for the target's specified logical unit number. Under some
exceptional conditions, an initiator may send the ABORT message or the BUS
DEVICE RESET message instead of the IDENTIFY message, as the first message.
Only one logical unit number shall be identified for any one selection
sequence; a second IDENTIFY message with a new logical unit number shall not
be issued before the SCSI bus has been released (BUS FREE phase).
Whenever a physical path is established in an initiator that can accommodate
disconnection and reconnection, the initiator shall ensure that the active
pointers of the physical path are equal to the saved pointers for that
particular logical unit number. (An implied restore pointers operation occurs
as a result of connect or reconnect.)
SCSI devices that implement any message other than the COMMAND COMPLETE
message shall also implement the MESSAGE REJECT message.
Table 5-2
Message Codes
==============================================================================
Code Type Description Direction
------------------------------------------------------------------------------
00H M COMMAND COMPLETE In
01H O EXTENDED MESSAGE In Out
02H O SAVE DATA POINTER In
03H O RESTORE POINTERS In
04H O DISCONNECT In
05H O INITIATOR DETECTED ERROR Out
06H O ABORT Out
07H O MESSAGE REJECT In Out
08H O NO OPERATION Out
09H O MESSAGE PARITY ERROR Out
0AH O LINKED COMMAND COMPLETE In
0BH O LINKED COMMAND COMPLETE (WITH FLAG) In
0CH O BUS DEVICE RESET Out
0DH _ 7FH R Reserved Codes
80H _ FFH O IDENTIFY In Out
==============================================================================
Key: In = Target to initiator, Out = Initiator to target.
5.5.2 Messages. The single byte messages (Table 5-2) are listed along with
their code values and their definitions.
COMMAND COMPLETE 00H (Mandatory). This message is sent from a target to an
initiator to indicate that the execution of a command (or series of linked
commands) has terminated and that valid status has been sent to the initiator.
After successfully sending this message, the target shall go to the BUS FREE
phase by releasing BSY.
NOTE: The command may have been executed successfully or unsuccessfully as
indicated in the status.
EXTENDED MESSAGE 01H (Optional). This message is sent from either the
initiator or the target as the first byte of a multiple-byte message. (See
5.5.3 for descriptions of extended messages.)
SAVE DATA POINTER 02H (Optional). This message is sent from a target to
direct the initiator to save a copy of the present active data pointer for the
currently attached logical unit. (See 5.4 for a definition of pointers.)
RESTORE POINTERS 03H (Optional). This message is sent from a target to direct
the initiator to restore the most recently saved pointers (for the currently
attached logical unit) to the active state. Pointers to the command, data,
and status locations for the logical unit shall be restored to the active
pointers. Command and status pointers shall be restored to the beginning of
the present command and status areas. The data pointer shall be restored to
the value at the beginning of the data area in the absence of a SAVE DATA
POINTER message or to the value at the point at which the last SAVE DATA
POINTER message occurred for that logical unit.
DISCONNECT 04H (Optional). This message is sent from a target to inform an
initiator that the present physical path is going to be broken (the target
plans to disconnect by releasing BSY), but that a later reconnect will be
required in order to complete the current operation. If the initiator detects
the BUS FREE phase (other than as a result of a RESET condition) without first
receiving a DISCONNECT or COMMAND COMPLETE message, the initiator shall
consider this as a catastrophic error condition. If the target intentionally
creates this condition, the target shall clear the current command. This
message shall not cause the initiator to save the data pointer. Note: If
DISCONNECT messages are used break a long data transfer into two or more
shorter transfers, then a SAVE DATA POINTER should be issued before each
DISCONNECT message.
INITIATOR DETECTED ERROR 05H (Optional). This message is sent from an
initiator to inform a target that an error (e.g., parity error) has occurred
that does not preclude the target from retrying the operation. Although
present pointer integrity is not assured, a RESTORE POINTERS message or a
disconnect followed by a reconnect, shall cause the pointers to be restored to
their defined prior state.
ABORT 06H (Optional). This message is sent from the initiator to the target
to clear the present operation. If a logical unit has been identified, all
pending data and status for the issuing initiator from the effected logical
unit shall be cleared, and the target shall go to the BUS FREE phase. Pending
data and status for other initiators shall not be cleared. If a logical unit
has not been identified, the target shall go to the BUS FREE phase. No status
or ending message shall be sent for the operation. It is not an error to
issue this message to an logical unit that is not currently performing an
operation for the initiator.
MESSAGE REJECT 07H (Optional). This message is sent from either the initiator
or target to indicate that the last message it received was inappropriate or
has not been implemented.
In order to indicate its intentions of sending this message, the initiator
shall assert the ATN signal prior to its release of ACK for the REQ/ACK
handshake of the message that is to be rejected. When a target sends this
message, it shall change to MESSAGE IN phase and send this message prior to
requesting additional message bytes from the initiator. This provides an
interlock so that the initiator can determine which message is rejected.
This message shall be implemented if any other optional messages are
implemented.
NO OPERATION 08H (Optional). This message is sent from an initiator in
response to a target's request for a message when the initiator does not
currently have any other valid message to send.
MESSAGE PARITY ERROR 09H (Optional). This message is sent from the initiator
to the target to indicate that one or more bytes in the last message it
received had a parity error.
In order to indicate its intentions of sending this message, the initiator
shall assert the ATN signal prior to its release of ACK for the REQ/ACK
handshake of the message that has the parity error. This provides an
interlock so that the target can determine which message has the parity error.
LINKED COMMAND COMPLETE 0AH (Optional). This message is sent from a target to
an initiator to indicate that the execution of a linked command has completed
and that status has been sent. The initiator shall then set the pointers to
the initial state for the next linked command.
LINKED COMMAND COMPLETE (WITH FLAG) 0BH (Optional). This message is sent from
a target to an initiator to indicate that the execution of a linked command
(with the flag bit set to one) has completed and that status has been sent.
The initiator shall then set the pointers to the initial state of the next
linked command. Typically this message would be used to cause an interrupt in
the initiator between two linked commands.
BUS DEVICE RESET 0CH (Optional). This message is sent from an initiator to
direct a target to clear all current commands on that SCSI device. This
message forces the SCSI device to an initial state with no operations pending
for any initiator. Upon recognizing this message, the target shall go to the
BUS FREE phase.
Reserved 0DH to 7FH. These message codes are reserved for future
standardization.
IDENTIFY 80H to FFH (Optional). These messages are sent by either the
initiator or the target to establish the physical path connection between an
initiator and target for a particular logical unit.
Bit 7. This bit is always set to one to distinguish these messages from the
other messages.
Bit 6. This bit is only set to one by the initiator. When set to one, it
indicates that the initiator has the ability to accommodate disconnection and
reconnection.
Bits 5-3. Reserved.
Bits 2-0. These bits specify a logical unit number in a target.
Only one logical unit number shall be identified for any one selection
sequence; a second IDENTIFY message with a new logical unit number shall not
be issued before the bus has been released (BUS FREE phase).
When sent from a target to an initiator during reconnection, an implied
RESTORE POINTERS message shall be performed by the initiator prior to
completion of this message.
5.5.3 Extended Messages (Optional). A value of one in the first byte of a
message indicates the beginning of a multiple-byte extended message. The
minimum number of bytes sent for an extended message is three. The extended
message format and the extended message codes are shown in Tables 5-3 and 5-4,
respectively.
Table 5-3
Extended Message Format
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
----------|----------|-------------------------------------------------------|
1 | nH | Extended message length |
----------|----------|-------------------------------------------------------|
2 | yH | Extended message code |
----------|----------|-------------------------------------------------------|
3 _ nH+1 | xH | Extended message arguments |
==============================================================================
The extended message length specifies the length in bytes of the extended
message code plus the extended message arguments to follow. Therefore, the
total length of the message is equal to the extended message length plus two.
A value of zero for the extended message length indicates 256 bytes follow.
The extended message codes are listed in Table 5-4. The extended message
arguments are specified for the defined extended messages in Sections 5.5.4
through 5.5.6.
Table 5-4
Extended Message Codes
==============================================================================
Code (yH) Description
------------------------------------------------------------------------------
00H MODIFY DATA POINTER (Optional)
01H SYNCHRONOUS DATA TRANSFER REQUEST (Optional)
02H EXTENDED IDENTIFY (Optional)
03H _ 7FH Reserved
80H _ FFH Vendor Unique
==============================================================================
5.5.4 MODIFY DATA POINTER Message (Optional)
Table 5-5
MODIFY DATA POINTER
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
-----|---------|-------------------------------------------------------------|
1 | 05H | Extended message length |
-----|---------|-------------------------------------------------------------|
2 | 00H | MODIFY DATA POINTER code |
-----|---------|-------------------------------------------------------------|
3 | xH | Argument (MSB) |
-----|---------|-------------------------------------------------------------|
4 | xH | Argument |
-----|---------|-------------------------------------------------------------|
5 | xH | Argument |
-----|---------|-------------------------------------------------------------|
6 | xH | Argument (LSB) |
==============================================================================
The MODIFY DATA POINTER message (Table 5-5) is sent from the target to the
initiator and requests that the signed argument be added (two's complement) to
the value of the current data pointer.
5.5.5 SYNCHRONOUS DATA TRANSFER REQUEST Message (Optional)
Table 5-6
SYNCHRONOUS DATA TRANSFER REQUEST
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
-----|---------|-------------------------------------------------------------|
1 | 03H | Extended message length |
-----|---------|-------------------------------------------------------------|
2 | 01H | SYNCHRONOUS DATA TRANSFER REQUEST code |
-----|---------|-------------------------------------------------------------|
3 | mH | Transfer period (mH times 4 nanoseconds) |
-----|---------|-------------------------------------------------------------|
4 | xH | REQ/ACK offset |
==============================================================================
A pair of SYNCHRONOUS DATA TRANSFER REQUEST messages (Table 5-6) are
exchanged between an initiator and a target whenever an SCSI device that can
support synchronous data transfer recognizes that it has not communicated with
the other SCSI device since receiving the last "hard" RESET condition or a BUS
DEVICE RESET message. The SCSI devices may also exchange messages to
establish synchronous data transfer when requested to do so. The message
exchange establishes the transfer period and the REQ/ACK offset. The transfer
period is the minimum time between leading edges of successive REQ pulses and
of successive ACK pulses.
The REQ/ACK offset is the maximum number of REQ pulses that may be
outstanding before its corresponding ACK pulse is received at the target. A
REQ/ACK offset value of zero shall indicate asynchronous mode; a value of FFH
shall indicate unlimited offset.
If the initiator recognizes that negotiation is required, it asserts ATN
and, if the target implements message transfers, sends a SYNCHRONOUS DATA
TRANSFER REQUEST message indicating an REQ/ACK offset and minimum transfer
period. The REQ/ACK offset is chosen to prevent initiator buffer overflows,
while the minimum transfer period is chosen to meet the data handling
requirements of the initiator. The target responds in any of the following
ways:
Target Response Implied Agreement
------------------------------------- --------------------------------------
(1) REQ/ACK offset less than or equal REQ/ACK offset equal to target value.
to the requested value.
Minimum transfer period equal to Minimum transfer period equal to
or greater than requested period. the target value.
(2) REQ/ACK offset equal to zero. Asynchronous transfer.
(3) MESSAGE REJECT. Asynchronous transfer.
If the target recognizes that negotiation is required, it sends a
SYNCHRONOUS DATA TRANSFER REQUEST message to the initiator. The REQ/ACK
offset is selected to prevent buffer and offset counter overflows, while the
minimum transfer period is chosen to meet the data handling requirements of
the target. The initiator responds in any of the following ways if the target
chooses an REQ/ACK offset equal to FFH:
Initiator Response Implied Agreement
------------------------------------- --------------------------------------
(1) REQ/ACK offset equal to FFH. REQ/ACK offset unlimited.
Minimum transfer period equal to Minimum transfer period equal to
or greater than requested period. the initiator value.
(2) REQ/ACK offset equal to 00H. Asynchronous transfer. The target may
renegotiate for an REQ/ACK offset less
than FFH and greater than 00H.
(3) MESSAGE REJECT. Asynchronous transfer.
The initiator responds in any of the following ways if the target selects an
REQ/ACK offset less than FFH:
Initiator Response Implied Agreement
------------------------------------- --------------------------------------
(1) REQ/ACK offset less than or equal REQ/ACK offset equals initiator value.
to the requested value.
Minimum transfer period equal to Minimum transfer period equal to the
or greater than requested value. initiator value.
(2) REQ/ACK offset equal to zero. Asynchronous transfer.
(3) MESSAGE REJECT. Asynchronous transfer.
The implied agreement shall remain in effect until a BUS DEVICE RESET
message is received, until a "hard" RESET condition occurs, or until one of
the two SCSI devices elects to modify the agreement. Renegotiation at every
selection is not recommended, since a significant performance impact is
likely. The default mode of data transfer is asynchronous mode. The default
mode is entered at power on, after a BUS DEVICE RESET message, or after a
"hard" RESET condition. The SYNCHRONOUS DATA TRANSFER REQUEST message
exchange can only take place following a SELECTION phase that includes the
SCSI IDs for both the initiator and the target. Violation of this rule may
make data transfer impossible owing to disagreements among SCSI devices about
the data transfer mode.
5.5.6 EXTENDED IDENTIFY Message (Optional)
Table 5-7
EXTENDED IDENTIFY
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
-----|---------|-------------------------------------------------------------|
1 | 02H | Extended message length |
-----|---------|-------------------------------------------------------------|
2 | 02H | EXTENDED IDENTIFY code |
-----|---------|-------------------------------------------------------------|
3 | xxH | Sub-logical unit number |
==============================================================================
The EXTENDED IDENTIFY message (Table 5-7) is optional and may be sent by a
target or an initiator. It may be used in conjunction with the normal
IDENTIFY message in order to expand the logical unit number addressing in a
target. The sub-logical unit number specifies the encoded eight-bit sub-
logical unit number used to identify one of 256 sub-logical units within the
logical unit. This allows up to 2048 units to be addressed on a single
target.
6. SCSI Commands
This section defines the SCSI command structure and gives several examples.
The command definitions assume a data structure providing the appearance at
the interface of a contiguous set of logical blocks of a fixed or explicitly
defined data length. The SCSI device maps the physical characteristics of the
attached peripheral devices to one of several logical structures defined by
the device type code.
A single command may transfer one or more logical blocks of data. Multiple
commands may be linked if they are sent to the same logical unit. A target
may disconnect from the SCSI bus to allow activity by other SCSI devices while
a logical unit is being prepared to transfer data.
Upon command completion (successful or unsuccessful), the target returns a
status byte to the initiator. Since most error and exception conditions
cannot be adequately described with a single status byte, one status code,
CHECK CONDITION, indicates that additional information is available. The
initiator may issue a REQUEST SENSE command to retrieve this additional
information.
By keeping to a minimum the functions essential to communicate via this
protocol, a wide range of peripheral devices of varying capability can operate
in the same environment.
Because subsets of the full architecture may be implemented, optional
functions are noted.
6.1 Command Implementation Requirements. The first byte of any SCSI command
shall contain an operation code as defined in this document. Three bits (bits
7 - 5) of the second byte of each SCSI command specify the logical unit if it
is not specified using the IDENTIFY message (see 5.5.2). The last byte of all
SCSI commands shall contain a control byte as defined in 6.2.6.
6.1.1 Reserved. Reserved bits, fields, bytes, and code values are set aside
for future standardization. Their use and interpretation will be specified by
future extensions to this standard. A reserved bit, field, or byte shall be
set to zero, or in accordance with a future extension to this standard. A
target that receives a reserved bit, field, or byte that is not zero or
receives a reserved code value shall terminate the command with a CHECK
CONDITION status and, if extended sense is implemented, the sense key shall be
set to ILLEGAL REQUEST. It shall also be acceptable for a target to interpret
the bit, field, byte, or code value in accordance with a future extension to
this standard.
6.1.2 Operation Code Types
Operation
Code Type Description
--------- -------------------------------------------------------------------
M Mandatory - Commands so designated shall be implemented in order to
meet the minimum requirement of this standard.
E Extended - Commands so designated shall be implemented in addition
to mandatory commands to meet the extended requirement of this
standard.
O Optional - Commands so designated, if implemented, shall be
implemented as defined in this standard.
V Vendor unique - Operation codes so designated are available for
vendor defined commands. See the vendor specifications where
compatibility is desired.
R Reserved - Operation codes so designated shall not be used. They
are reserved for future extensions to this standard.
6.1.3 Unit Attention Condition. A unit attention condition for a logical
unit shall begin for each initiator whenever the removable medium may have
been changed or the target has been reset (by a BUS DEVICE RESET message or a
"hard" RESET condition). The unit attention condition shall persist for each
initiator until that initiator issues a command to the logical unit other than
REQUEST SENSE or INQUIRY for which the target shall return CHECK CONDITION
status. If the next command from that initiator to the logical unit
(following the CHECK CONDITION status) is REQUEST SENSE, and if the target
supports extended sense, then the UNIT ATTENTION sense key shall be returned.
(If any command other than REQUEST SENSE is received, the unit attention
condition is lost.)
If an INQUIRY command is received from an initiator with a pending unit
attention condition (before the target reports CHECK CONDITION status), the
target shall perform the INQUIRY command and shall not clear the unit
attention condition.
If a REQUEST SENSE command is received from an initiator with a pending unit
attention condition (before the target reports CHECK CONDITION status), then
the target may either:
(1) report any pending sense data and preserve the unit attention condition
(2) discard any pending sense data, report UNIT ATTENTION sense key, and
clear the unit attention condition for that initiator.
If an initiator issues a command other than INQUIRY or REQUEST SENSE while a
unit attention condition exists for that initiator, the target shall not
perform the command and shall report CHECK CONDITION status.
6.2 Command Descriptor Block. A request to a peripheral device is performed
by sending a command descriptor block to the target. For several commands,
the request is accompanied by a list of parameters sent during the DATA OUT
phase. See the specific commands for detailed information.
The command descriptor block always has an operation code as the first byte
of the command. This is followed by a logical unit number, command parameters
(if any), and a control byte.
For all commands, if there is an invalid parameter in the command descriptor
block, then the target shall terminate the command without altering the
medium.
6.2.1 Operation Code. The operation code (Table 6-1) of the command
descriptor block has a group code field and a command code field. The three-
bit group code field provides for eight groups of command codes. The five-bit
command code field provides for thirty-two command codes in each group. Thus,
a total of 256 possible operation codes exist. Operation codes are defined in
Sections 7 through 13.
The group code specifies one of the following groups:
Group 0 - six-byte commands (see Table 6-2)
Group 1 - ten-byte commands (see Table 6-3)
Group 2 - reserved
Group 3 - reserved
Group 4 - reserved
Group 5 - twelve-byte commands (see Table 6-4)
Group 6 - vendor unique
Group 7 - vendor unique
(This page is intentionally blank.)
Table 6-1
Operation Code
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Group Code | Command Code |
==============================================================================
Table 6-2
Typical Command Descriptor Block for Six-byte Commands
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number |Logical Block Address (if required) (MSB) |
-----|-----------------------------------------------------------------------|
2 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
3 | Logical Block Address (if required) (LSB) |
-----|-----------------------------------------------------------------------|
4 | Transfer Length (if required) |
-----|-----------------------------------------------------------------------|
5 | Control Byte |
==============================================================================
Table 6-3
Typical Command Descriptor Block for Ten-byte Commands
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved | RelAdr |
-----|-----------------------------------------------------------------------|
2 | Logical Block Address (if required) (MSB) |
-----|-----------------------------------------------------------------------|
3 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
4 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
5 | Logical Block Address (if required) (LSB) |
-----|-----------------------------------------------------------------------|
6 | Reserved |
-----|-----------------------------------------------------------------------|
7 | Transfer Length (if required) (MSB) |
-----|-----------------------------------------------------------------------|
8 | Transfer Length (if required) (LSB) |
-----|-----------------------------------------------------------------------|
9 | Control Byte |
==============================================================================
Table 6-4
Typical Command Descriptor Block for Twelve-byte Commands
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved | RelAdr |
-----|-----------------------------------------------------------------------|
2 | Logical Block Address (if required) (MSB) |
-----|-----------------------------------------------------------------------|
3 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
4 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
5 | Logical Block Address (if required) (LSB) |
-----|-----------------------------------------------------------------------|
6 | Reserved |
-----|-----------------------------------------------------------------------|
7 | Reserved |
-----|-----------------------------------------------------------------------|
8 | Reserved |
-----|-----------------------------------------------------------------------|
9 | Transfer Length (if required) (MSB) |
-----|-----------------------------------------------------------------------|
10 | Transfer Length (if required) (LSB) |
-----|-----------------------------------------------------------------------|
11 | Control Byte |
==============================================================================
6.2.2 Logical Unit Number. The logical unit number addresses one of up to
eight physical or virtual devices attached to a target. This method of
addressing is provided for systems that do not implement the IDENTIFY message.
A target that accepts an IDENTIFY message shall use the logical unit number
specified within the message. In this case, the target shall ignore the
logical unit number specified within the command descriptor block.
(Implementors note: It is a good practice for initiators that implement the
IDENTIFY message to specify the same logical unit number in the command
descriptor block.)
6.2.3 Logical Block Address. The logical block address on logical units
shall begin with block zero and be contiguous up to the last logical block on
that logical unit.
Group 0 command descriptor blocks contain 21-bit logical block addresses.
Groups 1 and 5 command descriptor blocks contain 32-bit logical block
addresses.
The logical block concept implies that the initiator and target shall have
previously established the number of data bytes per logical block. This may
be established through the use of the READ CAPACITY command or the MODE SENSE
command or by prior arrangement.
6.2.4 Relative Address Bit. The relative address (RelAdr) bit of the
group 1 and group 5 commands is set to one to indicate that the logical block
address portion of the command descriptor block is a two's complement
displacement. This negative or positive displacement is to be added to the
logical block address last accessed on the logical unit to form the logical
block address for this command. This feature is only available when linking
commands. The feature requires that a previous command in the linked group
have accessed a block of data on the logical unit. (For an example of the
operation of this function, see Section 6.3.3.)
6.2.5 Transfer Length. The transfer length specifies the amount of data to
be transferred, usually the number of blocks. For several commands the
transfer length indicates the requested number of bytes to be sent as defined
in the command description. For these commands the transfer length field may
be identified by a different name. See the following descriptions and the
individual command descriptions for further information.
Commands that use one byte for transfer length allow up to 256 blocks of
data to be transferred by one command. A transfer length value of 1 to 255
indicates the number of blocks that shall transferred. A value of zero
indicates 256 blocks.
Commands that use two bytes for transfer length allow up to 65,535 blocks of
data to be transferred by one command. In this case, a transfer length of
zero indicates that no data transfer shall take place. A value of 1 to 65,535
indicates the number of blocks that shall be transferred.
For several commands more than two bytes are allocated for transfer length.
Refer to the specific command description for further information.
The transfer length of the commands that are used to send a list of
parameters to a target is called the parameter list length. The parameter
list length specifies the number of bytes sent during the DATA OUT phase.
The transfer length of the commands that are used to return sense data (e.g.
REQUEST SENSE, INQUIRY, MODE SENSE, etc) to an initiator is called the
allocation length. The allocation length specifies the number of bytes that
the initiator has allocated for returned data. The target shall terminate the
DATA IN phase when allocation length bytes have been transferred or when all
available sense data have been transferred to the initiator, whichever is
less.
6.2.6 Control Byte. The control byte is the last byte of every command
descriptor block. A typical control byte is described in Table 6-5.
Table 6-5
Control Byte
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
Last | Vendor unique | Reserved | Flag | Link |
==============================================================================
Bit Description
----- ----------------------------------------------------------------------
7 _ 6 Vendor unique
5 _ 2 Reserved
1 Flag bit - If the link bit is zero, then the flag bit shall be set to
zero. If the link bit is one, and if the command terminates
successfully, the target shall send LINKED COMMAND COMPLETE message if
the flag bit is zero and shall send LINKED COMMAND COMPLETE (WITH
FLAG) message if the flag bit is one. Typically, this bit is used to
cause an interrupt in the initiator between linked commands.
0 Link bit - This bit is set to one to indicate that the initiator
desires an automatic link to the next command upon successful
completion of the current command. Implementation of linked commands
is optional. If the link bit is one, targets that implement linked
commands, upon successful termination of the command, shall return
INTERMEDIATE status and shall then send one of the two messages
defined by the flag bit (above).
Targets that do not implement linked commands shall return a CHECK
CONDITION status and, if extended sense is implemented, shall set the
sense key to ILLEGAL REQUEST if either of the link and flag bits are
set to one.
6.3 Command Examples
6.3.1 Single Command Example. A typical operation on the SCSI bus is likely
to include a single READ command to a peripheral device. This operation is
described in detail starting with a request from the initiator. This example
assumes that no linked commands and no malfunctions or errors occur.
The initiator has active pointers and a set of stored pointers representing
active disconnected SCSI devices (an initiator without disconnect capability
does not require stored pointers). The initiator sets up the active pointers
for the operation requested, arbitrates for the SCSI bus, and selects the
target. Once this process is completed, the target assumes control of the
operation.
The target obtains the command from the initiator (in this case, a READ
command). The target interprets the command and executes it. In this case,
the target gets the data from the peripheral device and sends it to the
initiator. At the completion of the READ command, the target sends a status
byte to the initiator. To end the operation, the target sends a COMMAND
COMPLETE message to the initiator.
6.3.2 Disconnect Example. In the above single command example, the length
of time necessary to obtain the data may require a time-consuming physical
seek. In order to improve system throughput, the target may disconnect from
the initiator, freeing the SCSI bus to allow other requests to be sent to
other logical units. To do this, the initiator needs to be reselectable and
capable of restoring the pointers upon reconnection. The target needs to be
capable of arbitrating for the SCSI bus and reselecting the initiator.
After the target has received the READ command (and has determined that
there will be a delay), it disconnects by sending a DISCONNECT message and
releasing BSY.
When the data are ready to be transferred, the target reconnects to the
initiator. As a result of this reconnection, the initiator restores the
pointers to their most recent saved values (which, in this case, are the
initial values) and the target continues (as in the single command example) to
finish the operation. The initiator recognizes that the operation is complete
when COMMAND COMPLETE message is received.
If target wishes to disconnect after transferring part of the data (e.g.,
while crossing a cylinder boundary), it may do so by sending a SAVE DATA
POINTER message and a DISCONNECT message to the initiator and then
disconnecting. When reconnection is completed, the current data pointer value
is restored to its value immediately prior to the SAVE DATA POINTER message.
On those occasions when an error or exception condition occurs and the
target elects to repeat the information transfer, the target may repeat the
transfer by either issuing a RESTORE POINTERS message or by disconnecting
without issuing a SAVE DATA POINTER message. When reconnection is completed,
the most recent saved pointer values are restored.
6.3.3 Linked Command Example. The link function defines a relationship
between commands that when combined with the relative address bit, allows
previous operations to modify subsequent operations. Link makes high-
performance functions possible by providing a relative addressing capability
and allowing multiple command execution without invoking the functional
component of the initiator.
If the desired data address (in the previously described READ command
example) is unknown, but a search key defined as some particular bytes of a
field is known, then by linking the READ command to a SEARCH DATA EQUAL
command, the data can be quickly and effectively transferred to the initiator.
A LINKED COMMAND COMPLETE message is sent from the target to the initiator
to indicate linked command completion. The initiator then updates the stored
pointers so that subsequent requests from the target will reference the next
command of the chain. Command processing of linked and single commands is
simular except that relative addressing is permitted in linked commands.
Linked commands shall be addressed to a single logical unit.
For example, the successful completion of a SEARCH DATA EQUAL command causes
the target to fetch the linked READ command from the initiator. If the
relative address bit in the READ command has been set to one, and the address
field of the READ command is set to zero, the target transfers the
successfully searched block to the initiator.
This American National Standard provides the mechanical, electrical, and
functional requirements for a small computer input/output bus and command sets
for peripheral device types commonly used with small computers.
The small computer system interface, described in this standard, is a local
I/O bus that can be operated at data rates up to 4 megabytes per second
depending upon circuit implementation choices. The primary objective of the
interface is to provide host computers with device independence within a
class of devices. Thus, different disk drives, tape drives, printers, and
even communication devices can be added to the host computer(s) without
requiring modifications to generic system hardware or software. Provision
is made for the addition of nongeneric features and functions through vendor
unique fields and codes.
The interface uses logical rather than physical addressing for all data
blocks. For direct access devices, each logical unit may be interrogated to
determine how many blocks it contains. A logical unit may coincide with all
or part of a peripheral device.
Provision is made for cable lengths up to 25 meters using differential
drivers and receivers. A single-ended driver and receiver configuration is
defined for cable lengths of up to 6 meters and is primarily intended for
applications within a cabinet.
The interface protocol includes provision for the connection of multiple
initiators (SCSI devices capable of initiating an operation) and multiple
targets (SCSI devices capable of responding to a request to perform an
operation). Optional distributed arbitration (i.e., bus-contention logic) is
built into the architecture of SCSI. A priority system awards interface
control to the highest priority SCSI device that is contending for use of the
bus. The time to complete arbitration is independent of the number of devices
that are contending and can be completed in less than 10 microseconds.
The physical characteristics are described in Section 4. There are two
electrical alternatives: single-ended and differential. Single-ended and
differential devices are electrically different and shall not be mixed on the
same bus. In addition, there are several options: shielded or unshielded
connectors may be used and parity may or may not be implemented.
Section 5 describes the logical characteristics of the interface. An
arbitration option is defined to permit multiple initiators and to permit
concurrent I/O operations. All SCSI devices are required to be capable of
operating with the defined asynchronous transfer protocol. In addition, an
optional synchronous transfer protocol is defined. Section 5 also specifies a
message protocol for control of the interface. In most cases, messages are
not directly apparent to the host computer software. Only one message,
COMMAND COMPLETE, is mandatory; all others are optional and are not
necessarily implemented. Note that some options (e.g., synchronous transfer)
require the implementation of certain messages.
The SCSI command structure is specified in Section 6. Commands are
classified as mandatory (M), extended (E), optional (O), or vendor unique (V).
SCSI devices shall implement all mandatory commands defined for the
appropriate device type and may implement other commands as well. Extended
SCSI devices shall implement all extended plus all mandatory commands and may
implement other commands as well. Extended SCSI devices contain commands that
facilitate the writing of self-configuring software drivers that can
"discover" all necessary attributes without prior knowledge of specific
peripheral characteristics (such as storage capacity). Extended commands for
direct access devices also implement a very large logical block address space
(232 blocks), although mandatory commands for direct access devices implement
a somewhat smaller logical block address space (221 blocks).
Section 7 specifies those commands that have a consistent meaning for all
device types.
Sections 8 through 13 contain commands for direct-access (e.g., magnetic
disk), sequential-access (e.g., magnetic tape), printer, processor, write-
once-read-multiple (e.g., optical disk), and read-only direct-access devices,
respectively. The commands in each of these sections are unique to the device
type, or they have interpretations, fields, or features that are specific for
the device type. Thus, for example, although the WRITE command is used for
several device types, it has a somewhat different form for each type, with
different parameters and meanings. Therefore, it is specified separately for
each device type.
Section 14 describes the status byte for all device types. Status is
returned by targets at the end of each command.
Appendixes A through C provide examples of SCSI signal sequences, timing,
and phase sequences. Appendix D contains information on recommended shielded
connectors. Appendix E contains information on conformance statements.
Appendix F contains information on other standards related to medium types and
density codes for flexible disks and magnetic tapes. Appendix G contains
information on future extensions to SCSI that are being considered by X3T9.2
However, the appendixes are not part of this standard.
2. Referenced Standard
This standard is intended for use in conjunction with EIA RS-485-1983,
Standard for Electrical Characteristics of Generators and Receivers for use in
Balanced Digital Multipoint Systems.1
3. Glossary and Conventions
3.1 Glossary
byte. In this standard, this term indicates an 8-bit (octet) byte.
command descriptor block (CDB). The structure used to communicate requests
from an initiator to a target.
connect. The function that occurs when an initiator selects a target to start
an operation.
____________
1 Available from the Electronic Industries Association, 2001 Eye Street NW,
Washington, D.C. 20006.
disconnect. The function that occurs when a target releases control of the
SCSI bus, allowing it to go to the BUS FREE phase.
initiator. An SCSI device (usually a host system) that requests an operation
to be performed by another SCSI device.
INTERMEDIATE status. A status code sent from a target to an initiator upon
completion of each command in a set of linked commands except the last command
in the set.
logical unit. A physical or virtual device addressable through a target.
logical unit number. An encoded three-bit identifier for the logical unit.
LSB. Least significant byte.
LUN. Logical unit number.
mm. Millimeter.
ms. Millisecond.
MSB. Most significant byte.
ns. Nanosecond.
one. A true signal value.
peripheral device. A peripheral that can be attached to an SCSI device
(e.g., magnetic-disk, printer, optical-disk, or magnetic-tape).
reconnect. The function that occurs when a target selects an initiator to
continue an operation after a disconnect.
reserved. The term used for bits, bytes, fields, and code values that are set
aside for future standardization.
SCSI address. The octal representation of the unique address (0-7) assigned
to an SCSI device. This address would normally be assigned and set in the
SCSI device during system installation.
SCSI ID. The bit-significant representation of the SCSI address referring to
one of the signal lines DB(7-0).
SCSI device. A host computer adapter or a peripheral controller or an
intelligent peripheral that can be attached to the SCSI bus.
signal assertion. The act of driving a signal to the true state.
signal negation. The act of driving a signal to the false state or allowing
the cable terminators to bias the signal to the false state (by placing the
driver in the high impedance condition).
signal release. The act of allowing the cable terminators to bias the signal
to the false state (by placing the driver in the high impedance condition).
status. One byte of information sent from a target to an initiator upon
completion of each command.
target. An SCSI device that performs an operation requested by an initiator.
us. Microsecond.
vendor unique. In this standard, this term indicates bits, fields, or code
values that are vendor specific.
xxH. Numbers followed by capital H subscript are hexadecimal values. All
other numbers are decimal values.
zero. A false signal value.
3.2 Editorial Conventions. Certain words and terms used in this standard have
a specific meaning beyond the normal English meaning. These words and terms
are defined either in the glossary (see 3.1) or in the text where they first
appear (e.g., initiator). Names of signals, phases, conditions, messages,
commands, statuses, and sense keys are in all uppercase (e.g., REQUEST SENSE).
Lowercase is used for words having the normal English meaning.
4. Physical Characteristics
This section contains the physical definition of the SCSI. The connectors,
cables, signals, terminators, and bus timing needed to implement SCSI are
covered.
4.1 Physical Description. SCSI devices are daisy-chained together using a
common cable. Both ends of the cable are terminated. All signals are common
between all SCSI devices. Two driver/receiver alternatives are available:
(1) Single-ended drivers and receivers, which allow a maximum cable length
of six meters (primarily for connection within a cabinet)
(2) Differential drivers and receivers, which allow a maximum cable length
of 25 meters (primarily for connection outside of a cabinet)
4.2 Cable Requirements. An ideal impedance match with cable terminators
implies a cable characteristic impedance of 132 ohms (singled-ended option) or
122 ohms (differential option). In general, cables with this high of a
characteristic impedance are not available; however, impedances that are
somewhat lower are satisfactory. A characteristic impedance of 100 ohms + 10%
is recommended for unshielded flat or twisted pair ribbon cable. A
characteristic impedance greater than 90 ohms is preferred for shielded
cables; however, most available cables have a somewhat lower characteristic
impedance. To minimize discontinuities and signal reflections, cables of
different impedances should not be used in the same bus. Implementations may
require trade-offs in shielding effectiveness, cable length, the number of
loads, transfer rates, and cost to achieve satisfactory system operation.
A minimum conductor size of 28 AWG shall be employed to minimize noise
effects and ensure proper distribution of optional terminator power.
4.2.1 Single-Ended Cable. A 50-conductor flat cable or 25-signal twisted-
pair cable shall be used. The maximum cable length shall be 6.0 meters.
A stub length of no more than 0.1 meters is allowed off the mainline
interconnection within any connected equipment.
SCSI bus termination may be internal to the SCSI devices that are at the
ends of the cable.
4.2.2 Differential Cable. A 50-conductor cable or 25-signal twisted-pair
cable shall be used. The maximum cable length shall be 25 meters.
A stub length of no more than 0.2 meters is allowed off the mainline
interconnection within any connected equipment.
SCSI bus termination may be internal to the SCSI devices that are at the
ends of the cable.
4.3 Connector Requirements. Nonshielded connectors are specified. The
nonshielded connectors are typically used for in-cabinet applications.
Appendix D defines recommended shielded connectors and their pin assignments.
These connectors are typically used for external applications where
electromagnetic compatibility (EMC) and electrostatic discharge (ESD)
protection may be required. Either type of connector may be used with the
single-ended or differential drivers.
The nonshielded SCSI device connector (Figure 4-1) shall be a 50-conductor
connector consisting of two rows of 25 male pins with adjacent pins 2.54 mm
(0.1 in) apart. A shroud and header body should be used. The nonmating
portion of the connector is shown for reference only.
The nonshielded cable connector (Figure 4-2) shall be a 50-conductor
connector consisting of two rows of 25 female contacts with adjacent contacts
2.54 mm (0.1 in) apart. It is recommended that keyed connectors be used.
The unshielded connector pin assignments shall be as shown in Table 4-1 for
single-ended drivers and as shown in Table 4-2 for differential drivers.
Figure 4-1a. Nonshielded SCSI Device Connector
==============================================================================
Dimensions Millimeters Inches
------------------------------------------------------------------------------
D1 2.54 0.100
D2* 82.80 3.260
D3 2.54 0.100
D4 4.83 0.190
D5* 8.51 0.335
D6* 72.64 2.860
D7* 78.74 3.100
D8* 13.94 0.549
D9 4.19 0.165
D10 6.09 0.240
D11 6.60 0.260
==============================================================================
NOTES:
(1) Fifty Contacts on 2.54-mm (0.100-inch) spacing = 60.96 mm (2.40 inch).
(2) Tolerances + 0.127 mm (0.005 inch) noncumulative.
(3) Dimensions listed with asterisks (*) are shown for reference only.
Figure 4-1b. Nonshielded SCSI Device Connector
(Editors note: Figures 4-1a and 4-1b are to be combined into a single figure
during the final editing.)
Figure 4-2a. Nonshielded Cable Connector
==============================================================================
Dimensions Millimeters Inches
------------------------------------------------------------------------------
C1 2.5400 0.100
C2 60.9600 2.400
C3 2.5400 0.100
C4 8.3570 0.329
C5 3.3025 0.130
C6 68.0720 2.680
C7 6.0960 0.240
C8* 8.1530 0.321
C9* 13.4870 0.531
C10* 3.8100 0.150
C11* 1.2700 0.050
C12* 6.0960 0.240
C13 32.3850 1.275
C14 3.3020 0.130
C15 7.4930 0.295
C16 2.6670 0.105
C17 1.6250 0.064
==============================================================================
NOTES:
(1) Fifty contacts on 1.27-mm (0.05-inch)* staggered spacing = 62.23 mm
(2.450 inch)*.
(2) Tolerances + 0.127 mm (0.005 inch) noncumulative.
(3) Dimensions listed with asterisks (*) are shown for reference only.
Figure 4-2b. Nonshielded Cable Connector
(Editors note: Figures 4-2a and 4-2b are to be combined into a single figure
during the final editing.)
(Pages 17-19 and 19.1-19.4 are deleted. The information previously contained
on these pages has been moved to Appendix D.)
Table 4-1
Single-Ended Pin Assignments
==============================================================================
Signal Pin Number
------------------------------------------------------------------------------
-DB(0) 2
-DB(1) 4
-DB(2) 6
-DB(3) 8
-DB(4) 10
-DB(5) 12
-DB(6) 14
-DB(7) 16
-DB(P) 18
GROUND 20
GROUND 22
GROUND 24
TERMPWR 26
GROUND 28
GROUND 30
-ATN 32
GROUND 34
-BSY 36
-ACK 38
-RST 40
-MSG 42
-SEL 44
-C/D 46
-REQ 48
-I/O 50
==============================================================================
NOTES:
(1) All odd pins except pin 25 shall be connected to ground. Pin 25 should
be left open. Some products designed prior to the generation of this standard
connected this pin to ground.
(2) The minus sign next to the signals indicates active low.
Table 4-2
Differential Pin Assignments
==============================================================================
Signal Name Pin Number Signal Name
------------------------------------------------------------------------------
SHIELD GROUND 1 2 GROUND
+DB(0) 3 4 -DB(0)
+DB(1) 5 6 -DB(1)
+DB(2) 7 8 -DB(2)
+DB(3) 9 10 -DB(3)
+DB(4) 11 12 -DB(4)
+DB(5) 13 14 -DB(5)
+DB(6) 15 16 -DB(6)
+DB(7) 17 18 -DB(7)
+DB(P) 19 20 -DB(P)
DIFFSENS 21 22 GROUND
GROUND 23 24 GROUND
TERMPWR 25 26 TERMPWR
GROUND 27 28 GROUND
+ATN 29 30 -ATN
GROUND 31 32 GROUND
+BSY 33 34 -BSY
+ACK 35 36 -ACK
+RST 37 38 -RST
+MSG 39 40 -MSG
+SEL 41 42 -SEL
+C/D 43 44 -C/D
+REQ 45 46 -REQ
+I/O 47 48 -I/O
GROUND 49 50 GROUND
==============================================================================
NOTE:
(1) SHIELD GROUND is optional on some cables. (Implementors note: Some
shielded flat ribbon cables use pin 1 as a connection to the shield.)
4.4 Electrical Description
NOTE: For these measurements, SCSI bus termination is assumed to be external
to the SCSI device. An SCSI device may have the provision for allowing
optional internal termination.
4.4.1 Single-Ended Alternative. All assigned signals shall be terminated
with 220 ohms to +5 volts (nominal) and 330 ohms to ground at each end of the
cable. (See Figure 4-5.) All signals shall use open-collector or three-state
drivers.
4.4.1.1 Output Characteristics. Each signal driven by an SCSI device
shall have the following output characteristics when measured at the SCSI
device's connector:
Signal assertion = 0.0 volts dc to 0.4 volts dc
Minimum driver output capability = 48 milliamps (sinking) at 0.5 volts dc
Signal negation = 2.5 volts dc to 5.25 volts dc
4.4.1.2 Input Characteristics. Each signal received by an SCSI device
shall have the following input characteristics when measured at the SCSI
device's connector:
Signal true = 0.0 volts dc to 0.8 volts dc
Maximum total input load = -0.4 milliamps at 0.4 volts dc
Signal false = 2.0 volts dc to 5.25 volts dc
Minimum input hysteresis = 0.2 volts dc
4.4.2 Differential Alternative. All signals consist of two lines denoted
+SIGNAL and -SIGNAL. A signal is true when +SIGNAL is more positive than
-SIGNAL, and a signal is false when -SIGNAL is more positive than +SIGNAL.
All assigned signals shall be terminated at each end of the cable as shown in
Figure 4-6.
NOTE: As an option, the DIFFSENS signal of the connector is reserved for an
active high enable for the differential drivers. If a single-ended device or
terminator is inadvertently connected, this signal is grounded, disabling the
drivers. (See Figure 4-7.)
4.4.2.1 Output Characteristics. Each signal driven by an SCSI device
shall have the following output characteristics when measured at the SCSI
device's connector:
VOL (Low-level output voltage) = 2.0 V maximum at IOL (Low-level output
current) = 55 milliamps.
VOH (High-level output voltage) = 3.0 V minimum at IOH (High-level output
current) = -55 milliamps.
VOD (Differential voltage) = 1.0 V minimum with common-mode voltage ranges
from -7 volts dc to +12 volts dc.
VOL and VOH shall be as measured between the output terminal and the SCSI
device's logic ground reference.
The output characteristics shall additionally conform to EIA RS-485-1983.
4.4.2.2 Input Characteristics. Each signal received by an SCSI device
shall have the following input characteristics when measured at the SCSI
device's connector:
II (Input current on either input) = + 2.0 milliamps maximum.
NOTE: These characteristics include both receivers and passive drivers.
This requirement shall be met with the input voltage varying between
-7 volts dc and +12 volts dc, with power on or off, and with the hysteresis
equaling 35 millivolts, minimum.
The input characteristics shall additionally conform to EIA RS-485-1983.
4.4.3 Terminator Power (Optional). Single-ended SCSI devices providing
terminator power (TERMPWR) shall have the following characteristics:
VTerm = 4.0 volts dc to 5.25 volts dc
800 milliamps minimum source drive capability
1.0 milliamp maximum sink capability (except for the purposes of
providing power to an internal terminator) with 1.0 amp
recommended current limiting (e.g., a fuse).
Differential SCSI devices providing terminator power (TERMPWR) shall have
the following characteristics:
VTerm = 4.0 volts dc to 5.25 volts dc
600 milliamps minimum source drive capability
1.0 milliamp maximum sink capability (except for the purposes of
providing power to an internal terminator) with 1.0 amp
recommended current limiting (e.g., a fuse).
The use of keyed connectors is recommended in SCSI devices that provide
terminator power to prevent accidental grounding or misconnection of
terminator power.
SCSI devices that supply terminator power shall do so through a diode or
similar semiconductor that prevents the backflow of power to the SCSI device.
Figure 4-5. Termination for Single-Ended Devices
Figure 4-6. Termination for Differential Devices
Figure 4-7. Differential Driver Protection Circuit (Optional)
4.5 SCSI Bus. Communication on the SCSI bus is allowed between only two SCSI
devices at any given time. There is a maximum of eight SCSI devices. Each
SCSI device has an SCSI ID bit assigned as shown in Figure 4-8.
When two SCSI devices communicate on the SCSI bus, one acts as an initiator
and the other acts as a target. The initiator originates an operation and the
target performs the operation. An SCSI device usually has a fixed role as an
initiator or target, but some devices may be able to assume either role.
An initiator may address up to eight peripheral devices that are connected
to a target. An option allows the addressing of up to 2,048 peripheral
devices per target using extended messages. Three sample system
configurations are shown in Figure 4-9.
DB(7) DB(6) DB(5) DB(4) DB(3) DB(2) DB(1) DB(0) <-- DATA BUS
| | | | | | | |
| | | | | | | SCSI ID = 0
| | | | | | |
| | | | | | SCSI ID = 1
| | | | | |
| | | | | SCSI ID = 2
| | | | |
| | | | SCSI ID = 3
| | | |
| | | SCSI ID = 4
| | |
| | SCSI ID = 5
| |
| SCSI ID = 6
|
SCSI ID = 7
Figure 4-8. SCSI ID Bits
Figure 4-9. Sample SCSI Configurations
Up to eight SCSI devices can be supported on the SCSI bus. They can be any
combination of initiators and targets.
Certain SCSI bus functions are assigned to the initiator and certain SCSI
bus functions are assigned to the target. The initiator may arbitrate for the
SCSI bus and select a particular target. The target may request the transfer
of COMMAND, DATA, STATUS, or other information on the DATA BUS, and in some
cases it may arbitrate for the SCSI bus and reselect an initiator for the
purpose of continuing an operation.
Information transfers on the DATA BUS are asynchronous and follow a defined
REQ/ACK handshake protocol. One byte of information may be transferred with
each handshake. An option is defined for synchronous data transfer.
4.6 SCSI Bus Signals. There are a total of eighteen signals. Nine are used
for control and nine are used for data. (Data signals include the parity
signal option). These signals are described as follows:
BSY (BUSY). An "OR-tied" signal that indicates that the bus is being used.
SEL (SELECT). A signal used by an initiator to select a target or by a target
to reselect an initiator.
C/D (CONTROL/DATA). A signal driven by a target that indicates whether
CONTROL or DATA information is on the DATA BUS. True indicates CONTROL.
I/O (INPUT/OUTPUT). A signal driven by a target that controls the direction
of data movement on the DATA BUS with respect to an initiator. True indicates
input to the initiator. This signal is also used to distinguish between
SELECTION and RESELECTION phases.
MSG (MESSAGE). A signal driven by a target during the MESSAGE phase.
REQ (REQUEST). A signal driven by a target to indicate a request for a
REQ/ACK data transfer handshake.
ACK (ACKNOWLEDGE). A signal driven by an initiator to indicate an
acknowledgment for a REQ/ACK data transfer handshake.
ATN (ATTENTION). A signal driven by an initiator to indicate the ATTENTION
condition.
RST (RESET). An "OR-tied" signal that indicates the RESET condition.
DB(7-0,P) (DATA BUS). Eight data-bit signals, plus a parity-bit signal that
form a DATA BUS. DB(7) is the most significant bit and has the highest
priority during the ARBITRATION phase. Bit number, significance, and priority
decrease downward to DB(0). A data bit is defined as one when the signal
value is true and is defined as zero when the signal value is false.
Data parity DB(P) is odd. The use of parity is a system option (i.e., a
system is configured so that all SCSI devices on a bus generate parity and
have parity detection enabled, or all SCSI devices have parity detection
disabled or not implemented). Parity is not valid during the ARBITRATION
phase.
4.6.1 Signal Values. Signals may assume true or false values. There are
two methods of driving these signals. In both cases, the signal shall be
actively driven true, or asserted. In the case of OR-tied drivers, the driver
does not drive the signal to the false state, rather the bias circuitry of the
bus terminators pulls the signal false whenever it is released by the drivers
at every SCSI device. If any driver is asserted, then the signal is true. In
the case of non-OR-tied drivers, the signal may be actively driven false, or
negated. In this standard, wherever the term negated is used, it means that
the signal may be actively driven false, or may be simply released (in which
case the bias circuitry pulls it false), at the option of the implementor.
The advantage to actively drive signals false is that the transition from true
to false occurs more quickly, and noise margins may be somewhat improved; this
may permit somewhat faster data transfer.
4.6.2 OR-Tied Signals. The BSY and RST signals shall be OR-tied only. In
the ordinary operation of the bus, these signals are simultaneously driven
true by several drivers. No signals other than BSY, RST, and DB(P) are
simultaneously driven by two or more drivers, and any signal other than BSY
and RST may employ OR-tied or non-OR-tied drivers. DB(P) shall not be driven
false during the ARBITRATION phase. There is no operational problem in mixing
OR-tied and non-OR-tied drivers on signals other than BSY and RST.
4.6.3 Signal Sources. Table 4-3 indicates which type of SCSI device is
allowed to source each signal. No attempt is made to show if the source is
driving asserted, driving negated, or is passive. All SCSI device drivers
that are not active sources shall be in the passive state. Note that the RST
signal may be sourced by any SCSI device at any time.
Table 4-3
Signal Sources
==============================================================================
Signals
----------------------------------------------------------
C/D, I/O,
Bus Phase BSY SEL MSG, REQ ACK/ATN DB(7-0,P)
------------------------------------------------------------------------------
BUS FREE None None None None None
ARBITRATION All Winner None None SCSI ID
SELECTION I&T Initiator None Initiator Initiator
RESELECTION I&T Target Target Initiator Target
COMMAND Target None Target Initiator Initiator
DATA IN Target None Target Initiator Target
DATA OUT Target None Target Initiator Initiator
STATUS Target None Target Initiator Target
MESSAGE IN Target None Target Initiator Target
MESSAGE OUT Target None Target Initiator Initiator
==============================================================================
All: The signal shall be driven by all SCSI devices that are
actively arbitrating.
SCSI ID: A unique data bit (the SCSI ID) shall be driven by each SCSI
device that is actively arbitrating; the other seven data bits
shall be released (i.e., not driven) by this SCSI device. The
parity bit (DB(P)) may be undriven or driven to the true state,
but shall never be driven to the false state during this phase.
I&T: The signal shall be driven by the initiator, target, or both, as
specified in the SELECTION phase and RESELECTION phase.
Initiator: If this signal is driven, it shall be driven only by the active
initiator.
None: The signal shall be released; that is, not be driven by any SCSI
device. The bias circuitry of the bus terminators pulls the
signal to the false state.
Winner: The signal shall be driven by the one SCSI device that wins
arbitration.
Target: If the signal is driven, it shall be driven only by the active
target.
4.7 SCSI Bus Timing. Unless otherwise indicated, the delay-time measurements
for each SCSI device, shown in 4.7.1 through 4.7.14, shall be calculated from
signal conditions existing at that SCSI device's own SCSI bus connection.
Thus, these measurements (except cable skew delay) can be made without
considering delays in the cable.
4.7.1 Arbitration Delay (2.2 microseconds). The minimum time an SCSI device
shall wait from asserting BSY for arbitration until the DATA BUS can be
examined to see if arbitration has been won. There is no maximum time.
4.7.2 Assertion Period (90 nanoseconds). The minimum time that a target
shall assert REQ while using synchronous data transfers. Also, the minimum
time that an initiator shall assert ACK while using synchronous data
transfers.
4.7.3 Bus Clear Delay (800 nanoseconds). The maximum time for an SCSI
device to stop driving all bus signals after:
(1) The BUS FREE phase is detected (BSY and SEL both false for a bus settle
delay)
(2) SEL is received from another SCSI device during the ARBITRATION phase
(3) The transition of RST to true.
NOTE: For the first condition above, the maximum time for an SCSI device to
clear the bus is 1200 nanoseconds from BSY and SEL first becoming both false.
If an SCSI device requires more than a bus settle delay to detect BUS FREE
phase, it shall clear the bus within a bus clear delay minus the excess time.
4.7.4 Bus Free Delay (800 nanoseconds). The minimum time that an SCSI
device shall wait from its detection of the BUS FREE phase (BSY and SEL both
false for a bus settle delay) until its assertion of BSY when going to the
ARBITRATION phase.
4.7.5 Bus Set Delay (1.8 microseconds). The maximum time for an SCSI device
to assert BSY and its SCSI ID bit on the DATA BUS after it detects BUS FREE
phase (BSY and SEL both false for a bus settle delay) for the purpose of
entering the ARBITRATION phase.
4.7.6 Bus Settle Delay (400 nanoseconds). The time to wait for the bus to
settle after changing certain control signals as called out in the protocol
definitions.
4.7.7 Cable Skew Delay (10 nanoseconds). The maximum difference in
propagation time allowed between any two SCSI bus signals when measured
between any two SCSI devices.
4.7.8 Data Release Delay (400 nanoseconds). The maximum time for an
initiator to release the DATA BUS signals following the transition of the I/O
signal from false to true.
4.7.9 Deskew Delay (45 nanoseconds). The minimum time required for deskew
of certain signals.
4.7.10 Hold Time (45 nanoseconds). The minimum time added between the
assertion of REQ or ACK and the changing of the data lines to provide hold
time in the initiator or target, respectively, while using synchronous data
transfers.
4.7.11 Negation Period (90 nanoseconds). The minimum time that a target
shall negate REQ while using synchronous data transfers. Also, the minimum
time that an initiator shall negate ACK while using synchronous data
transfers.
4.7.12 Reset Hold Time (25 microseconds). The minimum time for which RST is
asserted. There is no maximum time.
4.7.13 Selection Abort Time (200 microseconds). The maximum time that a
target (or initiator) shall take from its most recent detection of being
selected (or reselected) until asserting a BSY response. This timeout is
required to ensure that a target (or initiator) does not assert BSY after a
SELECTION (or RESELECTION) phase has been aborted. This is not the selection
timeout period; see Sections 5.1.3.5 and 5.1.4.2 for a complete description.
4.7.14 Selection Timeout Delay (250 milliseconds, recommended). The minimum
time that an initiator (or target) should wait for a BSY response during the
SELECTION (or RESELECTION) phase before starting the timeout procedure. Note
that this is only a recommended time period. The specifications for the
peripheral devices shall be consulted for the actual timing requirements.
4.7.15 Transfer Period (set during a MESSAGE phase). The Transfer Period
specifies the minimum time allowed between the leading edges of successive REQ
pulses and of successive ACK pulses while using synchronous data transfers.
(See Sections 5.1.5.2 and 5.5.5.)
5. Logical Characteristics
5.1 SCSI Bus Phases. The SCSI architecture includes eight distinct phases:
BUS FREE phase
ARBITRATION phase
SELECTION phase
RESELECTION phase
COMMAND phase \
DATA phase \ These phases are collectively termed the
STATUS phase / information transfer phases.
MESSAGE phase /
The SCSI bus can never be in more than one phase at any given time. Unless
otherwise noted in the following descriptions, signals that are not mentioned
shall not be asserted.
5.1.1 BUS FREE Phase. The BUS FREE phase is used to indicate that no SCSI
device is actively using the SCSI bus and that it is available for subsequent
users.
SCSI devices shall detect the BUS FREE phase after SEL and BSY are both
false for at least a bus settle delay.
SCSI devices shall release all SCSI bus signals within a bus clear delay
after BSY and SEL become continuously false for a bus settle delay. If an
SCSI device requires more than a bus settle delay to detect the BUS FREE phase
then it shall release all SCSI bus signals within a bus clear delay minus the
excess time to detect the BUS FREE phase. The total time to clear the SCSI
bus shall not exceed a bus settle delay plus a bus clear delay.
5.1.2 ARBITRATION Phase. The ARBITRATION phase allows one SCSI device to
gain control of the SCSI bus so that it can assume the role of an initiator or
target.
NOTE: Implementation of the ARBITRATION phase is a system option. Systems
that do not implement this option can have only one initiator. The
ARBITRATION phase is required for systems that use the RESELECTION phase.
The procedure for an SCSI device to obtain control of the SCSI bus is as
follows:
(1) The SCSI device shall first wait for the BUS FREE phase to occur. The
BUS FREE phase is detected whenever both BSY and SEL are simultaneously and
continuously false for a minimum of a bus settle delay. (Implementors Note:
This bus settle delay is necessary because a transmission line phenomenon
known as a "wire-OR glitch" may cause BSY to briefly appear false, even though
it is being driven true.)
(2) The SCSI device shall wait a minimum of a bus free delay after
detection of the BUS FREE phase (i.e. after BSY and SEL are both false for a
bus settle delay) before driving any signal.
(3) Following the bus free delay in Step (2), the SCSI device may arbitrate
for the SCSI bus by asserting both BSY and its own SCSI ID, however the SCSI
device shall not arbitrate (i.e. assert BSY and its SCSI ID) if more than a
bus set delay has passed since the BUS FREE phase was last observed.
(Implementors Note: There is no maximum delay before asserting BSY and the
SCSI ID following the bus free delay in Step (2) as long as the bus remains in
the BUS FREE phase. However, SCSI devices that delay longer than a bus settle
delay plus a bus set delay from the time when BSY and SEL first become false
may fail to participate in arbitration when competing with faster SCSI
devices.)
(4) After waiting at least an arbitration delay (measured from its
assertion of BSY) the SCSI device shall examine the DATA BUS. If a higher
priority SCSI ID bit is true on the DATA BUS (DB(7) is the highest), then the
SCSI device has lost the arbitration and the SCSI device may release its
signals and return to Step (1). If no higher priority SCSI ID bit is true on
the DATA BUS, then the SCSI device has won the arbitration and it shall assert
SEL. Any other SCSI device that is participating in the ARBITRATION phase has
lost the arbitration and shall release BSY and its SCSI ID bit within a bus
clear delay after SEL becomes true. An SCSI device that loses arbitration may
return to Step (1).
(5) The SCSI device that wins arbitration shall wait at least a bus clear
delay plus a bus settle delay after asserting SEL before changing any signals.
NOTE: The SCSI ID bit is a single bit on the DATA BUS that corresponds to the
SCSI device's unique SCSI address. All other seven DATA BUS bits shall be
released by the SCSI device. Parity is not valid during the ARBITRATION
phase. During the ARBITRATION phase, DB(P) may be undriven or driven to the
true state, but shall not be driven to the false state.
5.1.3 SELECTION Phase. The SELECTION phase allows an initiator to select a
target for the purpose of initiating some target function (e.g., READ or WRITE
command).
NOTE: During the SELECTION phase the I/O signal shall be negated so that this
phase can be distinguished from the RESELECTION phase.
5.1.3.1 Nonarbitrating Systems. In systems with the ARBITRATION phase
not implemented, the initiator shall first detect the BUS FREE phase and then
wait a minimum of a bus clear delay. Then, except in certain single initiator
environments with initiators employing the single initiator option (see
5.1.3.4), the initiator shall assert the desired target's SCSI ID and its own
initiator SCSI ID on the DATA BUS. After two deskew delays the initiator
shall assert SEL.
5.1.3.2 Arbitrating Systems. In systems with ARBITRATION phase
implemented, the SCSI device that won the arbitration has both BSY and SEL
asserted and has delayed at least a bus clear delay plus a bus settle delay
before ending the ARBITRATION phase. The SCSI device that won the arbitration
becomes an initiator by releasing I/O. Except in certain single initiator
environments with initiators employing the single initiator option (see
5.1.3.4), the initiator shall set the DATA BUS to a value which is the OR of
its SCSI ID bit and the target's SCSI ID bit. The initiator shall then wait
at least two deskew delays and release BSY. The initiator shall then wait at
least a bus settle delay before looking for a response from the target.
5.1.3.3 All Systems. In all systems, the target shall determine that it
is selected when SEL and its SCSI ID bit are true and BSY and I/O are false
for at least a bus settle delay. The selected target may examine the DATA BUS
in order to determine the SCSI ID of the selecting initiator unless the
initiator employed the single initiator option (see 5.1.3.4). The selected
target shall then assert BSY within a selection abort time of its most recent
detection of being selected; this is required for correct operation of the
timeout procedure. In systems with parity implemented, the target shall not
respond to a selection if bad parity is detected. Also, if more than two SCSI
ID bits are on the DATA BUS, the target shall not respond to selection.
At least two deskew delays after the initiator detects BSY is true, it shall
release SEL and may change the DATA BUS.
5.1.3.4 Single Initiator Option. Initiators that do not implement the
RESELECTION phase and do not operate in the multiple initiator environment are
allowed to set only the target's SCSI ID bit during the SELECTION phase. This
makes it impossible for the target to determine the initiator's SCSI ID.
5.1.3.5 Selection Timeout Procedure. Two optional selection timeout
procedures are specified for clearing the SCSI bus if the initiator waits a
minimum of a selection timeout delay and there has been no BSY response from
the target:
(1) Optionally, the initiator shall assert the RST signal (see 5.2.2).
(2) Optionally, the initiator shall continue asserting SEL and shall release
the DATA BUS. If the initiator has not detected BSY to be true after at least
a selection abort time plus two deskew delays, the initiator shall release SEL
allowing the SCSI bus to go to the BUS FREE phase. SCSI devices shall ensure
that when responding to selection that the selection was still valid within a
selection abort time of their assertion of BSY. Failure to comply with this
requirement could result in an improper selection (two targets connected to
the same initiator, wrong target connected to an initiator, or a target
connected to no initiator).
5.1.4 RESELECTION Phase (Optional). RESELECTION is an optional phase that
allows a target to reconnect to an initiator for the purpose of continuing
some operation that was previously started by the initiator but was suspended
by the target, (i.e., the target disconnected by allowing a BUS FREE phase to
occur before the operation was complete).
5.1.4.1 RESELECTION. RESELECTION can only be used in systems that have
ARBITRATION phase implemented.
Upon completing the ARBITRATION phase, the winning SCSI device has both BSY
and SEL asserted and has delayed at least a bus clear delay plus a bus settle
delay. The winning SCSI device becomes a target by asserting the I/O signal.
The winning SCSI device shall also set the DATA BUS to a value that is the OR
of its SCSI ID bit and the initiator's SCSI ID bit. The target shall wait at
least two deskew delays and release BSY. The target shall then wait at least
a bus settle delay before looking for a response from the initiator.
The initiator shall determine that it is reselected when SEL, I/O, and its
SCSI ID bit are true and BSY is false for at least a bus settle delay. The
reselected initiator may examine the DATA BUS in order to determine the SCSI
ID of the reselecting target. The reselected initiator shall then assert BSY
within a selection abort time of its most recent detection of being
reselected; this is required for correct operation of the timeout procedure.
In systems with parity implemented, the initiator shall not respond to a
RESELECTION if bad parity is detected. Also, the initiator shall not respond
to a RESELECTION if more than two SCSI ID bits are on the DATA BUS.
After the target detects BSY, it shall also assert BSY and wait at least two
deskew delays and then release SEL. The target may then change the I/O signal
and the DATA BUS. After the reselected initiator detects SEL false, it shall
release BSY. The target shall continue asserting BSY until the target is
ready to relinquish the SCSI bus.
NOTE: When the target is asserting BSY, a transmission line phenomenon known
as a "wire-OR glitch" may cause BSY to appear false for up to a round-trip
propagation delay following the release of BSY by the initiator. This is the
reason why the BUS FREE phase is recognized only after both BSY and SEL are
continuously false for a minimum of a bus settle delay. Cables longer than 25
meters should not be used even if the chosen driver, receiver, and cable
provide adequate noise margins, because they increase the duration of the
glitch and could cause SCSI devices to inadvertently detect the BUS FREE
phase.
5.1.4.2 RESELECTION Timeout Procedure. Two optional RESELECTION timeout
procedures are specified for clearing the SCSI bus during a RESELECTION phase
if the target waits a minimum of a selection timeout period and there has been
no BSY response from the initiator:
(1) Optionally, the target shall assert the RST signal (see 5.2.2).
(2) Optionally, the target shall continue asserting SEL and I/O and shall
release all DATA BUS signals. If the target has not detected BSY to be true
after at least a selection abort time plus two deskew delays, the target shall
release SEL and I/O allowing the SCSI bus to go to the BUS FREE phase. SCSI
devices that respond to RESELECTION shall ensure that the RESELECTION was
still valid within a selection abort time of their assertion of BSY. Failure
to comply with this requirement could result in an improper reselection (two
initiators connected to the same target or the wrong initiator connected to a
target).
5.1.5 Information Transfer Phases.
NOTE: The COMMAND, DATA, STATUS, and MESSAGE phases are all grouped together
as the information transfer phases because they are all used to transfer data
or control information via the DATA BUS. The actual contents of the
information is beyond the scope of this section.
The C/D, I/O, and MSG signals are used to distinguish between the different
information transfer phases. (See Table 5-1.) The target drives these three
signals and therefore controls all changes from one phase to another. The
initiator can request a MESSAGE OUT phase by asserting ATN, while the target
can cause the BUS FREE phase by releasing MSG, C/D, I/O, and BSY.
Table 5-1
Information Transfer Phases
==============================================================================
Signal
-----------
MSG C/D I/O Phase Name Direction Of Transfer Comment
------------------------------------------------------------------------------
0 0 0 DATA OUT Initiator to target \ Data
0 0 1 DATA IN Initiator from target / Phase
0 1 0 COMMAND Initiator to target
0 1 1 STATUS Initiator from target
1 0 0 *
1 0 1 *
1 1 0 MESSAGE OUT Initiator to target \ Message
1 1 1 MESSAGE IN Initiator from target / Phase
==============================================================================
Key: 0 = False, 1 = True, * = Reserved for future standardization.
The information transfer phases use one or more REQ/ACK handshakes to
control the information transfer. Each REQ/ACK handshake allows the transfer
of one byte of information. During the information transfer phases BSY shall
remain true and SEL shall remain false. Additionally, during the information
transfer phases, the target shall continuously envelope the REQ/ACK
handshake(s) with C/D, I/O, and MSG in such a manner that these control
signals are valid for a bus settle delay before the assertion of REQ of the
first handshake and remain valid until the negation of ACK at the end of the
last handshake.
5.1.5.1 Asynchronous Information Transfer. The target shall control the
direction of information transfer by means of the I/O signal. When I/O is
true, information shall be transferred from the target to the initiator. When
I/O is false, information shall be transferred from the initiator to the
target.
If I/O is true (transfer to the initiator), the target shall first drive
DB(7-0,P) to their desired values, delay at least one deskew delay plus a
cable skew delay, then assert REQ. DB(7-0,P) shall remain valid until ACK is
true at the target. The initiator shall read DB(7-0,P) after REQ is true,
then signal its acceptance of the data by asserting ACK. When ACK becomes
true at the target, the target may change or release DB(7-0,P) and shall
negate REQ. After REQ is false the initiator shall then negate ACK. After
ACK is false the target may continue the transfer by driving DB(7-0,P) and
asserting REQ, as described above.
If I/O is false (transfer to the target) the target shall request
information by asserting REQ. The initiator shall drive DB(7-0,P) to their
desired values, delay at least one deskew delay plus a cable skew delay and
assert ACK. The initiator shall continue to drive DB(7-0,P) until REQ is
false. When ACK becomes true at the target, the target shall read DB(7-0,P),
then negate REQ. When REQ becomes false at the initiator, the initiator may
change or release DB(7-0,P) and shall negate ACK. The target may continue the
transfer by asserting REQ, as described above.
5.1.5.2 Synchronous Data Transfer (Optional). Synchronous data transfer
is optional, and may be used only in the data phase if previously agreed to by
the initiator and target through the message system (see SYNCHRONOUS DATA
TRANSFER REQUEST message, 5.5.5). The messages determine the use of
synchronous mode by both SCSI devices and establish a REQ/ACK offset and a
transfer period.
The REQ/ACK offset specifies the maximum number of REQ pulses that can be
sent by the target in advance of the number of ACK pulses received from the
initiator, establishing a pacing mechanism. If the number of REQ pulses
exceeds the number of ACK pulses by the REQ/ACK offset, the target shall not
assert REQ until the next ACK pulse is received. A requirement for successful
completion of the data phase is that the number of ACK and REQ pulses be
equal.
The target shall assert the REQ signal for a minimum of an assertion period.
The target shall wait at least the greater of a transfer period from the last
transition of REQ to true or a minimum of a negation period from the last
transition of REQ to false before asserting the REQ signal.
The initiator shall send one pulse on the ACK signal for each REQ pulse
received. The initiator shall assert the ACK signal for a minimum of an
assertion period. The initiator shall wait at least the greater of a transfer
period from the last transition of ACK to true or for a minimum of a negation
period from the last transition of ACK to false before asserting the ACK
signal.
If I/O is true (transfer to the initiator), the target shall first drive
DB(7-0,P) to their desired values, wait at least one deskew delay plus one
cable skew delay, then assert REQ. DB(7-0,P) shall be held valid for a
minimum of one deskew delay plus one cable skew delay plus one hold time after
the assertion of REQ. The target shall assert REQ for a minimum of an
assertion period. The target may then negate REQ and change or release DB(7-
0,P). The initiator shall read the value on DB(7-0,P) within one hold time of
the transition of REQ to true. The initiator shall then respond with an ACK
pulse.
If I/O is false (transfer to the target), the initiator shall transfer one
byte for each REQ pulse received. After receiving a REQ pulse, the initiator
shall first drive DB(7-0,P) to their desired values, delay at least one deskew
delay plus one cable skew delay, then assert ACK. The initiator shall hold
DB(7-0,P) valid for at least one deskew delay plus one cable skew delay plus
one hold time after the assertion of ACK. The initiator shall assert ACK for
a minimum of an assertion period. The initiator may then negate ACK and may
change or release DB(7-0,P). The target shall read the value of DB(7-0,P)
within one hold time of the transition of ACK to true.
5.1.6 COMMAND Phase. The COMMAND phase allows the target to request command
information from the initiator.
The target shall assert the C/D signal and negate the I/O and MSG signals
during the REQ/ACK handshake(s) of this phase.
5.1.7 Data Phase. The data phase is a term that encompasses both the DATA
IN phase and the DATA OUT phase.
5.1.7.1 DATA IN Phase. The DATA IN phase allows the target to request
that data be sent to the initiator from the target.
The target shall assert the I/O signal and negate the C/D and MSG signals
during the REQ/ACK handshake(s) of this phase.
5.1.7.2 DATA OUT Phase. The DATA OUT phase allows the target to request
that data be sent from the initiator to the target.
The target shall negate the C/D, I/O, and MSG signals during the REQ/ACK
handshake(s) of this phase.
5.1.8 STATUS Phase. The STATUS phase allows the target to request that
status information be sent from the target to the initiator.
The target shall assert C/D and I/O and negate the MSG signal during the
REQ/ACK handshake of this phase.
5.1.9 Message Phase. The message phase is a term that references either a
MESSAGE IN, or a MESSAGE OUT phase. Multiple messages may be sent during
either phase. The first byte transferred in either of these phases shall be
either a single-byte message or the first byte of a multiple-byte message.
Multiple-byte messages shall be wholly contained within a single message
phase.
5.1.9.1 MESSAGE IN Phase. The MESSAGE IN phase allows the target to
request that message(s) be sent to the initiator from the target.
The target shall assert C/D, I/O, and MSG during the REQ/ACK handshake(s) of
this phase.
5.1.9.2 MESSAGE OUT Phase. The MESSAGE OUT phase allows the target to
request that message(s) be sent from the initiator to the target. The target
may invoke this phase at its convenience in response to the ATTENTION
condition (see 5.2.1) created by the initiator.
The target shall assert C/D and MSG and negate I/O during the REQ/ACK
handshake(s) of this phase. The target shall handshake byte(s) in this phase
until ATN goes false, unless an error occurs (see MESSAGE REJECT, 5.5.2).
If the target detects one or more parity error(s) on the message byte(s)
received, it may indicate its desire to retry the message(s) by asserting REQ
after detecting ATN has gone false and prior to changing to any other phase.
The initiator, upon detecting this condition, shall resend all of the previous
message byte(s) sent during this phase. When resending more than one message
byte, the initiator shall assert ATN prior to asserting ACK on the first byte
and shall maintain ATN asserted until the last byte is sent as described in
5.2.1.
If the target receives all of the message byte(s) successfully (i.e., no
parity errors), it shall indicate that it does not wish to retry by changing
to any information transfer phase other than the MESSAGE OUT phase and
transfer at least one byte. The target may also indicate that it has
successfully received the message byte(s) by changing to the BUS FREE phase
(e.g., ABORT or BUS DEVICE RESET messages).
5.1.10 Signal Restrictions Between Phases. When the SCSI bus is between two
information transfer phases, the following restrictions shall apply to the
SCSI bus signals:
(1) The BSY, SEL, REQ, and ACK signals shall not change.
(2) The C/D, I/O, MSG, and DATA BUS signals may change. When switching the
DATA BUS direction from out (initiator driving) to in (target driving), the
target shall delay driving the DATA BUS by at least a data release delay plus
a bus settle delay after asserting the I/O signal and the initiator shall
release the DATA BUS no later than a data release delay after the transition
of the I/O signal to true. When switching the DATA BUS direction from in
(target driving) to out (initiator driving), the target shall release the DATA
BUS no later than a deskew delay after negating the I/O signal.
(3) The ATN and RST signals may change as defined under the descriptions for
the ATTENTION condition (5.2.1) and RESET condition (5.2.2).
5.2 SCSI Bus Conditions. The SCSI bus has two asynchronous conditions; the
ATTENTION condition and the RESET condition. These conditions cause the SCSI
device to perform certain actions and can alter the phase sequence.
5.2.1 ATTENTION Condition. The ATTENTION condition allows an initiator to
inform a target that the initiator has a message ready. The target may get
this message at its convenience by performing a MESSAGE OUT phase.
The initiator creates the ATTENTION condition by asserting ATN at any time
except during the ARBITRATION or BUS FREE phases.
The target may respond with the MESSAGE OUT phase.
The initiator shall keep ATN asserted if more than one byte is to be
transferred. The initiator may negate the ATN signal at any time except it
shall not negate the ATN signal while the ACK signal is asserted during a
MESSAGE OUT phase. Normally, the initiator negates ATN while REQ is true and
ACK is false during the last REQ/ACK handshake of the MESSAGE OUT phase.
5.2.2 RESET Condition. The RESET condition is used to immediately clear all
SCSI devices from the bus. This condition shall take precedence over all
other phases and conditions. Any SCSI device may create the RESET condition
by asserting RST for a minimum of a reset hold time. During the RESET
condition, the state of all SCSI bus signals other than RST is not defined.
All SCSI devices shall release all SCSI bus signals (except RST) within a
bus clear delay of the transition of RST to true. The BUS FREE phase always
follows the RESET condition.
The effect of the RESET condition on uncompleted commands, SCSI device
reservations, and SCSI device operating modes is determined by whether the
SCSI device has implemented the "hard" RESET option or the "soft" RESET option
(one of which shall be implemented) as defined in 5.2.2.1 and 5.2.2.2.
5.2.2.1 "Hard" RESET Option. SCSI devices that implement the "hard"
RESET option, upon detection of the RESET condition, shall:
(1) Clear all uncompleted commands
(2) Release all SCSI device reservations
(3) Return any SCSI device operating modes (MODE SELECT, PREVENT/ALLOW
MEDIUM REMOVAL commands, etc) to their default conditions.
5.2.2.2 "Soft" RESET Option. SCSI devices that implement the "soft"
RESET option, upon detection of the RESET condition, shall:
(1) Attempt to complete any uncompleted commands that were fully identified
(2) Preserve all SCSI device reservations
(3) Preserve any SCSI device operating modes (MODE SELECT, PREVENT/ALLOW
MEDIUM REMOVAL commands, etc)
The "soft" RESET option allows a single initiator to reset the SCSI bus
without disturbing the operation of other initiators in a multiple initiator
system. To ensure proper operation the following conditions shall be met:
(1) An initiator shall not consider a command to be fully identified until
the IDENTIFY message is sent to the target and the target responds by changing
to any other information transfer phase and requests that at least one byte be
transferred.
(2) A target shall consider a command to be fully identified when it
successfully receives the IDENTIFY message.
(3) If an initiator selects a logical unit for which there already is an
active command for the same initiator, the target shall clear the original
command and perform the new command.
(4) If a target reselects an initiator to continue a command for which the
initiator has no record, the initiator shall abort that command by sending the
ABORT message.
(5) An initiator shall consider a command to be completed when it negates
ACK for a successfully received COMMAND COMPLETE message.
(6) A target shall consider a command to be completed when it detects the
false transition of ACK for the COMMAND COMPLETE message with the ATN signal
false.
(7) An initiator shall not negate ACK for the SAVE DATA POINTER message
until it has actually saved the data pointer for the operation.
(8) A target shall consider the data pointer to be saved when it detects the
false transition of ACK for the SAVE DATA POINTER message with the ATN signal
false.
(9) If the RESET condition occurs between the time that the target asserts
REQ for the SAVE DATA POINTER message and it detects the false transition of
ACK, the target shall terminate the command with a CHECK CONDITION status. If
extended sense is implemented, the target shall set the sense key to ABORTED
COMMAND. This is necessary because the target cannot determine whether the
data pointer has actually been saved.
NOTE: If the ATN signal is true in conditions (6) or (8), the target would
normally switch to MESSAGE OUT phase and attempt to transfer a message byte.
If the RESET condition occurs before it is able to successfully receive the
message byte, the target shall assume that the initiator may not have
successfully received the COMMAND COMPLETE message or the SAVE DATA POINTER
message. In the case of COMMAND COMPLETE message, the target shall reselect
the initiator and attempt to send the COMMAND COMPLETE message again. In the
case of the SAVE DATA POINTER message, the target shall reselect the initiator
and terminate the command as described in condition (9).
5.3 SCSI Bus Phase Sequences. The order in which phases are used on the SCSI
bus follows a prescribed sequence.
In all systems, the RESET condition can abort any phase and is always
followed by the BUS FREE phase. Also, any other phase can be followed by the
BUS FREE phase.
5.3.1 Nonarbitrating Systems. In systems where the ARBITRATION phase is not
implemented, the allowable sequences shall be as shown in Figure 5-1. The
normal progression is from the BUS FREE phase to SELECTION, and from SELECTION
to one or more of the information transfer phases (COMMAND, DATA, STATUS, or
MESSAGE).
5.3.2 Arbitrating Systems. In systems where the ARBITRATION phase is
implemented, the allowable sequences shall be as shown in Figure 5-2. The
normal progression is from the BUS FREE phase to ARBITRATION, from ARBITRATION
to SELECTION or RESELECTION, and from SELECTION or RESELECTION to one or more
of the information transfer phases (COMMAND, DATA, STATUS, or MESSAGE).
5.3.3 All Systems. There are no restrictions on the sequences between
information transfer phases. A phase type may even be followed by the same
phase type (e.g., a data phase may be followed by another data phase).
Figure 5-1. Phase Sequences without Arbitration
Figure 5-2. Phase Sequences with Arbitration
5.4 SCSI Pointers. Consider the system shown in Figure 5-3 in which an
initiator and target communicate on the SCSI bus in order to execute a
command.
------------------------- -------------------------
| Function | | Initiator|-----------------| Target | | Function |
| Origin | | Path | SCSI BUS | Path | | Execution|
| | | Control |-----------------| Control | | |
------------------------- -------------------------
Initiator Target
Figure 5-3. Simplified SCSI System
The SCSI architecture provides for two sets of three pointers within each
initiator. The pointers reside in the initiator path control. The first set
of pointers are known as the current (or active) pointers. These pointers are
used to represent the state of the interface and point to the next command,
data, or status byte to be transferred between the initiator's memory and the
target. There is only one set of current pointers in each initiator. The
current pointers are used by the target currently connected to the initiator.
The second set of pointers are known as the saved pointers. There is one
set of saved pointers for each command that is currently active (whether or
not it is currently connected). The saved command pointer always points to
the start of the command descriptor block (see 6.2) for the current command.
The saved status pointer always points to the start of the status area for the
current command. At the beginning of each command, the saved data pointer
points to the start of the data area. It remains at this value until the
target sends a SAVE DATA POINTER message (see 5.5.2) to the initiator. In
response to this message, the initiator stores the value of the current data
pointer into the saved data pointer. The target may restore the current
pointers to their saved values by sending a RESTORE POINTERS message (see
5.5.2) to the initiator. The initiator moves the saved value of each pointer
into the corresponding current pointer. Whenever an SCSI device disconnects
from the bus, only the saved pointer values are retained. The current pointer
values are restored from the saved values upon the next reconnection.
5.5 Message System Specification. The message system allows communication
between an initiator and target for the purpose of physical path management.
5.5.1 Message Protocol. All SCSI devices shall implement the COMMAND COMPLETE
message. A functional SCSI device can be constructed without using any of the
other messages if the logical unit number is specified in the command
descriptor block. The remainder of this section deals with the additional
requirements on SCSI devices that support messages other than COMMAND
COMPLETE.
SCSI devices indicate their ability to accommodate more than the COMMAND
COMPLETE message by asserting or responding to the ATN signal. The initiator
indicates this in the SELECTION phase by asserting ATN prior to the SCSI bus
condition of SEL true, and BSY false. The target indicates its ability to
accommodate more messages by responding to the ATTENTION condition with the
MESSAGE OUT phase after going through the SELECTION phase.
For SCSI devices that support messages other than COMMAND COMPLETE, the first
message sent by the initiator after the SELECTION phase shall be the IDENTIFY
message. This allows the establishment of the physical path for a particular
logical unit specified by the initiator. After the RESELECTION phase, the
target's first message shall be IDENTIFY. This allows the physical path to be
reestablished for the target's specified logical unit number. Under some
exceptional conditions, an initiator may send the ABORT message or the BUS
DEVICE RESET message instead of the IDENTIFY message, as the first message.
Only one logical unit number shall be identified for any one selection
sequence; a second IDENTIFY message with a new logical unit number shall not
be issued before the SCSI bus has been released (BUS FREE phase).
Whenever a physical path is established in an initiator that can accommodate
disconnection and reconnection, the initiator shall ensure that the active
pointers of the physical path are equal to the saved pointers for that
particular logical unit number. (An implied restore pointers operation occurs
as a result of connect or reconnect.)
SCSI devices that implement any message other than the COMMAND COMPLETE
message shall also implement the MESSAGE REJECT message.
Table 5-2
Message Codes
==============================================================================
Code Type Description Direction
------------------------------------------------------------------------------
00H M COMMAND COMPLETE In
01H O EXTENDED MESSAGE In Out
02H O SAVE DATA POINTER In
03H O RESTORE POINTERS In
04H O DISCONNECT In
05H O INITIATOR DETECTED ERROR Out
06H O ABORT Out
07H O MESSAGE REJECT In Out
08H O NO OPERATION Out
09H O MESSAGE PARITY ERROR Out
0AH O LINKED COMMAND COMPLETE In
0BH O LINKED COMMAND COMPLETE (WITH FLAG) In
0CH O BUS DEVICE RESET Out
0DH _ 7FH R Reserved Codes
80H _ FFH O IDENTIFY In Out
==============================================================================
Key: In = Target to initiator, Out = Initiator to target.
5.5.2 Messages. The single byte messages (Table 5-2) are listed along with
their code values and their definitions.
COMMAND COMPLETE 00H (Mandatory). This message is sent from a target to an
initiator to indicate that the execution of a command (or series of linked
commands) has terminated and that valid status has been sent to the initiator.
After successfully sending this message, the target shall go to the BUS FREE
phase by releasing BSY.
NOTE: The command may have been executed successfully or unsuccessfully as
indicated in the status.
EXTENDED MESSAGE 01H (Optional). This message is sent from either the
initiator or the target as the first byte of a multiple-byte message. (See
5.5.3 for descriptions of extended messages.)
SAVE DATA POINTER 02H (Optional). This message is sent from a target to
direct the initiator to save a copy of the present active data pointer for the
currently attached logical unit. (See 5.4 for a definition of pointers.)
RESTORE POINTERS 03H (Optional). This message is sent from a target to direct
the initiator to restore the most recently saved pointers (for the currently
attached logical unit) to the active state. Pointers to the command, data,
and status locations for the logical unit shall be restored to the active
pointers. Command and status pointers shall be restored to the beginning of
the present command and status areas. The data pointer shall be restored to
the value at the beginning of the data area in the absence of a SAVE DATA
POINTER message or to the value at the point at which the last SAVE DATA
POINTER message occurred for that logical unit.
DISCONNECT 04H (Optional). This message is sent from a target to inform an
initiator that the present physical path is going to be broken (the target
plans to disconnect by releasing BSY), but that a later reconnect will be
required in order to complete the current operation. If the initiator detects
the BUS FREE phase (other than as a result of a RESET condition) without first
receiving a DISCONNECT or COMMAND COMPLETE message, the initiator shall
consider this as a catastrophic error condition. If the target intentionally
creates this condition, the target shall clear the current command. This
message shall not cause the initiator to save the data pointer. Note: If
DISCONNECT messages are used break a long data transfer into two or more
shorter transfers, then a SAVE DATA POINTER should be issued before each
DISCONNECT message.
INITIATOR DETECTED ERROR 05H (Optional). This message is sent from an
initiator to inform a target that an error (e.g., parity error) has occurred
that does not preclude the target from retrying the operation. Although
present pointer integrity is not assured, a RESTORE POINTERS message or a
disconnect followed by a reconnect, shall cause the pointers to be restored to
their defined prior state.
ABORT 06H (Optional). This message is sent from the initiator to the target
to clear the present operation. If a logical unit has been identified, all
pending data and status for the issuing initiator from the effected logical
unit shall be cleared, and the target shall go to the BUS FREE phase. Pending
data and status for other initiators shall not be cleared. If a logical unit
has not been identified, the target shall go to the BUS FREE phase. No status
or ending message shall be sent for the operation. It is not an error to
issue this message to an logical unit that is not currently performing an
operation for the initiator.
MESSAGE REJECT 07H (Optional). This message is sent from either the initiator
or target to indicate that the last message it received was inappropriate or
has not been implemented.
In order to indicate its intentions of sending this message, the initiator
shall assert the ATN signal prior to its release of ACK for the REQ/ACK
handshake of the message that is to be rejected. When a target sends this
message, it shall change to MESSAGE IN phase and send this message prior to
requesting additional message bytes from the initiator. This provides an
interlock so that the initiator can determine which message is rejected.
This message shall be implemented if any other optional messages are
implemented.
NO OPERATION 08H (Optional). This message is sent from an initiator in
response to a target's request for a message when the initiator does not
currently have any other valid message to send.
MESSAGE PARITY ERROR 09H (Optional). This message is sent from the initiator
to the target to indicate that one or more bytes in the last message it
received had a parity error.
In order to indicate its intentions of sending this message, the initiator
shall assert the ATN signal prior to its release of ACK for the REQ/ACK
handshake of the message that has the parity error. This provides an
interlock so that the target can determine which message has the parity error.
LINKED COMMAND COMPLETE 0AH (Optional). This message is sent from a target to
an initiator to indicate that the execution of a linked command has completed
and that status has been sent. The initiator shall then set the pointers to
the initial state for the next linked command.
LINKED COMMAND COMPLETE (WITH FLAG) 0BH (Optional). This message is sent from
a target to an initiator to indicate that the execution of a linked command
(with the flag bit set to one) has completed and that status has been sent.
The initiator shall then set the pointers to the initial state of the next
linked command. Typically this message would be used to cause an interrupt in
the initiator between two linked commands.
BUS DEVICE RESET 0CH (Optional). This message is sent from an initiator to
direct a target to clear all current commands on that SCSI device. This
message forces the SCSI device to an initial state with no operations pending
for any initiator. Upon recognizing this message, the target shall go to the
BUS FREE phase.
Reserved 0DH to 7FH. These message codes are reserved for future
standardization.
IDENTIFY 80H to FFH (Optional). These messages are sent by either the
initiator or the target to establish the physical path connection between an
initiator and target for a particular logical unit.
Bit 7. This bit is always set to one to distinguish these messages from the
other messages.
Bit 6. This bit is only set to one by the initiator. When set to one, it
indicates that the initiator has the ability to accommodate disconnection and
reconnection.
Bits 5-3. Reserved.
Bits 2-0. These bits specify a logical unit number in a target.
Only one logical unit number shall be identified for any one selection
sequence; a second IDENTIFY message with a new logical unit number shall not
be issued before the bus has been released (BUS FREE phase).
When sent from a target to an initiator during reconnection, an implied
RESTORE POINTERS message shall be performed by the initiator prior to
completion of this message.
5.5.3 Extended Messages (Optional). A value of one in the first byte of a
message indicates the beginning of a multiple-byte extended message. The
minimum number of bytes sent for an extended message is three. The extended
message format and the extended message codes are shown in Tables 5-3 and 5-4,
respectively.
Table 5-3
Extended Message Format
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
----------|----------|-------------------------------------------------------|
1 | nH | Extended message length |
----------|----------|-------------------------------------------------------|
2 | yH | Extended message code |
----------|----------|-------------------------------------------------------|
3 _ nH+1 | xH | Extended message arguments |
==============================================================================
The extended message length specifies the length in bytes of the extended
message code plus the extended message arguments to follow. Therefore, the
total length of the message is equal to the extended message length plus two.
A value of zero for the extended message length indicates 256 bytes follow.
The extended message codes are listed in Table 5-4. The extended message
arguments are specified for the defined extended messages in Sections 5.5.4
through 5.5.6.
Table 5-4
Extended Message Codes
==============================================================================
Code (yH) Description
------------------------------------------------------------------------------
00H MODIFY DATA POINTER (Optional)
01H SYNCHRONOUS DATA TRANSFER REQUEST (Optional)
02H EXTENDED IDENTIFY (Optional)
03H _ 7FH Reserved
80H _ FFH Vendor Unique
==============================================================================
5.5.4 MODIFY DATA POINTER Message (Optional)
Table 5-5
MODIFY DATA POINTER
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
-----|---------|-------------------------------------------------------------|
1 | 05H | Extended message length |
-----|---------|-------------------------------------------------------------|
2 | 00H | MODIFY DATA POINTER code |
-----|---------|-------------------------------------------------------------|
3 | xH | Argument (MSB) |
-----|---------|-------------------------------------------------------------|
4 | xH | Argument |
-----|---------|-------------------------------------------------------------|
5 | xH | Argument |
-----|---------|-------------------------------------------------------------|
6 | xH | Argument (LSB) |
==============================================================================
The MODIFY DATA POINTER message (Table 5-5) is sent from the target to the
initiator and requests that the signed argument be added (two's complement) to
the value of the current data pointer.
5.5.5 SYNCHRONOUS DATA TRANSFER REQUEST Message (Optional)
Table 5-6
SYNCHRONOUS DATA TRANSFER REQUEST
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
-----|---------|-------------------------------------------------------------|
1 | 03H | Extended message length |
-----|---------|-------------------------------------------------------------|
2 | 01H | SYNCHRONOUS DATA TRANSFER REQUEST code |
-----|---------|-------------------------------------------------------------|
3 | mH | Transfer period (mH times 4 nanoseconds) |
-----|---------|-------------------------------------------------------------|
4 | xH | REQ/ACK offset |
==============================================================================
A pair of SYNCHRONOUS DATA TRANSFER REQUEST messages (Table 5-6) are
exchanged between an initiator and a target whenever an SCSI device that can
support synchronous data transfer recognizes that it has not communicated with
the other SCSI device since receiving the last "hard" RESET condition or a BUS
DEVICE RESET message. The SCSI devices may also exchange messages to
establish synchronous data transfer when requested to do so. The message
exchange establishes the transfer period and the REQ/ACK offset. The transfer
period is the minimum time between leading edges of successive REQ pulses and
of successive ACK pulses.
The REQ/ACK offset is the maximum number of REQ pulses that may be
outstanding before its corresponding ACK pulse is received at the target. A
REQ/ACK offset value of zero shall indicate asynchronous mode; a value of FFH
shall indicate unlimited offset.
If the initiator recognizes that negotiation is required, it asserts ATN
and, if the target implements message transfers, sends a SYNCHRONOUS DATA
TRANSFER REQUEST message indicating an REQ/ACK offset and minimum transfer
period. The REQ/ACK offset is chosen to prevent initiator buffer overflows,
while the minimum transfer period is chosen to meet the data handling
requirements of the initiator. The target responds in any of the following
ways:
Target Response Implied Agreement
------------------------------------- --------------------------------------
(1) REQ/ACK offset less than or equal REQ/ACK offset equal to target value.
to the requested value.
Minimum transfer period equal to Minimum transfer period equal to
or greater than requested period. the target value.
(2) REQ/ACK offset equal to zero. Asynchronous transfer.
(3) MESSAGE REJECT. Asynchronous transfer.
If the target recognizes that negotiation is required, it sends a
SYNCHRONOUS DATA TRANSFER REQUEST message to the initiator. The REQ/ACK
offset is selected to prevent buffer and offset counter overflows, while the
minimum transfer period is chosen to meet the data handling requirements of
the target. The initiator responds in any of the following ways if the target
chooses an REQ/ACK offset equal to FFH:
Initiator Response Implied Agreement
------------------------------------- --------------------------------------
(1) REQ/ACK offset equal to FFH. REQ/ACK offset unlimited.
Minimum transfer period equal to Minimum transfer period equal to
or greater than requested period. the initiator value.
(2) REQ/ACK offset equal to 00H. Asynchronous transfer. The target may
renegotiate for an REQ/ACK offset less
than FFH and greater than 00H.
(3) MESSAGE REJECT. Asynchronous transfer.
The initiator responds in any of the following ways if the target selects an
REQ/ACK offset less than FFH:
Initiator Response Implied Agreement
------------------------------------- --------------------------------------
(1) REQ/ACK offset less than or equal REQ/ACK offset equals initiator value.
to the requested value.
Minimum transfer period equal to Minimum transfer period equal to the
or greater than requested value. initiator value.
(2) REQ/ACK offset equal to zero. Asynchronous transfer.
(3) MESSAGE REJECT. Asynchronous transfer.
The implied agreement shall remain in effect until a BUS DEVICE RESET
message is received, until a "hard" RESET condition occurs, or until one of
the two SCSI devices elects to modify the agreement. Renegotiation at every
selection is not recommended, since a significant performance impact is
likely. The default mode of data transfer is asynchronous mode. The default
mode is entered at power on, after a BUS DEVICE RESET message, or after a
"hard" RESET condition. The SYNCHRONOUS DATA TRANSFER REQUEST message
exchange can only take place following a SELECTION phase that includes the
SCSI IDs for both the initiator and the target. Violation of this rule may
make data transfer impossible owing to disagreements among SCSI devices about
the data transfer mode.
5.5.6 EXTENDED IDENTIFY Message (Optional)
Table 5-7
EXTENDED IDENTIFY
==============================================================================
Byte | Value | Description |
==============================================================================
0 | 01H | Extended message |
-----|---------|-------------------------------------------------------------|
1 | 02H | Extended message length |
-----|---------|-------------------------------------------------------------|
2 | 02H | EXTENDED IDENTIFY code |
-----|---------|-------------------------------------------------------------|
3 | xxH | Sub-logical unit number |
==============================================================================
The EXTENDED IDENTIFY message (Table 5-7) is optional and may be sent by a
target or an initiator. It may be used in conjunction with the normal
IDENTIFY message in order to expand the logical unit number addressing in a
target. The sub-logical unit number specifies the encoded eight-bit sub-
logical unit number used to identify one of 256 sub-logical units within the
logical unit. This allows up to 2048 units to be addressed on a single
target.
6. SCSI Commands
This section defines the SCSI command structure and gives several examples.
The command definitions assume a data structure providing the appearance at
the interface of a contiguous set of logical blocks of a fixed or explicitly
defined data length. The SCSI device maps the physical characteristics of the
attached peripheral devices to one of several logical structures defined by
the device type code.
A single command may transfer one or more logical blocks of data. Multiple
commands may be linked if they are sent to the same logical unit. A target
may disconnect from the SCSI bus to allow activity by other SCSI devices while
a logical unit is being prepared to transfer data.
Upon command completion (successful or unsuccessful), the target returns a
status byte to the initiator. Since most error and exception conditions
cannot be adequately described with a single status byte, one status code,
CHECK CONDITION, indicates that additional information is available. The
initiator may issue a REQUEST SENSE command to retrieve this additional
information.
By keeping to a minimum the functions essential to communicate via this
protocol, a wide range of peripheral devices of varying capability can operate
in the same environment.
Because subsets of the full architecture may be implemented, optional
functions are noted.
6.1 Command Implementation Requirements. The first byte of any SCSI command
shall contain an operation code as defined in this document. Three bits (bits
7 - 5) of the second byte of each SCSI command specify the logical unit if it
is not specified using the IDENTIFY message (see 5.5.2). The last byte of all
SCSI commands shall contain a control byte as defined in 6.2.6.
6.1.1 Reserved. Reserved bits, fields, bytes, and code values are set aside
for future standardization. Their use and interpretation will be specified by
future extensions to this standard. A reserved bit, field, or byte shall be
set to zero, or in accordance with a future extension to this standard. A
target that receives a reserved bit, field, or byte that is not zero or
receives a reserved code value shall terminate the command with a CHECK
CONDITION status and, if extended sense is implemented, the sense key shall be
set to ILLEGAL REQUEST. It shall also be acceptable for a target to interpret
the bit, field, byte, or code value in accordance with a future extension to
this standard.
6.1.2 Operation Code Types
Operation
Code Type Description
--------- -------------------------------------------------------------------
M Mandatory - Commands so designated shall be implemented in order to
meet the minimum requirement of this standard.
E Extended - Commands so designated shall be implemented in addition
to mandatory commands to meet the extended requirement of this
standard.
O Optional - Commands so designated, if implemented, shall be
implemented as defined in this standard.
V Vendor unique - Operation codes so designated are available for
vendor defined commands. See the vendor specifications where
compatibility is desired.
R Reserved - Operation codes so designated shall not be used. They
are reserved for future extensions to this standard.
6.1.3 Unit Attention Condition. A unit attention condition for a logical
unit shall begin for each initiator whenever the removable medium may have
been changed or the target has been reset (by a BUS DEVICE RESET message or a
"hard" RESET condition). The unit attention condition shall persist for each
initiator until that initiator issues a command to the logical unit other than
REQUEST SENSE or INQUIRY for which the target shall return CHECK CONDITION
status. If the next command from that initiator to the logical unit
(following the CHECK CONDITION status) is REQUEST SENSE, and if the target
supports extended sense, then the UNIT ATTENTION sense key shall be returned.
(If any command other than REQUEST SENSE is received, the unit attention
condition is lost.)
If an INQUIRY command is received from an initiator with a pending unit
attention condition (before the target reports CHECK CONDITION status), the
target shall perform the INQUIRY command and shall not clear the unit
attention condition.
If a REQUEST SENSE command is received from an initiator with a pending unit
attention condition (before the target reports CHECK CONDITION status), then
the target may either:
(1) report any pending sense data and preserve the unit attention condition
(2) discard any pending sense data, report UNIT ATTENTION sense key, and
clear the unit attention condition for that initiator.
If an initiator issues a command other than INQUIRY or REQUEST SENSE while a
unit attention condition exists for that initiator, the target shall not
perform the command and shall report CHECK CONDITION status.
6.2 Command Descriptor Block. A request to a peripheral device is performed
by sending a command descriptor block to the target. For several commands,
the request is accompanied by a list of parameters sent during the DATA OUT
phase. See the specific commands for detailed information.
The command descriptor block always has an operation code as the first byte
of the command. This is followed by a logical unit number, command parameters
(if any), and a control byte.
For all commands, if there is an invalid parameter in the command descriptor
block, then the target shall terminate the command without altering the
medium.
6.2.1 Operation Code. The operation code (Table 6-1) of the command
descriptor block has a group code field and a command code field. The three-
bit group code field provides for eight groups of command codes. The five-bit
command code field provides for thirty-two command codes in each group. Thus,
a total of 256 possible operation codes exist. Operation codes are defined in
Sections 7 through 13.
The group code specifies one of the following groups:
Group 0 - six-byte commands (see Table 6-2)
Group 1 - ten-byte commands (see Table 6-3)
Group 2 - reserved
Group 3 - reserved
Group 4 - reserved
Group 5 - twelve-byte commands (see Table 6-4)
Group 6 - vendor unique
Group 7 - vendor unique
(This page is intentionally blank.)
Table 6-1
Operation Code
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Group Code | Command Code |
==============================================================================
Table 6-2
Typical Command Descriptor Block for Six-byte Commands
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number |Logical Block Address (if required) (MSB) |
-----|-----------------------------------------------------------------------|
2 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
3 | Logical Block Address (if required) (LSB) |
-----|-----------------------------------------------------------------------|
4 | Transfer Length (if required) |
-----|-----------------------------------------------------------------------|
5 | Control Byte |
==============================================================================
Table 6-3
Typical Command Descriptor Block for Ten-byte Commands
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved | RelAdr |
-----|-----------------------------------------------------------------------|
2 | Logical Block Address (if required) (MSB) |
-----|-----------------------------------------------------------------------|
3 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
4 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
5 | Logical Block Address (if required) (LSB) |
-----|-----------------------------------------------------------------------|
6 | Reserved |
-----|-----------------------------------------------------------------------|
7 | Transfer Length (if required) (MSB) |
-----|-----------------------------------------------------------------------|
8 | Transfer Length (if required) (LSB) |
-----|-----------------------------------------------------------------------|
9 | Control Byte |
==============================================================================
Table 6-4
Typical Command Descriptor Block for Twelve-byte Commands
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved | RelAdr |
-----|-----------------------------------------------------------------------|
2 | Logical Block Address (if required) (MSB) |
-----|-----------------------------------------------------------------------|
3 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
4 | Logical Block Address (if required) |
-----|-----------------------------------------------------------------------|
5 | Logical Block Address (if required) (LSB) |
-----|-----------------------------------------------------------------------|
6 | Reserved |
-----|-----------------------------------------------------------------------|
7 | Reserved |
-----|-----------------------------------------------------------------------|
8 | Reserved |
-----|-----------------------------------------------------------------------|
9 | Transfer Length (if required) (MSB) |
-----|-----------------------------------------------------------------------|
10 | Transfer Length (if required) (LSB) |
-----|-----------------------------------------------------------------------|
11 | Control Byte |
==============================================================================
6.2.2 Logical Unit Number. The logical unit number addresses one of up to
eight physical or virtual devices attached to a target. This method of
addressing is provided for systems that do not implement the IDENTIFY message.
A target that accepts an IDENTIFY message shall use the logical unit number
specified within the message. In this case, the target shall ignore the
logical unit number specified within the command descriptor block.
(Implementors note: It is a good practice for initiators that implement the
IDENTIFY message to specify the same logical unit number in the command
descriptor block.)
6.2.3 Logical Block Address. The logical block address on logical units
shall begin with block zero and be contiguous up to the last logical block on
that logical unit.
Group 0 command descriptor blocks contain 21-bit logical block addresses.
Groups 1 and 5 command descriptor blocks contain 32-bit logical block
addresses.
The logical block concept implies that the initiator and target shall have
previously established the number of data bytes per logical block. This may
be established through the use of the READ CAPACITY command or the MODE SENSE
command or by prior arrangement.
6.2.4 Relative Address Bit. The relative address (RelAdr) bit of the
group 1 and group 5 commands is set to one to indicate that the logical block
address portion of the command descriptor block is a two's complement
displacement. This negative or positive displacement is to be added to the
logical block address last accessed on the logical unit to form the logical
block address for this command. This feature is only available when linking
commands. The feature requires that a previous command in the linked group
have accessed a block of data on the logical unit. (For an example of the
operation of this function, see Section 6.3.3.)
6.2.5 Transfer Length. The transfer length specifies the amount of data to
be transferred, usually the number of blocks. For several commands the
transfer length indicates the requested number of bytes to be sent as defined
in the command description. For these commands the transfer length field may
be identified by a different name. See the following descriptions and the
individual command descriptions for further information.
Commands that use one byte for transfer length allow up to 256 blocks of
data to be transferred by one command. A transfer length value of 1 to 255
indicates the number of blocks that shall transferred. A value of zero
indicates 256 blocks.
Commands that use two bytes for transfer length allow up to 65,535 blocks of
data to be transferred by one command. In this case, a transfer length of
zero indicates that no data transfer shall take place. A value of 1 to 65,535
indicates the number of blocks that shall be transferred.
For several commands more than two bytes are allocated for transfer length.
Refer to the specific command description for further information.
The transfer length of the commands that are used to send a list of
parameters to a target is called the parameter list length. The parameter
list length specifies the number of bytes sent during the DATA OUT phase.
The transfer length of the commands that are used to return sense data (e.g.
REQUEST SENSE, INQUIRY, MODE SENSE, etc) to an initiator is called the
allocation length. The allocation length specifies the number of bytes that
the initiator has allocated for returned data. The target shall terminate the
DATA IN phase when allocation length bytes have been transferred or when all
available sense data have been transferred to the initiator, whichever is
less.
6.2.6 Control Byte. The control byte is the last byte of every command
descriptor block. A typical control byte is described in Table 6-5.
Table 6-5
Control Byte
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
Last | Vendor unique | Reserved | Flag | Link |
==============================================================================
Bit Description
----- ----------------------------------------------------------------------
7 _ 6 Vendor unique
5 _ 2 Reserved
1 Flag bit - If the link bit is zero, then the flag bit shall be set to
zero. If the link bit is one, and if the command terminates
successfully, the target shall send LINKED COMMAND COMPLETE message if
the flag bit is zero and shall send LINKED COMMAND COMPLETE (WITH
FLAG) message if the flag bit is one. Typically, this bit is used to
cause an interrupt in the initiator between linked commands.
0 Link bit - This bit is set to one to indicate that the initiator
desires an automatic link to the next command upon successful
completion of the current command. Implementation of linked commands
is optional. If the link bit is one, targets that implement linked
commands, upon successful termination of the command, shall return
INTERMEDIATE status and shall then send one of the two messages
defined by the flag bit (above).
Targets that do not implement linked commands shall return a CHECK
CONDITION status and, if extended sense is implemented, shall set the
sense key to ILLEGAL REQUEST if either of the link and flag bits are
set to one.
6.3 Command Examples
6.3.1 Single Command Example. A typical operation on the SCSI bus is likely
to include a single READ command to a peripheral device. This operation is
described in detail starting with a request from the initiator. This example
assumes that no linked commands and no malfunctions or errors occur.
The initiator has active pointers and a set of stored pointers representing
active disconnected SCSI devices (an initiator without disconnect capability
does not require stored pointers). The initiator sets up the active pointers
for the operation requested, arbitrates for the SCSI bus, and selects the
target. Once this process is completed, the target assumes control of the
operation.
The target obtains the command from the initiator (in this case, a READ
command). The target interprets the command and executes it. In this case,
the target gets the data from the peripheral device and sends it to the
initiator. At the completion of the READ command, the target sends a status
byte to the initiator. To end the operation, the target sends a COMMAND
COMPLETE message to the initiator.
6.3.2 Disconnect Example. In the above single command example, the length
of time necessary to obtain the data may require a time-consuming physical
seek. In order to improve system throughput, the target may disconnect from
the initiator, freeing the SCSI bus to allow other requests to be sent to
other logical units. To do this, the initiator needs to be reselectable and
capable of restoring the pointers upon reconnection. The target needs to be
capable of arbitrating for the SCSI bus and reselecting the initiator.
After the target has received the READ command (and has determined that
there will be a delay), it disconnects by sending a DISCONNECT message and
releasing BSY.
When the data are ready to be transferred, the target reconnects to the
initiator. As a result of this reconnection, the initiator restores the
pointers to their most recent saved values (which, in this case, are the
initial values) and the target continues (as in the single command example) to
finish the operation. The initiator recognizes that the operation is complete
when COMMAND COMPLETE message is received.
If target wishes to disconnect after transferring part of the data (e.g.,
while crossing a cylinder boundary), it may do so by sending a SAVE DATA
POINTER message and a DISCONNECT message to the initiator and then
disconnecting. When reconnection is completed, the current data pointer value
is restored to its value immediately prior to the SAVE DATA POINTER message.
On those occasions when an error or exception condition occurs and the
target elects to repeat the information transfer, the target may repeat the
transfer by either issuing a RESTORE POINTERS message or by disconnecting
without issuing a SAVE DATA POINTER message. When reconnection is completed,
the most recent saved pointer values are restored.
6.3.3 Linked Command Example. The link function defines a relationship
between commands that when combined with the relative address bit, allows
previous operations to modify subsequent operations. Link makes high-
performance functions possible by providing a relative addressing capability
and allowing multiple command execution without invoking the functional
component of the initiator.
If the desired data address (in the previously described READ command
example) is unknown, but a search key defined as some particular bytes of a
field is known, then by linking the READ command to a SEARCH DATA EQUAL
command, the data can be quickly and effectively transferred to the initiator.
A LINKED COMMAND COMPLETE message is sent from the target to the initiator
to indicate linked command completion. The initiator then updates the stored
pointers so that subsequent requests from the target will reference the next
command of the chain. Command processing of linked and single commands is
simular except that relative addressing is permitted in linked commands.
Linked commands shall be addressed to a single logical unit.
For example, the successful completion of a SEARCH DATA EQUAL command causes
the target to fetch the linked READ command from the initiator. If the
relative address bit in the READ command has been set to one, and the address
field of the READ command is set to zero, the target transfers the
successfully searched block to the initiator.
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