1. Scope
This American National Standard defines an input/output bus for
interconnecting computers and peripheral devices. The standard defines
extensions to the Small Computer System Interface (X3.131-1986), referred to
herein as SCSI-1. It also provides more complete standardization of the
previously defined command sets. The document includes the necessary
specification of the mechanical, electrical, and functional characteristics of
the interface to allow inter-operability of devices meeting the standard.
This standard is referred to herein as SCSI-2. The term SCSI is used wherever
it is not necessary to distinguish between the two versions of SCSI.
SCSI is a local I/O bus that can be operated over a wide range of data
rates. 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, optical media drives, and other devices can be
added to the host computers without requiring modifications to generic system
hardware or software. Provision is made for the addition of special features
and functions through the use of vendor unique fields and codes. Reserved
fields and codes are provided for future standardization.
A second key objective of SCSI-2 is to provide compatibility with those
SCSI-1 devices that support bus parity and that meet conformance level 2 of
SCSI-1. While some previously vendor unique commands and parameters have been
defined by the SCSI-2 standard, devices meeting SCSI-1 and SCSI-2 can co-exist
on the same bus. It is intended that those operating systems providing
support for both command sets be able to operate in environments mixing SCSI-1
and SCSI-2 devices. Properly conforming SCSI-1 devices, both initiators and
targets, should respond in an acceptable manner to reject SCSI-2 protocol
extensions. All SCSI-2 protocol extensions are designed to be permissive of
such rejections and to allow the SCSI-1 device to continue operation without
requiring the use of the extension.
A third key objective of SCSI-2 is to move device-dependent intelligence out
to the SCSI-2 devices. This requires the definition of a command set that
allows a sophisticated operating system to obtain all required initialization
information from the attached SCSI-2 devices. The formalized sequence of
requests identify the type of attached SCSI-2 device, the characteristics of
the device, and all the changeable parameters supported by the device.
Further requests can determine the readiness of the device to operate, the
types of media supported by the device, and all other pertinent system
information. Those parameters not required by the operating system for
operation, initialization, or system tuning are not exposed to the SCSI-2
interface, but are managed by the SCSI-2 device itself.
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.
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). 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 should not be mixed on the
same bus.
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.
Section 5 describes the logical characteristics of the interface.
Arbitration 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.
The SCSI command and status structure is specified in Section 6. Commands
are classified as mandatory (M), optional (O), or vendor unique (V). SCSI
devices are required to implement all mandatory commands defined for the
appropriate device type and may implement other commands as well. 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).
Many commands also implement a very large logical block address space (232
blocks), although some commands 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 17 contain commands for direct-access (e.g., magnetic
disk), sequential-access (e.g., magnetic tape), printer, processor, write-once
(e.g., optical disk), CD-ROM devices, scanner devices, optical memory devices,
medium changer devices, and communications 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.
Starting with section 7 and for each section on a specific device type the
section is constructed of at least four sub sections. The first subsection is
the model for the device type. The model establishes the framework for
interpreting the commands for the device type. The attributes and
capabilities of the device type are discussed and examples are given. The
second subsection defines the commands applicable to the device type. The
third subsection defines the parameters applicable to the device type. These
are the diagnostic parameters, log parameters, mode parameters and vital
product data parameters that are transmitted as part of the appropriate
commands. Most of the parameters are formatted into pages. The fourth
subsection is the glossary of terms that apply to the device type.
Appendixes A through C provide examples of SCSI signal sequences, timing,
and phase sequences. Appendix D contains information on other standards
related to medium types and density codes for flexible disks and magnetic
tapes. Appendix E describes data integrity in command queuing environments.
Appendix F describes normal procedures following a power-on condition.
Appendix G describes the worst case skew times for a fast SCSI implementation.
Appendix H contains information on other SCSI standardization activities.
Appendix I contains the additional sense codes and operation codes in
numerical order. Appendix J contains the vendor identification codes as of
the date of this document. However, the appendixes are not a required part of
this standard.
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