Note: This document is a copy of one found at another site. It contained some problems with the HTML code and was also a good introduction to SCSI in all its forms and terms that I downloaded it to my UNCA CSCI account.

SCSI - An Introduction

By Brad Stamas

Abstract: The purpose of this article is to provide an introduction and high-level understanding of the Small Computer System Interface or SCSI. This article contains a brief history of SCSI as well as descriptions of the key concepts and terms used in the various SCSI specifications. Also included in this article are a glossary of SCSI terms and references to other documents providing more in-depth description of SCSI or applications of SCSI. This article is in large part a summary of the various SCSI specifications referenced in the sources section.

Table of Contents:

Introduction
History and Evolution
SCSI-1
SCSI-2
SCSI-3
Appendices
Sources
Glossary of Terms

Introduction

The Small Computer System Interface (SCSI) is an ANSI standard that defines an input/output bus and logical interfaces supporting the bus for interconnecting computers and peripheral devices. The primary objective of SCSI is to provide a device independent mechanism to attach and access devices to host computers. SCSI is designed to provide an efficient peer-to-peer I/O bus that supports multiple devices, including one or more hosts. Thus, through a single SCSI interface 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.

SCSI is a local I/O bus that can be operated over a wide range of data rates. The SCSI specification also allows for a number of different physical interconnect configurations. A SCSI bus's geographic reach and speed of data transfer is dependent upon the particular physical configuration chosen. For configurations utilizing parallel copper wire interconnection schemes, the maximum SCSI bus length may be as much as 25 meters or as little as 1.5 meters, while the maximum data transfer rates may range from 5 MBytes/sec up to 80 MBytes/sec. For configurations utilizing a serial optical interconnect scheme, SCSI devices may be separated by as much as 3 kilometers with a maximum data transfer rate of up to 100 MBytes/sec today and potentially more in the future.

SCSI is the predominant high-speed bus technology used to interconnect systems to peripheral devices and to build peripheral subsystems. SCSI interfaces are available on almost all systems that support Unix, NT, or any of their variants. SCSI interfaces are available on systems ranging in size from desktop PC's to multiprocessing supercomputers. Traditionally, SCSI has been used to interconnect devices within a single chassis or to interconnect a few external peripheral devices located nearby a computer. As the amount and importance of data residing on these peripheral devices grew, so did the visibility of, the importance of and the reliance upon SCSI grow. Today, there are mission critical applications running on Unix and NT systems that require access to large amounts of data. It is imperative that the storage devices serving these mission critical applications also support access to large amounts of data with a high level of data availability. It is within this context that SCSI, an interface that has existed for a number of years has become more important to users and vendors of enterprise class storage.

History and Evolution

To understand SCSI and where it is going it is necessary to know a little bit about its history. The development of SCSI can be traced back to SASI, the Shugart Associates System Interface. SASI was developed in the late 1970's by Shugart Associates as a 8-bit parallel device-independent peripheral or system bus for use in small and medium sized computers. The SASI interface defined a logical level rather than a device level interface to disk. A logical view allowed the system's view of the disk device to be independent of the physical geometry of the disk device. This logical view allowed various companies to independently develop systems and peripheral devices that could be used together. It also allowed these companies to integrate technology and cost saving advancements rapidly. This concept would prove to be instrumental in the successful development of "open" system platforms in the future.

In 1978, the ANSI group that handles I/O standards began the discussion of standardizing interfaces to small computers. SASI was presented to the ANSI group who initially rejected it in favor of another specification already moving forward within the standards organization. Following the ANSI rejection, system manufacturer NCR worked together with Shugart Associates to improve SASI. In 1982 SASI was again submitted to ANSI which then used it as the foundation for the Small Computer System Interface (SCSI) standard. By 1983, the SCSI specification had matured to the point that NCR began shipping the first SCSI chipset. The original version (also referred to as SCSI-1) was approved by ANSI in 1986. Extensions to the original specification (SCSI-2) were finalized in 1994. An ANSI committee is currently working on SCSI-3. To date, portions of SCSI-3 have already been agreed upon, while others are still under discussion.

SCSI and SCSI-1

The original SCSI, or more appropriately SCSI-1, specification defined a logical interface that worked in consort with a shared 8-bit wide bus that operated at up to 5 MHz. Eight (8) different devices can be attached to one and other on a SCSI-1 physical bus. Devices may be daisy chained together to form a bus or the bus may be implemented as a backplane. In the case of a daisy chain, the devices are cabled together and the bus is terminated at each end (as in Figure 1.). In the case of a backplane implementation, devices are plugged into the backplane or a stub attached to the backplane.

Any two devices on the SCSI bus can communicate by setting up a connection, exchanging control information, and transferring data between one and other. The device that initiates the connection is called the Initiator. The device that is the target of the Initiator's connection is called the Target. The SCSI specification allows any device attached to the bus to support both Initiator and Target functions. It should be noted here that within the context of SCSI the term "device" is used to describe anything, host system or peripheral system, that attaches to the SCSI bus. In practice, however, it is generally the host system interfaces that initiate communications over the bus while peripherals (disk, tape, etc.) are generally the targets of these communications. As peripheral-to-peripheral communication is, for the most part, a non-exploited capability, the terms Initiator and Target are used synonymously to refer to a host system interface and a peripheral system interface respectively.

Individual devices on a SCSI bus are distinguished from one and other through a unique SCSI identification number or SCSI ID. SCSI ID's are mapped to assigned ID bits. As the SCSI-1 bus is only 8 bits wide, a maximum of 8 devices can be addressed on the bus (ID 0 through ID 7). To alleviate these addressing constraints somewhat, SCSI allows each Target to be sub-divided in to Logical Units (LUN). The maximum number of logical units per Target is 8 (LUN 0 through LUN 8). Dividing a target into logical units is useful if the target is a controller supporting multiple sub-units (certain RAID subsystems for example) or if the target also supports a separate control or management interface. In practice most SCSI-1 tape and a large number of disk devices support only a single LUN (LUN 0).

The bus described in the original SCSI specification operated at up to 5 MHz. That is to also say that there were 5 million data transfer periods or cycles per second. Some documents use the term "megatransfers" to describe the cycle rate of the bus. As the original SCSI bus was 8-bits (1 Byte) wide, the bus could transfer data at 5 MByte/sec. The 5 MByte/second rate is a computed rate(1 Byte per period x 5 million transfers periods per second). While this data transfer rate was theoretically achievable, the throughput rate of devices on a SCSI-1 bus never approached this limit for a number of reasons. These reasons include: 1) The specification allows transfer rates below 5 MHz and the rate used is defined by the slower of the two communication devices; 2) SCSI allows data to be transferred in an interlocked (asynchronous) fashion that results in a much reduced throughput; 3) Data is not transferred during the periods required to set up a connection and pass both command and status between two devices so the achieved aggregate throughput is less than the computed data transfer rate; and 4) Most devices and host interfaces implementing SCSI-1 initially could not sustain data transfer rates of 5 MBytes/sec.

The SCSI standard supports two electrical interface configurations: "single-ended" and "differential". Basically, a single ended interface is designed to use fewer wires to support a SCSI bus than a differential interface. Fewer wires allowed for smaller connectors, less complex driver/receiver design, and less cost. While these characteristics restricted the placement of devices and the total length of a single-ended bus to a maximum of 6 meters, they also supported low cost, integrated interface designs. Single-ended interfaces, therefore, are generally used when SCSI is implemented within a single cabinet. Differential interfaces are more expensive than single-ended interfaces as they are designed to span distances up to 25 meters. Differential interfaces are generally used as the external connection between a host and a peripheral subsystem. A single ended interface or bus can not be connected to a differential interface or bus without the use of a special converter.

SCSI-2

SCSI-2 was developed in order to address SCSI-1 problem areas, extend the functionality of SCSI, and to keep pace with technology changes. Major goals of SCSI-2 included the following: increase performance, improve compatibility, increase the number of addressable devices, and improve functionality. These goals were achieved in part by expanding the bus width, increasing the bus data transfer rate, adding/organizing commands, and defining compatibility requirements.

SCSI-2 increased the maximum data transfer cycle rate to 10 MHz or 10 million transfers per second. A bus that operated from 5 MHz to 10 MHz is said to be "Fast". In order to accommodate the faster data transfer cycle rate, the maximum single-ended bus length was reduced to 3 meters. There was, however, no need to reduce the 25 meter maximum length of a differential bus to accommodate Fast.

The SCSI-2 specification also allowed the bus to be widened from 8-bits (1-byte) to 16 bits (two-byte) or 32 bits (four-byte) wide. Although the specification allows a 32 bit wide bus, for various reasons, mostly only 16 bit wide buses have been implemented to date. As a result, the term "Wide" is used to refer to a 16 bit (2 byte) wide bus. It should also be noted here that while not explicitly defined, the term Narrow is sometimes used to refer to an 8 bit wide bus and to differentiate between an 8 bit and 16 bit wide bus. Widening the bus allowed for increased addressing and data transfer rates. Up to 16 devices can be addressed on a Wide bus. Data transfer rates on a Wide bus are twice that of the equivalent 8 bit wide bus.

Given the added attributes of Fast and Wide, there are now Fast SCSI, Wide SCSI, and Fast Wide SCSI. A Fast SCSI bus is 8 bits in width, can support 8 devices, and has a maximum data transfer rate of 10 MBytes/sec.(8 bits or 1 Byte x 10 million data transfer cycles per second). A Wide SCSI bus is 16 bits in width, can support 16 devices, and has a maximum data transfer rate of 10 MBytes/sec. (16 bits or 2 Bytes x 5 million data transfer cycles per second). A Fast Wide SCSI bus is 16 bits in width, can support 16 devices, and has a maximum data transfer rate of 20 MBytes/sec. (16 bits or 2 Bytes x 10 million data transfer cycles per second). As with SCSI-1, the realizable throughput rates of devices attached to the bus will be less than maximum data transfer rate. This is again due to overhead attributed to setting up and managing bus communications as well as limits on device speeds. It should, however, be noted that a larger number of devices implementing SCSI-2 also support buffered operations which allow the actual data transfer portion to operate at near Fast or Fast Wide rates.

Along with the increased number of bus types, SCSI-2 also defined mechanisms to maintain compatibility between SCSI-2 and SCSI-1. Compatibility is assured through protocol definitions and cabling specifications. SCSI-2 is upward compatible. Devices supporting SCSI-2 and SCSI-1 can be intermixed on the same bus and will use the common minimum attributes to communicate. That is SCSI-2 devices will communicate with SCSI-1 devices as if they are SCSI-1 devices (speeds, protocols, addressing, interface lines with SCSI-1 restrictions applying) and Fast Wide SCSI devices will communicate with Fast SCSI devices as if they are Fast SCSI.

SCSI-3

SCSI-3 was started for the same reasons as SCSI-2: to address problem areas, extend the functionality of SCSI, and to keep pace with technology changes. As with SCSI-2, increasing performance and addressing were key goals of SCSI-3. To accommodate higher performance and greater addressing requirements, the support of serial interfaces (1394, Fibre Channel, and Serial Storage Architecture) and faster parallel interfaces would need to be defined. Resolution of these and other of complex issues along with the ability to support multiple serial interfaces required significant protocol changes. As the SCSI-2 document was already very large, it was proposed that the large SCSI-2 document be broken down into smaller pieces to form SCSI-3. Various layers of SCSI from the logical command interface through the various physical interfaces and the protocols supporting them are now defined by separate but related specifications. Whereas the SCSI-1 and SCSI-2 specifications defined everything from the logical command interface down to the physical interconnect, SCSI-3 became more of an architecture encompassing various specifications. The major components of SCSI-3 are illustrated in the following diagram:

Some key points to note about SCSI-3 are that it segments SCSI into layers with the logical interfaces defined at the top and the physical interfaces at the bottom. This layering allows the command sets supported by different device types (disk, tape, printer, medium changer, etc.) to be further defined and it defines common access semantics to be used by the top layer. These definitions preserve the logical view allowing a system's view of the device to be independent of the physical geometry and interface established in SCSI-1. In fact, many applications that interface to SCSI-2 will, without change, be able to interface to systems and peripherals that implement components of SCSI-3 architecture.

Below the layers describing common access and commands are the various protocols used to support the physical interfaces. SCSI-3 supports multiple types of physical interfaces each having its own addressing, interconnect, and communication characteristics. The multiple interface types exist in part to address differing connectivity requirements (addressing, speed, length, expense) and in part to support various manufacturer investments and preferences. Interface types can be categorized as being either serial or parallel.

In serial interfaces, data and control bits are communicated sequentially, instead of in parallel, on a single wire or fiber optic. SCSI-3 currently supports 3 serial interfaces: 1394 (also known as FireWire), Fibre Channel, and Serial Storage Architecture (SSA). Although it is beyond the scope of this paper to provide a thorough comparison of these three serial alternatives, it should be noted that the serial interfaces, when compared to SCSI-2, address at least two of the following: reduced manufacturing cost, smaller footprint on the interface, enhanced interconnect options (point-to-point, switched, fabric, torus ring), increased data transfer rates, longer interconnect (more than 1 kilometer with Fibre Channel optical interfaces), and more reliable connection technology. Regardless of the differences, the basic function of each of the serial interfaces is to allow SCSI commands, data, and status to be exchanged between interconnected devices.

SCSI-3 also continues to accommodate parallel copper interfaces. Advances in driver and receiver technology have allowed data transfer cycles per second to be increased to 20 MHz (Ultra) and again up to 40 MHz. (Ultra2). As with Fast, Ultra and Ultra2 are supported in either 8 bit (Ultra SCSI, Ultra2 SCSI) or 16 bit (Wide Ultra SCSI, Wide Ultra2 SCSI) definitions. The maximum number of addressable devices remains controlled by the bus width at 8 devices for the 8 bit wide bus definitions (Ultra SCSI and Ultra2 SCSI) and at 16 devices for the 16 bit wide bus definitions (Wide Ultra SCSI and Wide Ultra2 SCSI). The maximum data transfer rates for Ultra SCSI, Wide Ultra SCSI, Ultra2 SCSI, and Wide Ultra2 SCSI are 20 MBytes/sec, 40 MBytes/sec, 40 MBytes/sec, and 80 MBytes per second respectively.

Increases in the data transfer rate have come at the expense of a reduction in cable length. Ultra is limited to 3 meters in single-ended configurations and 25 meters in differential. At the 40 MHz data transfer cycle rate of Ultra2, single-ended and differential are not defined. To support Ultra2, a new transceiver technology, Low Voltage Differential (LVD), was developed which combines favorable aspects of both single-ended and differential. LVD supports Ultra2 rates at distances up to 12 meters. LVD also supports all the other Fast, Wide, or Ultra SCSI variants at distances up to 12 meters

Conclusion

SCSI originated as a standard means to support device independent, reliable, high-speed data transfer between any two devices interconnected on a shared bus. SCSI has evolved from a monolithic specification into a layered architecture that exploits multiple physical interface technologies and supporting protocols. As communications and computing technologies have advanced, the original SCSI specification has been updated to accommodate enhancements in function and performance. At the same time SCSI is being applied to serial interface technologies that promise enhanced connectivity and performance, parallel interface technologies are being improved to increase data transfer rates. While the support of multiple physical interface types has increased the complexity of SCSI, it also allows SCSI to be the applied in a wide variety of situations. Today, SCSI is one of the most common high-speed communications interfaces implemented. SCSI interfaces of one type or another are supported by most every computer manufacturer in the world. The scalability of SCSI has supported its growth in the past as it will support its growth and continued popularity in the future.

Appendices -

Table: SCSI Bus Width and Maximum Data Rates

Table: SCSI Maximum Bus Length and Device Addressing.

SCSI Technologies: Comparison of Maximums:

Sources

1. Ancott Corporation, Basics of SCSI; Second edition; 1993
2. American National Standards for Information System - Small Computer System Interface:, SCSI-1, X3.131-1986.
3. American National Standards for Information System - Small Computer System Interface; SCSI-II, X3.131-1994, Rev 10L.
4. Linn, John; Adapting SCSI to New Devices, Embedded Systems Programming, November 1995.
5. Nearline SCSI-2 Reference Manual, StorageTek, November 1994.
6. Ridge, Peter M., The Book of SCSI, 1995.
7. SCSI Basics, ENDL Publications, 1995.
8. Terminology for SCSI Parallel Interface Technology, SCSI Trade Association, 1996.
9. Ultra SCSI on the ROAD Seminar, SCSI Trade Association, 1996.

Glossary of Terms

This Glossary defines terms used in the context of the Small Computer System Interface or SCSI.

A-cable

A 50-wire cable used for 8-bit SCSI-1 buses.

ANSI

American National Standards Institute. A standards-setting, non-government organization, which develops and publishes standards for "voluntary" use in the USA.

arbitration

The process of selecting one respondent from a collection of several candidates that request the use of the SCSI bus concurrently.

Asynchronous transfer

A method of sending data that requires an acknowledgment from the receiver for each byte of data that is sent before the next one is sent. Asynchronous transfers are slower than synchronous transfers for this reason.

B-cable

A 68-wire cable used for 16 bit SCSI-2 buses.

CCS

Common Command Set. CCS describes the core set of commands supported by SCSI. CCS is a collection of command device commands and is a part of SCSI-2. The common commands are a subset of SCSI-1. As SCSI-1 allowed too many vendor specific features, CCS was designed to improve compatibility between SCSI devices from different vendors.

CDB

Command Descriptor Block. The 6-byte, 10-byte, or 12-byte structure used to communicate commands from a SCSI initiator to target.

command

An instruction transferred from SCSI initiator to SCSI target, typically containing function codes, an address, flags, and possibly other information. Commands are used to control the operation of or movement of data between devices on a SCSI bus.

connect

The function that occurs when a SCSI initiator selects a SCSI target to start an operation, or when a SCSI target selects an SCSI initiator to continue an operation.

data rate

The rate at which data is transferred across the SCSI bus during the data transfer phase. The data rate achieved is dependent upon the bus width (8-bit or 16-bit) and the transfer rate. For example an 8-bit bus operating at 10 Mhz (or Fast) has a data rate of 10 Megabytes per second.

device

Any single unit on the SCSI bus identifiable by a unique SCSI address. In the context of SCSI, the term device can be applied to hosts as well as peripherals. A SCSI device can act as an initiator or a target.

differential interface

A SCSI bus configuration in which each signal is sent on two wires. The signal is derived by taking the difference in voltage between the two wires, effectively eliminating unwanted noise in the wire. A differential interface is designed to be more (as compared to a single-ended configuration) noise immune and supports longer, up to 25 meter, SCSI bus lengths. Differential interfaces are generally used to externally interconnect systems and peripheral devices.

disconnect

The action that occurs when a SCSI target releases control of the SCSI bus allowing the bus to go to the Bus Free phase.

Fast

A term used to describe a SCSI-2 defined synchronous transmission rate of between 5 MHz and 10 MHz.

Fast-20

A simile for Ultra SCSI.

Fast SCSI

A term used to describe an 8-bit wide bus operating Fast (between 5 and 10 MHz). The maximum data rate of a Fast SCSI device or bus is 10 Mbytes/sec.

Fast and Wide SCSI

A term used to describe a 16-bit wide bus operating Fast (between 5 and 10 MHz). The maximum data rate of a Fast and Wide SCSI device or bus is 20 Mbytes/sec.

Fibre Channel

An ANSI standard that specifies high-speed serial communication between devices. Fibre channel supports serial communications across both copper and optical connections. Fibre Channel is used as one of the serial bus architectures supported by SCSI-3.

host

A processor or system usually consisting of a CPU and memory. A host communicates with other devices such as peripherals and other hosts on a SCSI bus. On the SCSI bus, a host is known by its SCSI address or SCSI ID.

initiator

A SCSI device (usually a host system) that requests an operation to be performed by another SCSI device (target).

I/O

Input / Output.

IEEE

Institute of Electrical and Electronics Engineers. An organization that promotes electrical and electronics standards.

logical unit

A virtual device addressable through a target SCSI device. A physical SCSI device can have more than one logical unit. Logical unit are supported only by SCSI targets.

LUN

Logical Unit Number. The value used to identify a logical unit of a SCSI device. In the SCSI-2 specification, there may be up to 8 logical units for each SCSI device address. These logical units are numbered from 0-7.

MByte or Megabyte

A measurement of approximately one million bytes.

MHz

Megahertz. A measurement of one million cycles per second. Used to describe the cycle rate of the SCSI bus.

narrow

A term used to describe a SCSI bus that is 8-bits wide.

P1394

An IEEE standard serial bus also known as FireWire. The intent of P1394 is to provide a physical bus that was competitive with SCSI parallel buses in both cost and speed, had no configuration switches, no terminators, a flexible topology, and deterministic latency.

P-cable

A 68-wire cable used for 16-bit SCSI-3 buses. P-cables can be used with Q-cables for 32-bit SCDSI-3 buses.

P-to-A transition cable

An adapter used to connect 8-bit SCSI-1 devices using A-cables to a 16-bit or 32 bit SCS-3 device using P-cables.

parallel

The transfer of bits over multiple wires at one time. Accomplished by devoting a separate wire for each bit of a byte.

peripheral device

A device that can be attached to a host computer or server using a SCSI bus. Typical peripheral devices include disk drives, tape drives, scanners, and printers.

port

A connection to a bus. The single-byte (narrow or 8-bit) SCSI bus (SCSI-1) allows eight ports. The 16-bit wide SCS-2 Wide bus allows sixteen ports.

Q-cable

A 68-wire cable used in conjunction with a P-cable for 32-bit SCSI buses.

SASI

Shugart Associates Standard Interface. An interface designed by Shugart Assoc. in 1980. It was the predecessor of SCSI.

SCSI

An acronym for Small Computer System Interface (pronouced "skuzzy"). An Industry standard for connecting peripheral devices and their controllers to a microprocessor. The SCSI defines both hardware and software standards for communication between a host computer and a peripheral. Computers and peripheral devices designed to meet SCSI specifications should work together. See also SCSI-1,SCSI-2, or SCSI-3.

SCSI-1

The first version of SCSI as defined by ANSI - X3.131-1986.

SCSI-2

The successor to SCSI-1 as defined by ANSI - X3.131-1992. SCSI-2 is upward compatible from SCSI-1. SCSI-2 specification allows for faster transmission rates, a wider bus (up to 32-bit wide), increased device addressing, and improved functionality.

SCSI-3

The SCSI-3 specification is designed to further improve functionality and accommodate high-speed serial transmission interfaces. To do so, SCSI was effectively "layered" logically. This layering allowed the software interfaces to remain relatively unchanged while accommodating new physical interconnect schemes based upon serial interconnects such as Fibre Channel and Serial Storage Architecture (SSA).

SCSI address

is an octal representation of the unique address assigned to a SCSI device. Also called a SCSI ID.

SCSI ID

See SCSI address.

serial

The transfer of bits over a single wire or fibre optic in a sequential fashion.

Serial Storage Architecture (SSA)

An architecture defining a serial physical interface and supporting protocol developed by IBM and now supported by the ANSI X3T10.1 committee. SSA supports point-to-point, loop, and multiple-ring topologies. Each node of an SSA topology can support up to 2 full duplex ports operating at 20 MBytes/sec per port. SCSI commands, data transfers, and responses are transported across an SSA topology.

single-ended interface

One of the SCSI bus electrical signal configurations. On a ingle-ended configuration, each signal is carried by a singnal wire. Single-ended buses are more susceptible to noise than differential buses. Traditionally, a single-ended interface was designed to use fewer (as compared to a differential configuration) pins, chips and PCB area. It cost less, was smaller, but it also is more sensitive to noise, more restrictive in physical devices spacing, and more restrictive in length (up to 6 meters). Single-ended interfaces are generally found inside subsystems that use SCSI to interconnect internal components (for example RAID arrays).

synchronous transmission

A transmission in which the sending and receiving devices operate continuously at the same frequency. Synchronous transmission allows many bytes of data to be sent before acknowledgment is received from the target. Data can be transferred synchronously faster than is can be transferred asynchronously. In SCSI, only data can be sent synchronously. Commands, messages, and status are sent asynchronously.

target

A SCSI device (usually a peripheral) that performs an operation as requested by a SCSI initiator.

Ultra

A term used to describe a SCSI defined synchronous transmission rate of between 10 MHz and 20 MHz.

Ultra SCSI

A term used to describe a 8-bit wide bus operating Ultra (between 10 MHz and 20 MHz). The maximum data rate of a Ultra SCSI device or bus is 20 Mbytes/sec. Sometimes referred to as Fast-20.

Ultra2

A term used to describe a SCSI defined synchronous transmission rate of between 20 MHz and 40 MHz.

Ultra2 SCSI

A term used to describe a 8-bit wide bus operating Ultra (between 20 MHz and 40 MHz). The maximum data rate of a Ultra SCSI device or bus is 40 Mbytes/sec.

Wide

A term used to describe a SCSI bus that is 16-bits in width.

Wide Ultra SCSI

A term used to describe a 16-bit wide bus operating Ultra (between 10 MHz and 20 MHz). The maximum data rate of a Ultra SCSI device or bus is 40 Mbytes/sec.

Wide Ultra2 SCSI

A term used to describe a 16-bit wide bus operating Ultra (between 20 MHz and 40 MHz). The maximum data rate of a Ultra SCSI device or bus is 80 Mbytes/sec.

X3.131-1986

The document describing the specifications of the SCSI-1 standard.

X3.131-1994

The document describing the specifications of the SCSI-2 standard.

X3T10

The ANSI committee responsible for organizing, defining, and promoting SCSI standards.