SCSI stands for "Small Computer System Interface", and is a standard interface for transferring data between devices on a computer bus. SCSI is pronounced "scuzzy" when spoken aloud; occasional attempts to promulgate the more flattering pronunciation "sexy" have never succeeded.

Shugart Technology (the company formed by Alan Shugart after leaving Shugart Associates) introduced SCSI in 1979, and initially referred to it as SASI (Shugart Associates System Interface). After a number of other companies (NCR being the first, in 1981) decided to adopt SASI, SASI received the new name "SCSI." NCR also helped start the standards process the next year, and in 1986, ANSI approved the SCSI spec (as X3.131-1986). Since then, SCSI has developed as an industry-wide standard, capable of being applied to virtually any computer system (there were even SCSI implementations for the venerable Commodore 64 home computer).

To attach a computer to the host bus requires a SCSI host adapter which controls the data transfer on the SCSI bus; the peripheral side must feature a SCSI controller (the SCSI controller is generally embedded—integral to the peripheral—in all but the earliest SCSI devices). SCSI is most commonly used for hard disks and tape storage devices, but also connects a wide range of other devices, including scanners, CD-ROM drives, CD writerss, and DVD drives. In fact, the entire SCSI standard promotes device independence, which means that theoretically anything can be made SCSI (SCSI printers actually exist).

SCSI has evolved over the years. Before summarizing the evolution, a distinction should be made between the terminology used in the SCSI standard itself, as promulgated by the T10 committee of INCITS, and common parlance, as codified by the SCSI trade association, SCSITA.

As of 2003, there have only been three SCSI standards: SCSI-1, SCSI-2, and SCSI-3. All SCSI standards have been modular, defining various capabilities which manufacturers can include or not. Individual vendors and SCSITA have given names to specific combinations of capabilities. For example, no term "Ultra SCSI" is defined in the standard, but is used to refer to SCSI implementations that signal at twice the rate of "Fast SCSI." Such a signalling rate is not compliant with SCSI-2 but is one option allowed by SCSI-3. Similarly, no version of the standard requires low-voltage-differential (LVD) signalling, but products called Ultra-2 SCSI include this capability. This terminology is helpful to consumers, because "Ultra-2 SCSI" device has a better-defined set of capabilities than simply identifying it as "SCSI-3."

No version of the standard has ever specified what kind of connector should be used. The connectors used by vendors have tended to evolve over time. Although SCSI-1 devices typically used bulky Blue Ribbon ("Centronics") connectors, and SCSI-2 devices typically "Mini-D" connectors, it is not correct to refer to these as "SCSI-1" and "SCSI-2" connectors.

The mainstream implementations of SCSI (in chronological order) are as follows, using common parlance:

  • SCSI-1: The original standard that was derived from SASI and formally adopted in 1986 by ANSI. SCSI-1 features an 8 bit bus (with parity), running asychronously at 3.5 MB/s or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 meters (a little more than 18 feet -- compare that to the 18 inch (1.5 meter) limit of the ATA interface). A variation on the original standard included a high voltage differential (HVD) implementation whose maximum cable length was many times that of the single-ended version.

  • SCSI-2: This standard was introduced in 1989 and gave rise to the Fast SCSI and Wide SCSI variants. Fast SCSI doubled the maximum transfer rate to 10 MB/s and Wide SCSI doubled the bus width to 16 bits on top of that (to reach 20 MB/s). However, these improvements came at the minor cost of a reduced maximum cable length to 3 meters. SCSI-2 also specified a 32-bit version of Wide SCSI, which used 2 16-bit cables per bus; this was largely ignored by SCSI device makers, and was officially retired in SCSI-3.

  • SCSI-3: Before Adaptec and later SCSITA codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI and fast-20 SCSI, were introduced in 1992. The bus speed doubled again to 20 MB/s for "narrow" (8 bit) systems and 40 MB/s for wide. The maximum cable length stayed at 3 meters but ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems).

  • Ultra-2 SCSI: This standard was introduced c. 1997 and featured a low voltage differential (LVD) bus. For this reason ultra-2 is sometimes referred to as "LVD SCSI." Using LVD technology, it became possible to allow a maximum bus cable length of 12 meters (more than 36 feet!), with much greater noise immunity. At the same time, the data transfer rate was increased to 80 MB/s. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by ultra-3 (ultra-160) SCSI.

  • Ultra-3 SCSI: Also known as ultra-160 SCSI and introduced toward the end of 1999, this version was basically an improvement on the ultra-2 standard, in that the transfer rate was doubled once more to 160 MB/s by the use of double transition clocking. Ultra-160 SCSI offered new features like cyclic redundancy check (CRC), an error correcting process, and domain validation.

  • Ultra-320: This is the ultra-160 standard with the data transfer rate doubled to 320 MB/s. Nearly all new SCSI hard drives being manufactured at the time of this writing (October 2003) are actually ultra-320 devices.

  • Ultra-640: Ultra-640 (otherwise known as fast-320) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. Ultra-640 doubles the interface speed yet again, this time to 640 MB/s.

Note: Ultra-2, ultra-160 and ultra-320 devices may be freely mixed on the LVD bus with no compromise in performance, as the host adapter will negotiate the operating speed and bus management requirements for each device. Single-ended devices should not be attached to the LVD bus, as doing so will force all devices to run at the slower single-ended speed. Support for single-ended interfaces has been deprecated in the SPI-5 standard (which describes Ultra-640), so future devices may not be electrically backward compatible.

SCSI devices are generally backward-compatible, i.e., it is possible to connect a ultra-3 SCSI hard disk to a ultra-2 SCSI controller and use it (though with reduced speed and feature set).

Each SCSI device (including the computer's [host adapter]) must be configured to have a unique SCSI ID on the bus. Also, the SCSI bus must be terminated with a terminator. Both active and passive terminators are in common use, with the active type much preferred (and required on LVD buses). Improper termination is a common problem with SCSI installations.

It is possible to convert a wide bus to a narrow one, with wide devices closer to the adapter. To do this properly requires a cable which terminates the wide part of the bus. This is sometimes referred to as a cable with high-9 termination. Specific commands allow the host to determine the active width of the bus. This arrangement is discouraged.

In the past, SCSI was very popular on all kinds of computers. SCSI remains popular on high-performance workstations, servers, and high-end peripherals. Desktop computers and notebooks more typically use the slower ATA/IDE interfaces for hard disks and USB (USB uses the SCSI command set for some operations) for other devices, since these interfaces, although less general-purpose, cost less to implement.

SCSI interface overview
Interface Bus speed (MBytes/s) Bus width (bits) Max. cable length (meters) Max. number of devices
Fast SCSI1081.5-38
Wide SCSI20161.5-316
Ultra SCSI2081.5-35-8
Ultra Wide SCSI40161.5-35-8
Ultra2 SCSI408128
Ultra2 Wide SCSI80161216
Ultra3 SCSI160161216
Ultra-320 SCSI320161216
iSCSIlimited only by IP networkN/AN/A??

SCSI Architecture Model

The original SCSI-1 standards defined the physical characteristics of the bus(es) and the electrical signalling sequences required to achieve a given action. The SCSI-2 standard expanded this and, a very important addition, specified the command set that defines the different things SCSI devices can do.

However, the SCSI command set itself is useful on its own, since it is mature and has a large body of knowledgeable users and designers. The SCSI-3 standard separated the SCSI command set from its traditional "parallel" implementation.

The logical structure of the SCSI command set is called the SCSI Architecture Model (SAM), and is included within the interface definitions for Fibre Channel, Serial Storage Architecture, InfiniBand, iScsi, USB, IEEE 1394 and Serial Attached SCSI. Thus these are all capable of using the SCSI command set for some operations.

iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged. iSCSI advocates project the iSCSI standard, an embedding of SCSI-3 over TCP/IP as displacing Fibre Channel in the long run, arguing that Ethernet data rates are currently increasing faster than data rates for Fibre Channel and similar disk-attachment technologies. iSCSI could thus address both the low-end and high-end markets with a single commodity-based technology. .

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