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SCSI (pronouned "scuzzy"), stands for "Small Computer System Interface", and provides a standard means for transferring data between devices[?] on a computer bus.

Shugart Associates originally introduced SCSI technology in 1979, and initially referred to it as SASI (for: "Shugart Associates System Interface"). After a number of other companies (NCR being the first, in 1981) also decided to adopt the same standard, 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.

To attach a computer to the 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 almost invariably an integral part of 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 writers[?], and DVD drives.

Several standardized versions of SCSI exist:

  • SCSI-1: The original standard, it features a maximum transfer rate of 5 MB/s and a maximum bus cable length of 6 meters.
  • SCSI-2: This standard introduced 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 cost of a reduced maximum cable length of only 3 meters.
  • SCSI-3: Also known as Ultra SCSI[?]. The bus speed doubled again. Maximum cable length stayed at 3 meters but Ultra SCSI has a reputation of extreme sensitivity to cable length. SCSI-3 actually encompasses a plethora of protocol standards including serial transfer techniques like IEEE-1394 (Apple Computer's FireWire standard), Fibre Optic protocols, etc, that transmit data serially at a very rapid rate.
  • Ultra2 SCSI: This standard introduced Low Voltage Differential (LVD), and for this reason sometimes appears under the name "LVD SCSI". Using this technology it became possible to allow a maximum bus cable length of 12 meters.
  • Ultra3 SCSI: This is the latest [as of when?] SCSI standard with a maximum transfer rate of 160 MB/s. SCSI-3 offers new features like cyclic redundancy check (CRC), domain validation[?], and double transition clocking[?] on top of Ultra2 SCSI capabilities. There is also a subset of Ultra3 called Ultra160. It has the same maximum transfer rate, but does not contain the entire set of Ultra3 instructions.

SCSI devices are generally backward-compatible, i.e. it is possible to connect a Ultra3 SCSI hard disk to a Ultra2 SCSI controller and use it (though with reduced speed and feature set). However, if all devices are not LVD-capable, the SCSI chain will only run at the speed of the slowest component.

Each SCSI device (including the computer's adapter) must be configured to have a unique SCSI ID on the bus.

The SCSI bus must be terminated with a terminator. Both active and passive terminators are in common use. Wrong termination is a very 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.

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 ATA/IDE interfaces for hard disks and USB for other devices, since these interfaces, although less general-purpose, cost less to implement.

Although SCSI-1 and SCSI-2 specified both the command set (i.e. the logic "0" and "1" signal definitions and sequencing) and physical signaling (i.e. the cable length, logic levels and termination), the SCSI command set itself is useful on its own, since it is mature and has a large body of knowledgeable users and designers. Therefore, partial uses of only the command set portion of SCSI have appeared. An example is Fibre Channel, although Fibre Channel is generally only used in high-end systems, such as RAID system attachments.

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

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
iSCSIlimited only by IP networkN/AN/A??

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