Small Computer System Interface

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Related to Small Computer System Interface: SCSI, Integrated Drive Electronics, Serial Attached SCSI

small computer system interface

[¦smȯl kəm¦pyüd·ər ‚sis·təm ′in·tər‚fās]
(computer science)
An interface standard or format for personal computers that allows the connection of up to seven peripheral devices. Abbreviated SCSI (scuzzy).
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Small Computer System Interface

(hardware, standard)
(SCSI) /skuh'zee/, /sek'si/ The most popular processor-independent standard, via a parallel bus, for system-level interfacing between a computer and intelligent devices including hard disks, floppy disks, CD-ROM, printers, scanners, and many more.

SCSI can connect multiple devices to a single SCSI adaptor (or "host adaptor") on the computer's bus. SCSI transfers bits in parallel and can operate in either asynchronous or synchronous modes. The synchronous transfer rate is up to 5MB/s. There must be at least one target and one initiator on the SCSI bus.

SCSI connections normally use "single ended" drivers as opposed to differential drivers. Single ended SCSI can suport up to six metres of cable. Differential ended SCSI can support up to 25 metres of cable.

SCSI was developed by Shugart Associates, which later became Seagate. SCSI was originally called SASI for "Shugart Associates System Interface" before it became a standard.

Due to SCSI's inherent protocol flexibility, large support infrastructure, continued speed increases and the acceptance of SCSI Expanders in applications it is expected to hold its market.

The original standard is now called "SCSI-1" to distinguish it from SCSI-2 and SCSI-3 which include specifications of Wide SCSI (a 16-bit bus) and Fast SCSI (10 MB/s transfer).

SCSI-1 has been standardised as ANSI X3.131-1986 and ISO/IEC 9316.

A problem with SCSI is the large number of different connectors allowed. Nowadays the trend is toward a 68-pin miniature D-type or "high density" connector (HD68) for Wide SCSI and a 50-pin version of the same connector (HD50) for 8-bit SCSI (Type 1-4, pin pitch 1.27 mm x 2.45 mm). 50-pin ribbon cable connectors are also popular for internal wiring (Type 5, pin pitch 2.54 mm x 2.54 mm). Apple Computer used a 25-pin connector on the Macintosh computer but this connector causes problems with high-speed equipment. Original SCSI implementations were highly incompatible with each other.

ASPI is a standard Microsoft Windows interface to SCSI devices.

Usenet newsgroup: news:comp.periphs.scsi.

news:comp.periphs.scsi. SCSI Trade Association & FAQ

This article is provided by FOLDOC - Free Online Dictionary of Computing (


(Small Computer System Interface) Pronounced "scuzzy," SCSI is an earlier hardware interface for up to 15 peripherals connected to one expansion card, called a "SCSI host adapter." Introduced in 1986 by Shugart Associates (see SASI), this parallel architecture was replaced by its serial successor (see serial attached SCSI).

SCSI hard drives were used in mainframes, servers and storage arrays in the late 1980s and 1990s because they were very robust. Initially the only kind chosen for multi-drive RAID configurations, less-costly IDE drives were eventually used (see RAID, IDE and SATA).

SCSI Was a Mini-Network
The SCSI bus connects up to 15 devices in a daisy chain topology, and any two can communicate at one time: host-to-peripheral and peripheral-to-peripheral. For more details, see SCSI Architecture Model, SCSI signaling, SCSI connectors and SCSI switch.

A "Scuzzy" Daisy Chain
SCSI enabled multiple peripherals to take up only one expansion slot in the computer.

SCSI Ports
To enable the daisy chain, all SCSI devices had one "in" and one "out" port plus a dial to set the device number. These SCSI-1 sockets are on the back of a 1990s disk drive. See SCSI connectors.
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