small computer system interface
(foldoc) | Small Computer System Interface
SASI
SCSI
(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 adapter
(or "host adapter") 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.
SCSI Trade Association & FAQ (http://scsita.org/).
["System" or "Systems"?]
(1999-03-30)
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