SCSI
SCSI (pronouned "scuzzy", sometimes like "sexy"), stands for "Small Computer
System Interface", and provides a standard means for transferring data
between devices on a computer bus.
Shugart Technology (the company formed by storage wizard Alan Shugart after
he left 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 writers,
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. The standards (in chronological order) are
as follows:
* 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: Also known as Ultra SCSI and fast-20 SCSI, it was 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). SCSI-3
actually encompasses a plethora of protocol standards including serial
transfer techniques like IEEE-1394 (Apple Computer's FireWire
standard), fiber optic protocols, etc, that transmit data serially at a
very rapid rate.
* 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.
The original SCSI standards specified the physical characteritics of the
bus(es) and the electrical signalling sequences required to achieve a given
action, as well as 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. Therefore, partial uses of only the command set portion of SCSI
have appeared. Fibre Channel, Serial Storage Architecture, InfiniBand,
iScsi, USB, IEEE 1394 and Serial Attached Scsi are all use the Scsi command
set for some operations.
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 Bus width Max. cable Max. number of
(MBytes/s) (bits) length (meters) devices
SCSI 5 8 6 8
Fast SCSI 10 8 1.5-3 8
Wide SCSI 20 16 1.5-3 16
Ultra SCSI 20 8 1.5-3 5-8
Ultra Wide
SCSI 40 16 1.5-3 5-8
Ultra2 SCSI 40 8 12 8
Ultra2 Wide
SCSI 80 16 12 16
Ultra3 SCSI 160 16 12 16
Ultra-320
SCSI 320 16 12 16
iSCSI limited only by N/A N/A ??
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