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SCSI stands for "Small Computer Systems Interface". It is a standard for parallel data transfers between a computer and its devices on a so-called SCSI bus. It was re-named from the proprietary Shugart Computer Systems Interface when a number of other companies also decided to adopt the same standard. Since then, SCSI has been developed as an industry-wide standard.

On the computer side there must be a SCSI adapter/controller which controls the data transfer on the SCSI bus. The most popular devices on a SCSI bus are hard disks but it is also possible to connect scanners, CD-ROMs, CD writers, DVD-ROMs, etc.

SCSI comes in several flavours (standards):

  • SCSI-1: This is 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 with the disadvantage of a reduced cable length of only 3 meters.
  • SCSI-3: Also known as Ultra SCSI. The bus speed was doubled again. Maximum cable length stays at 3 meters but Ultra SCSI is known to be very sensitive to cable length.
  • Ultra2 SCSI: This standard introduced Low Voltage Differential (LVD). That is why it is sometimes called LVD SCSI. Using this technology it was possible to allow a maximum bus cable length of 12 meters.
  • Ultra3 SCSI: This is the latest 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. 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 connect a Ultra3 SCSI harddisk to a Ultra2 SCSI controller and use it with reduced speed and feature set. However, if all devices are not LVD, the SCSI chain will only run at the speed of the slowest component.

Each SCSI device must be configured to have a unique SCSI ID on the bus.The SCSI adapter needs an ID for itself, too, as it appears as a device on the bus it controls.

The SCSI bus must be terminated with a terminator (there are both active and passive terminators) at both ends of the bus. 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.

In the past SCSI was very popular on all kinds of computers. Today, SCSI is mainly used on high-performance workstations or servers. Desktop computers and notebooks mostly use the cheaper ATA/IDE interface instead of SCSI.

Many SCSI interfaces are now implemented over Fiber Channel. However, Fiber Channel is generally only used in high-end systems, particularly RAID system attachments.

The newly introduced iSCSI standard, an embedding of SCSI-3 over TCP/IP is expected by some to replace Fiber Channel in the long run, as commodity Ethernet data rates are currently increasing faster than Fiber Channel and similar disk attach technology data rates, and iSCSI can address both the low-end and high-end markets with a single commodity-based technology.

However, iSCSI preserves the basic SCSI paradigm almost unchanged.

See also:

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??