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SCSI (Small Computer System Interface)

By Brien Posey

SCSI (pronounced SKUH-zee and sometimes colloquially known as "scuzzy"), the Small Computer System Interface, is a set of American National Standards Institute (ANSI) standard electronic interfaces that allow personal computers (PCs) to communicate with peripheral hardware such as disk drives, tape drives, CD-ROM drives, printers and scanners faster and more flexibly than previous parallel data transfer interfaces.

SCSI and devices

Although not all devices support all levels of SCSI, SCSI standards are generally backward-compatible. That is, if an older peripheral device is attached to a newer computer with support for a later standard, the older device will work at the older and slower data rate. In personal computing, SCSI interfaces have been replaced, for the most part, by Universal Serial Bus (USB). In the enterprise, SCSI is still used in server farms for hard drive controllers.

Common SCSI components

There are several components used in SCSI storage systems:

SCSI standards

Current SCSI technologies can transfer up to 640 megabytes per second (MBps).

Serial-attached SCSI

SAS products are compatible with devices that employ earlier SCSI technologies. The Serial Storage Architecture (SSA) standard can be used when SCSI performance is not adequate, as can iSCSI, which preserves the SCSI command set by embedding SCSI-3 (most SCSI-3 specifications start with the designation Ultra) over TCP/IP.

SAS has become a popular alternative to parallel SCSI in enterprise environments. Both serial and parallel SCSI are based on the SCSI command set. SAS offers the following distinct advantages over parallel SCSI:

SAS vs. SATA

Like SAS, SATA is a serial bus that replaces the aging parallel ATA (PATA) standard. The SATA-3 standard is rated at 6 gigabits per second (Gbps)/600 MBps, which is slightly slower than the Ultra640 SCSI standard. Similarly, SATA-2 has a transfer speed of 3 Gbps/300 MBps, which is just below that of Ultra320 SCSI.

SAS is backward-compatible with SATA-2 and above. A SATA-2 drive can be connected to a SAS backplane. This is possible because both SAS and SATA drives use the SCSI command set. Conversely, SAS drives cannot be connected to a SATA controller.

See also: SCSI controller

13 May 2015

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