History
The first two formal SCSI standards, SCSI-1 and SCSI-2, included parallel SCSI as a central part of the protocol. The SCSI-3 standard then split the framework into separate layers so parallel SCSI is now just one of a number of available implementations. See the main SCSI article for a complete list. As with all types of SCSI bus, parallel SCSI communication takes place between an initiator and a target.
The original SCSI-1 version of the parallel bus was 8 bits wide (plus a ninth parity bit). The SCSI-2 standard allowed for faster operation (10 MHz) and wider buses (16-bit or 32-bit). The 16-bit option became the most popular, as the 32-bit option was more expensive and was thus hardly ever used.
At 10 MHz with a bus width of 16 bits it is possible to achieve a data rate of 20 MB/s. Subsequent extensions to the SCSI standard allowed for faster speeds: 20 MHz, 40 MHz, 80 MHz, 160 MHz and most recently 320 MHz. At 320 MHz x 16 bits there is a theoretical maximum peak data rate of 640 MB/s.
Read more about this topic: Parallel SCSI
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