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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Standards | SCSI-3 ]

SCSI-3 Parallel Interface (SPI)

When the decision was made to expand the scope of SCSI-3 to include a number of different physical interfaces and protocols, what had been "just SCSI" had to be given a more specific name. Since "regular SCSI" uses a parallel bus (many wires transferring data in parallel), this technology became known as the SCSI-3 Parallel Interface or SPI. The first description of the parallel interface was accomplished in a rather confusing way, through the use of three different documents (see this page for more details on the standards documents):

  • Protocol: The protocol for parallel SCSI was defined in a document entitled SCSI-3 Interlocked Protocol (SIP).
  • Physical Layer: The physical layer was defined in the SCSI-3 Parallel Interface or SPI document, ANSI standard X3.253-1995. This specification only called for bus speeds of up to 10 MHz, which is so-called "Fast SCSI", first defined in SCSI-2.
  • Fast-20: This is an addendum to the original SPI document, published as ANSI standard X3.277-1996. It defined faster 20 MHz bus signaling, increasing maximum throughput to as much as 40 MB/s on the SCSI bus.

Taken collectively, these are sometimes called Ultra SCSI or Wide Ultra SCSI, which are really informal or marketing terms; sometimes, Ultra SCSI refers specifically to the faster signaling rates themselves. Aside from the faster signaling, which allows for speeds of up to 20 MB/s on narrow (8-bit) SCSI buses or 40 MB/s on wide (16-bit) buses, the other main change associated with SPI is the creation of new cabling. Wide buses previously required two cables, a cumbersome solution that was never widely accepted. SPI introduced the high-density, 68-pin "P" cable and connectors now widely used for faster SCSI buses.

This collection of documents was "retired" in 1999 and replaced with a single document, SPI-2.

Next: SCSI(-3) Parallel Interface - 2 (SPI-2)

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