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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):
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.