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)
