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SCSI(-3) Parallel Interface - 2 (SPI-2)
The second generation of the SCSI-3 parallel interface standard is called the SCSI(-3)
Parallel Interface - 2, or SPI-2. (The "-3" was dropped from
"SCSI-3" to uh... reduce confusion. Sure. :^) ) This ANSI standard, document
X3.302-1999, replaced the older SPI standard, and also
incorporated the SCSI-3 Interlocked Protocol (SIP) document. Thus, SPI-2 included
everything from the earlier SCSI-2, SPI, SIP and Fast-20 documents--as well as adding some
new features of course. SPI-2 formally replaced the earlier collection of SPI documents in
1999, and in doing so simplified matters significantly, since at least now everything
associated with parallel SCSI was back in one document.
Several important new technologies and features were defined as part of SPI-2; the most
important changes are the following:
- Fast-40 Data Transfer: SPI-2 defines another doubling of the maximum speed of the SCSI bus, from 20 MHz to 40 MHz, allowing
maximum throughput of 40 MB/s on a narrow (8-bit) channel or 80 MB/s on a wide (16-bit)
channel. The document also defines several restrictions associated with these faster
signaling speeds, such as the use of differential signaling.
- Low Voltage Differential Signaling: A new type of signaling for the
SCSI bus, called low voltage differential or LVD signaling, was
specified as part of SPI-2. LVD is an attempt to blend the best attributes of conventional
single-ended (SE) signaling and the older type of differential
signaling that is now called high voltage differential (HVD). LVD (or the older HVD)
is required to run the SCSI bus at Fast-40 speeds; you can read
more about it here.
- Multimode Operation: Specification is provided for a way to create
devices that will automatically work on both LVD and regular single-ended buses; such
units are called multimode devices. They are also
discussed in the section on LVD.
- SCA-2 Single Connector Attachment Connectors: An improvement to the
original SCA connectors, called SCA-2, was defined.
- Very High Density Connectors: SPI-2 defined a smaller version of the
older high-density 68-pin connectors. This new standard is called Very High Density
Cable Interconnect, abbreviated VHDCI. Read
more about them here.
 Note: The features
defined as part of SPI-2 are sometimes referred to by the informal (marketing) terms Ultra2 SCSI.and Wide
Ultra2 SCSI.
 Next: SCSI(-3) Parallel Interface - 3 (SPI-3)
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