There are two commonly used SCSI bus widths: narrow and wide. Narrow SCSI uses a data
pathway that is 8 bits wide, and was the first type of parallel SCSI defined in the
original SCSI-1 standard. Wide SCSI uses a data pathway 16
bits wide, and was first defined as part of SCSI-2. Since its
introduction, wide SCSI has been steadily increasing in popularity, since it allows a
doubling of bus bandwidth for any given signaling speed. It also allows the use of 16
devices on the SCSI bus, compared to the standard 8 devices for narrow SCSI.
Wide SCSI originally required the use of two cables: a 68-pin "B" cable in
addition to the regular 50-conductor "A" cable used for narrow SCSI. This use of
two cables was expensive and cumbersome, and the "A+B" configuration was
eventually replaced by a single 68-pin "P" cable. See here
for more on cabling issues.
Today, narrow SCSI is actually being left behind, as the need for extra performance has
led to the dominance of wide forms of SCSI, especially for hard disks. This has actually
led to some terminology difficulties. Traditionally, the narrow SCSI bus has been
considered the "regular" or default type, so "narrow" was not
generally mentioned in the name of SCSI type. For example, saying "Ultra SCSI" implied narrow operation; wide buses running
at Ultra speeds were called "Wide Ultra SCSI".
However, at around the time that Ultra2 SCSI was created,
narrow operation began to fall out of favor, and as a result most Ultra2 implementations
are wide, and many people stopped bothering with explicitly saying "Wide Ultra2 SCSI", even though this is the
technically accurate name.
Transfer modes faster than Ultra2 have done away with
narrow buses altogether. Presumably, if one is designing a device that needs throughput
enough to justify going to speeds faster than Ultra2, it would be silly to "give
away" half the throughput by going narrow instead of wide. Fast-80(DT)
and Fast-160(DT) signaling, as defined in the SPI-3
and SPI-4 standards respectively, are only for wide
implementation. As a result, all relevant marketing terms such as Ultra3, Ultra160, Ultra160+ and Ultra320
have wide bus operation implied, reversing the way it was with the earlier SCSI
flavors.
It is possible to mix narrow and wide SCSI on the same bus, but there are issues that
must be overcome to do so. These typically revolve around cabling, which is different for
narrow and wide SCSI, and also with termination. Adapters may be required to convert
between the narrow and wide cables. See this discussion for more
information. Note that there are also host adapters available that are specifically
designed to support both wide and narrow devices.
Note: A "very
wide" 32-bit form of SCSI was defined as part of the SCSI-2
standard, but was never accepted by the industry due to cost. It required the use of
two 68-conductor cables, for one thing! It was also non-standard; with the extra costs
involved, there was little interest in it. After laying the proverbial egg for several
years, it was finally withdrawn from the SCSI standard in SPI-3.
Next: SCSI Bus Speed
