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SCSI Bus Width
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