SCSI bus types

SCSI bus types From an electrical point of view, there are two incompatible bus types: single-ended and differential. This means that there are two different main groups of SCSI devices and controllers, which cannot be mixed on the same bus. It is possible however to use special converter hardware to transform a single-ended bus into a differential one (and vice versa). The differences between the bus types are explained in the next sections.

In SCSI related documentation there is a sort of jargon in use to abbreviate the different bus types and speeds.

Ultra: 40 Mbyte/s
FWD: Fast Wide Differential 20Mbyte/s
FND: Fast Narrow Differential 10Mbyte/s
FN: Fast Narrow 10Mbyte/s
SE: Single Ended 5Mbyte/s

Wide is a bit ambiguous, it can indicate 16 or 32 bit buses. The 32-bit variant is not (yet) in use, so wide usually means 16 bit.
Fast means that the timing on the bus is somewhat different, so that on a narrow (8 bit) bus 10 Mbyte/s are possible instead of 5 Mbyte/s for 'slow' SCSI. Ultra is faster again by a factor of 2.
It should be noted that the data lines > 8 are only used for data transfers and device addressing. The transfers of commands and status messages etc are only performed on the lowest 8 data lines. The standard allows narrow devices to operate on a wide bus. The useable bus width is negotiated between the devices. It is important to monitor device addressing closely when mixing wide and narrow.