This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
The max allowable transfer rate was raised to 10 MT/s (mega-transfers per second) in SCSI-2, referred to as Fast SCSI. Note that this is NOT required, devices running at ANY speed below that may claim to be SCSI-II compliant! Fast implies SCSI-II, not the other way around! Fast Narrow is thus 10 MB/sec. Both the initiator (computer) and target (peripheral) must support fast transfer for it to be of any use, but intermixing fast and slow devices on a bus presents no operational problems (only performance ones). A 16-bit bus (Wide SCSI) and 32-bit bus (double-wide SCSI) are specified in SCSI-2. The wide busses require the use of a second cable in SCSI-2. The first cable is 50 pins, known as the A cable; the 2nd is 68 pins, known as the B cable. I know of no one actually using 32-bit SCSI, but it would also run on an A/B cable pair. Slow (or Normal) Wide is thus 5 MT/s * 2 Bytes/T, 10 MB/sec. Fast Wide is 20 MB/sec. Fast Double Wide would be 40 MB/sec. In the SCSI-3 physical layer spec (SCSI-PH), a single 68-pin cable, known as the P cable, is allowable for 8 or 16-bit busses. This is the option most people who have implemented Wide SCSI have chosen for the cabling, even though their upper layer is generally SCSI-2. There is a small movement (heard here on the net occassionally) to promote an Ultra-SCSI high-speed bus, with a burst rate of something like 20 MT/sec on very short cables. At present it is unclear what will happen to this effort. There is also talk, in conjunction with a change to low-voltage differential signalling, to go to 40MT/sec.