Here are the various flavors of SCSI and some comments on them. Most of
this info is courtesy of some well informed, smart people on the PC
support list over at Linux Enthusiasts
and Professionals (LEAP), a central Florida Linux User's Group.
Here's the short short version.
SCSI-1 (Async) - 5Mhz, 8-bit, 4Mb/s, 7 devices, 50-pin. SCSI-1 (Sync) - 5Mhz, 8-bit, 5Mb/s, 7 devices, 50-pin. SCSI-2 (Wide) - 5Mhz, 16-bit, 10Mb/s, 15 devices, 68-pin. SCSI-2 (Fast) - 10Mhz, 8-bit, 10Mb/s, 7 devices, 50-pin. SCSI-2 (Fast/Wide) - 10Mhz, 16-bit, 20Mb/s, 15 devices, 68-pin. SCSI-3 (Ultra) - 20Mhz, 8-bit, 20Mb/s, 7 devices, 50-pin. SCSI-3 (Ultra/Wide) - 20Mhz, 16-bit, 40Mb/s, 7 devices, 68-pin. SCSI-3 (Ultra2) - 40Mhz, 8-bit, 40Mb/s, 7 devices, 50-pin. SCSI-3 (Ultra2/Wide) - 40Mhz, 16-bit, 80Mb/s, 15 devices, 68-pin. SCSI-3 (Ultra3(Ultra160)) - 40Mhz, 16-bit, 160Mb/s, 15 devices, 68-pin.
Devices and controller cards should be backwards compatable and will use the
least common protocol. So a SCSI 2 card will work with a SCSI 3 drive, but
the drive will only work at SCSI 2 rates.
Another good link at
Ars Technica.