The chart below shows a comparison of the SCSI variations:

Name Specification # of Devices Bus Width Bus Speed MBps
Asynchronous
SCSI
SCSI-1 8 8 bits 5 MHz 4 MBps
Synchronous
SCSI
SCSI-1 8 8 bits 5 MHz 5 MBps
Wide
SCSI
SCSI-2 16 16 bits 5 MHz 10 MBps
Fast
SCSI
SCSI-2 8 8 bits 10 MHz 10 MBps
Fast/Wide
SCSI
SCSI-2 16 16 bits 10 MHz 20 MBps
Ultra
SCSI
SCSI-3
SPI
8 8 bits 20 MHz 20 MBps
Ultra/Wide
SCSI
SCSI-3
SPI
8 16 bits 20 MHz 40 MBps
Ultra2
SCSI
SCSI-3
SPI-2
8 8 bits 40 MHz 40 MBps
Ultra2/Wide
SCSI
Fast-40
SPI-2
16 16 bits 40 MHz 80 MBps
Ultra3
SCSI
Fast-80
SPI-3
16 16 bits 40 MHz 160 MBps
Ultra320
SCSI
Fast-160
SPI-4
16 16 bits 80
MHz
320
MBps