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Brian Cryer's Glossary of IT Terms with Links


Small Computer Systems Interface. cf IDE. SCSI can be used to interface to any device that has a SCSI interface, this includes devices such as printers and scanners as well as the more traditional disk and tape drives.

SCSI adapters typically support one or two SCSI buses, each of which can support up to eight units. Each device on the SCSI bus is identified by a SCSI ID, which ranges from 0 to 7 (or sometimes 0 to 15). The SCSI adapter itself will occupy one of these SCSI IDs leaving 6 (or 14) available for other devices.

There are a number of different types of SCSI, reflecting its continuing development, and these are summarised in the following table:

SCSI Type Data Width Transfer Rate Max devices Max cable length (metres)
SCSI-1 8 bits 5MB/sec 7 6 m (SE) or 12 m (LVD)
SCSI-2 Fast (Narrow) 8 bits 10MB/sec 7 3 m (SE) or 12 m (LVD)
Fast Wide 16 bits 20MB/sec 15 3 m (SE) or 12 m (LVD)
SCSI-3 Ultra (narrow) 8 bits 20MB/sec 7 1.5 m (SE) or 12 m (LVD)
Ultra Wide 16 bits 40MB/sec 7 (SE) or 15 (LVD) 1.5m (SE) or 12m (LVD)
SCSI-4 Ultra 2 (narrow) 8 bits 40MB/sec 7 12m (LVD)
Ultra 2 Wide 16 bits 80MB/sec 15 12m (LVD)
Ultra 3 (narrow) 8 bits 80MB/sec 15 12m (LVD)
Ultra 3 (wide) 16 bits 160MB/sec 15 12m (LVD)
Ultra 320 (wide) 16 bits 320MB/sec 16 12m (LVD)


  1. The transfer rate is the maximum throughput of data on the bus, it does not indicate the maximum throughput of a device connected to the bus (for example the bus might be capable of 80MB/sec but a disk drive connected to it might be cable of only a fraction of that.)
  2. The maximum cable length (which is the length from the adapter to the last device or terminator) may reduce as devices are added.

For more information see: