FAQ's - SCSI VooDoo

With all the different type of SCSI, what does what? Although it may not be an issue for iMac and G3 B/W owners without SCSI port, here is a compiled list of various SCSI flavors.

A data path represents how many bits can be sent together to the drive. A wider path means a faster drive - but don't worry too much about the terminology. The speed in MBytes/s should be the main concern.

SCSI-1

8 bits data path, up to 5 MBytes/s. Cables: 6 meters long maximum 50 pins connectors to 50 pins Centronics

 

SCSI-2

8 bits data path, up to 5 MBytes/s SCSI-1 with an extended set of commands. Cables: 6 meters long maximum 50 pins connectors to Centronics or high-density

 

SCSI-2 FAST

SCSI-2 with double transfer rate (up to 10 MBytes/s) Cable: 3 meters maximum 50 pins connectors to Centronics or high-density

 

SCSI-2 FAST-WIDE

SCSI-2 FAST with 16 bit data path (up to 20 MBytes/s) Cable: 3 meters maximum 68 pin connector to 68 pin connector

 

SCSI-3 or ultra

SCSI-FAST with 4X transfer rate (up to 20 Mbytes/s) Cable: up to 1,5 m 50 pins connectors to Centronics or high-density

 

SCSI ultra wide

SCSI FAST WIDE with 4X transfer rate (up to 40 Mbytes/s) Cable up to 1,5 m special thin connector

 

SCSI ultra 2 wide

SCSI-FAST with lower signal voltage, differential (up to 80 Mbytes/s)

 

The SCSI buses on recent Macs are:

External SCSI: SCSI-1

Internal SCSI: FAST SCSI-2

SCSI drives negotiate their speed with the SCSI host (either a card or the motherboard), so all drives with an 8 bit data path should be compatible. Although most cards will support slower drives, a SCSI chain's speed will be limited to the speed of the slowest component.