SCSI : Computer Hardware Buyers’ Glossary

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SCSI
SCSI (Small Computer System Interface). SCSI controller cards can handle up to seven devices, usually hard disks, CD-ROMS, back up tape drives or scanners. SCSI hard disks are more expensive than the garden variety EIDE (Extended Integrated Drive Electronics) hard disks. SCSI won’t buy you any more speed if you use Windows-95. You have to use a true multi-tasking Operating System like OS/2 or NT to get the SCSI speed benefits. There are six flavours of SCSI. Home machines mostly use SATA (Serial ATA) instead of SCSI, even though they look like SCSI logically.
Types of SCSI
type bits pins async
MHz		 sync
MHz		 cable
meters		 raw
throughput
Mbytes/sec
SCSI 8 50 4 5 6 5
SCSI-2 8 50 8 10 3 10
wide SCSI-2 16 68 8 10 3 20
differential SCSI-2 16 68 8 10 25 20
Fast20 aka Ultra 1 16 68 8 20 3 40
Ultra 2 SCSI 16 68 8 40 12 80
Ultra 160 SCSI 16 68 8 80 12 160
The best SCSI controllers come from Adaptec. Their website is also a good place to find technical documentation about SCSI. It is possble to mix 50-pin and 68-pin SCSI devices on the same bus using a converter, however then all devices run at the slower speed. You can also use a controller with connectors for both 50 and 68-pin cables. Any new installations will use pure 68-pin cables.
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