MACINTOSH - SCSI DEVICES

Washington University in St. Louis, School of Medicine

SCSI Basics

SCSI (small computer system interface) is a standard developed by certain computer companies for connecting hard disks and other devices, such as scanners, CD-ROM drives, MO Drives, and some printers, to computers.

SCSI Cables

Used to connect SCSI devices to the computer or to each other. Three basic kind of cables --- 25-pin, 50pin, and 50-pin miniature. Most SCSI devices come with two SCSI ports in the back, so they can be daisy-chained.

SCSI ID numbers

Upto 7 SCSI devices (including the computer itself) can be chained together. Each device in the chain must have its own unique ID number (also known as address) from 0-6. Beware that the internal hard drive always has the SCSI ID '0' (zero).

On every Mac since the Plus you can specify which SCSI drive you want to be the startup device. If you don't do that the Mac will look first to SCSI ID 0 (the internal hard drive), then the floppy drive, then try to start from ID number 6, then 5, and on down to 1.

Terminators

A terminator is a little piece of hardware that keeps signals from echoing back and forth along the SCSI cabling, which can cause errors. The faster a Mac is, and the more devices there are attached to it, the more important termination is. Most new drives come with built-in termination and an on/off switch to turn it on or off.

The notes are based on The Macintosh Bible, 4th Edition, Arthur Naiman et. al., Peachpit Press, 1987-92.

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WWW Department of Molecular Microbiology, Box 8230 Washington University School of Medicine St. Louis, MO 63110-1093 USA	Tel 314-362-7059 FAX 314-362-1232 Last updated September 7, 1995.