Your Mac can be connected to external devices such as hard drives, removable cartridge drives (Zip, Jazz, Syquest, optical), tape drives, and scanners using SCSI cables. SCSI is an abbreviation for Small Computer System Interface. SCSI is an excellent way to connect computer peripherals. It is used as a standard by high-end UNIX workstations (Sun, Digital, and Silicon Graphics machines) as well as an add-on option for IBM-compatible PCs. However, it is not quite as a simple and user friendly as most Mac technology.
SCSI devices are connected in a chain from a single connector in the back of your Mac. Each SCSI device has two SCSI connectors, one to connect to the Mac (or to the previous device in the chain) and one to connect to the next device. You can have up to a total of seven devices linked one to another on a single SCSI chain or "bus", but your Mac already uses some of these spots for internal devices. Your hard drive itself is (probably) a SCSI device, as is any internal CD-ROM drive, and any other internal drives (Zip, second hard drive, etc.). This means if you have an internal SCSI hard drive, and a CD-ROM drive then you can only have five additonal SCSI devices connected to your Mac.
There are 3 basic rules that govern the set-up and troubleshooting of SCSI chains.
There are also some guidelines you should keep in mind, but we don't consider them rules because you can find exceptions and differences of opinions out there:
Setting up SCSI devices on a Mac
There isn't a strict rule about the order in which to hook up SCSI devices in a chain, but there are some guidelines. Fast devices that transfer a lot of data should be put closest to the Mac in the chain. Scanners and tape drives like to be last in the chain.
Each SCSI device must have a "driver" software that controls its operation in your Mac's Extensions folder (in the System folder). For example, the Iomega Zip drive uses the Iomega Driver.
Here is a basic procedure to follow for adding a new SCSI device to a Mac.
SCSI problems can cause a wide variety of symptoms on your Mac, from "Sad Macs" or flashing at boot to erratic disappearance of disks from the desktop. However, once set up correctly, a SCSI chain should not suddenly begin to malfunction.
If a SCSI device is not working:
SCSI is moody. You will occasionally find that each SCSI device works fine when connected by itself to your Mac, but there is a malfunction when a group of them are chained together. This is where the demons of SCSI Voodoo begin to haunt your computer.
It takes some fiddling to get the SCSI chain right and it sometimes feels like you have to appease the SCSI gods before it all works smoothly. Just be methodical in your set-up and troubleshooting. You will be able to get away with incorrect SCSI configurations from time to time, but they normally catch up with you, particularly when you move to a new shiny faster machine. The faster the machine is, the less tolerant.
First re-check the rules: ID#s, length of cables, termination. Then try tweaking the set up. Change the order of devices in the chain or try switching ID#s. Finally check for Extensions conflicts that involve SCSI drivers.
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