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SCSI

What Is SCSI? - SCSI is actually an acronym for Small Computer System Interface. It's a relatively fast (for it's time) bus for transfering data between different types of peripheral devices such as hard drives, CD-ROM drives, scanners, etc. A SCSI bus can be either internal, external, or cross the boundary from internal to external.

How many devices can be plugged into a SCSI bus? - A total of 7 devices can be plugged into a single SCSI bus, one of those devices will always be the computer or the SCSI card, because they too are devices on the SCSI bus.

How do I attach a device to the SCSI chain? - First, you need to make sure both your computer and the SCSI device that you plan to attach to your SCSI chain are off, both your computer and SCSI device can be damaged if this precaution is not taken. Next make sure that the cable type matches that of your SCSI port (either on your computer, SCSI card, or another device on the SCSI chain) so that you can, infact, add the device to your SCSI chain. Next you need to provide proper SCSI termination to the SCSI bus and set the SCSI ID number for the new device.

What types of cables are there? - Well, the port on the back of most Macs, except for PowerBooks, is a 25-pin port (DB-25 female). The port on the back of most PowerBooks is an HDI-30 port which is much more compact than that on desktop Macs. Another common type of port on SCSI devices is the larger Centronics-50 type port. You can also add SCSI-1 devices to your SCSI bus, these devices have a 50 pin port which is slightly smaller than the DB-25 port. Cables which can interface between all of these different types of ports are readily available from Apple dealers, on-line stores, and from mail-order catalogs. If you don't know what type of cable to get, contact a local Apple dealer or a local Macintosh User Group.

What is termination and why do I need it? - Termination makes sure that the data on the SCSI bus gets transfered properly. I could probably do a little better than that at explaining it, but other people have already written articles on this topic, the following explanation is from Termination Explained:

When SCSI was first released, the term "termination" became the hot topic. Termination is short for a concept called "termination power," which provides a minimal voltage used for the detection of SCSI devices on a chain. On a SCSI device, installing resistors creates a managed short between a set of wires, and shortcuts the circuit so that "termination power" does not pass beyond that device to another device. In this way, the circuit is completed, and signals can travel in a loop back to the controller card. When termination isn't present on a device, the loop is extended to the next device, instead of being completed on this device.

How do I terminate a SCSI bus? - Here I will only discuss terminating an external chain because Termination Explained does a better job of going in depth for those who wish to explore internal SCSI termination for SCSI busses which run both inside and outside of your computer's case.

All you need to do to terminate a SCSI chain is to place an active terminator at the end of the SCSI chain, on the last device. Note that some Macs don't provide SCSI termination power, so one of your external SCSI devices must provide it, but you can only have one device providing the termination power, so all other devices must have termination power disabled (usually by an authorized Apple dealer, unless you know how to do it yourself).