SCSI Primer

Summary: SCSI data sheet to help you understand the computerese. Some little things which took me too much time to really understand.


The SCSI interface (Small Computer System Interface) is a technical standard about the way to connect different types of devices to a computer. This interface was chosen on the Macintosh to connect hard disks since the beginning. For PCs, the SCSI interface remains a high level solution for big hard disks and other expensive devices.
If connecting a SCSI 1 device to a computer generally makes no problem, things become trickier when you have to connect SCSI 2 devices and when there are more than three devices on the bus.
If it is true that SCSI looks sometimes like a sort of black magic, one must note than half truthes or plain errors are often touted about SCSI. We tried to publish on this page only ascertained facts. However, if you discover an error, please let us know. We will correct it as soon as possible.

Logical Addresses (Devices IDs)

The logical addresses (0 to 7) are used to identify the devices connected to the bus. The host adapter (the board) is a device like others. To resolve conflicts arrising when several devices try to seize the bus at the same time, the standard states that the higher address has priority. This explains why the adapter is most of the time at address 7.
The logical address has nothing to see with the physical position of the devices on the bus (internal or external). The only rule is that two devices can't have the same address.


Terminators are resistances installed at each end of the bus (and not on any other devices). The role of these terminators is to allow the devices to ascertain the level (high or low) of each of the wires. You can think of the terminators as something necessary to allow the signal to be propagated and read, like a cello string must be tighten to give a sound under the bow.
There must be only two terminators, at each end. If the SCSI bus is internal, the terminators are on the board and on the last internal device. Likewise, if the SCSI bus is external, the terminators are on the board and on the last external device. If there are both internal and external devices, the terminator of the adapter must be inhibited.
There are theorically two kinds of terminators, active and passive ones. But there are also so called "semi-active" ones. The rule is to use the same kind at both ends.
May I add that you should not try to spare on this!

Pierre Duhem (ex-Logiciels & Services Duhem)
3, rue Pierre Haret - F-75009 Paris (France) - Tel. (+33) [0]149 700 455