SCSI Primer
Introduction
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.
Termination
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!
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