Installation Instructions: AM-642 STOP Converter
Page 5
Rev. A01
SCSI Bus Termination
To function properly, the SCSI bus on your computer must be terminated at each end. The SCSI
controller terminates one end of the bus; in most AMOS configurations, the other end of the bus is
terminated by plugging an external terminator into the external SCSI port. However, since the AM-642
plugs into the external port, you must terminate the bus by plugging a terminator into the terminator port
on the AM-642.
Narrow Bus Termination
On the narrow SCSI 2 bus, terminating the bus at the AM-642 is simple. If the AM-642 is the last device
on the bus, you merely plug the terminator supplied with the STOP converter into its terminator port.
Wide SCSI Bus Termination
Making sure the Wide SCSI bus is properly terminated is more complex, for two reasons:
•
All AMOS computers which support the Wide SCSI bus require active bus termination.
Therefore, you cannot use the terminator supplied with the AM-642, which is a passive
terminator.
•
The AM-642 is a narrow SCSI device. Therefore, any terminator plugged into it does not
terminate the “high” nine lines of the Wide SCSI bus. To properly terminate the bus, you need to
terminate these lines.
So, to properly terminate the Wide SCSI bus when using an AM-642, you must:
1. Between the external SCSI port and the AM-642, use an external wide-to-narrow SCSI cable
which actively terminates the high nine lines of the Wide SCSI bus. Alpha Micro offers this
cable in both three-foot and six-foot lengths (PDB-00440-80 and PDB-00440-81).
2. Plug a narrow active external terminator (PRA-00222-21) into the terminator port of the AM-
642. This terminates the remaining lines of the SCSI bus.
If you have more than one external device on the Wide SCSI-2 bus, terminate the high nine lines where
the bus changes from Wide to narrow, and the remaining lines at the end of the bus. Again, be sure to use
active terminators.
Be sure that only the last device on the bus has a terminator installed. If more than one device on
the SCSI bus has its termination enabled, your system will probably perform erratically! This is
true whether the other terminated device is in the computer chassis or in a separate subsystem
chassis.
SCSI bus performance, in general, degrades as you add more devices to the bus, and as the total length of
the SCSI cable increases. For most reliable performance, do not attach extra devices to the bus, and keep
the total cable length—internal and external—as short as possible.
