Power Computing Power 100, User Manual

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Connecting Peripheral Devices
42
Power 100/120 User Guide
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The way you assign SCSI IDs varies from device to device. On most external devices, the ID
is displayed on the back of the device. You usually change it by pressing small buttons above
or below the number display. For some external and most internal devices, changing the ID
requires setting switches or moving jumpers. Refer to the documentation that came with the
device for exact instructions.
The important thing to remember is that each device must have an ID that is unique within its
bus. If there is an ID conßict, your computer and the SCSI devices connected to it will
malfunction.
Ensuring proper termination
For a SCSI chain to work properly, it must be terminated correctly. The basic rule about
SCSI termination is simple: the device at the end of the chain must be
terminated . Devices
located between the computer and the end of the chain must be unterminated.
On the internal SCSI bus, any devices you install should be unterminated because the
computerÕs hard disk at the end of the bus is terminated and all of the internal connectors are
in the middle of the chain.
On the external bus, you need to consider where the device is placed in the chain and
whether it has an internal terminator. (Most SCSI devices use removable external
terminators, but some older devices have built-in terminators that are difÞcult to remove.)
■
If you have an internally terminated device, place it at the end of the chain and remove
external terminators from other devices in the chain.
■
If none of the devices is internally terminated, place an external terminator on the last
device in the chain and on no other device.
See the documentation that came with each device for information about how it is
terminated.
Connecting an external SCSI device
The external SCSI port, marked with
g
, provides high-speed communication between the
computer and devices such as hard disks, CD-ROM drives, scanners, printers, tape backup
drives, and so on.
To connect a single SCSI device (or the Þrst device in a chain), you need a SCSI cable with a
25-pin connector on one end and a 50-pin connector on the other. For each additional cable,
you need a cable with 50-pin connectors on both ends. The cables you use should be double-
shielded and have approximately 110-ohm impedance. Most SCSI problems are the result of
low-grade cables.
Note:
External SCSI devices which are connected to the computer must be turned on.
Devices on the SCSI bus which are turned off can cause system errors.