Connecting External Scsi Devices; Terminating The Scsi Bus - Adaptec AHA-3980 Quick Reference Installation Manual

Connecting External SCSI Devices

For each external SCSI device, obtain a high-quality
external cable with a single-ended impedance range of
80–110 ohms. The cables should be at least 30 cm
(11.8 inches) long. Use the cables to connect external
SCSI devices as follows:
1
Attach the SCSI connector at one end of an exter-
nal SCSI cable to the external Channel A connec-
tor (see the board layout on page 2). External
cable connectors can be plugged-in only one way.
2
Attach the connector at the other end of the external
cable to either SCSI connector on the SCSI device.
3
Connect other external SCSI devices sequentially
from one device to the next, with no branching
(see the following figure).

Terminating the SCSI Bus

Each end of the SCSI bus must be electrically termi-
nated. Follow these instructions:
Devices Attached
Internal devices
only (Channel A,
B, or C)
External devices
only (Channel A)
Both internal and
external devices
(Channel A)
1
See the device documentation for instructions.
External
SCSI Cables
Terminator
1. The channel is at one end of the bus. Channel termina-
tion is enabled by default; keep this setting.
2. The passthrough terminator provides termination at the
other end of the bus.
3. Disable termination on all other SCSI devices, including
any device attached to the passthrough terminator.
1. The channel is at one end of the bus. Channel termina-
tion is enabled by default; keep this setting.
2. Terminate the last external SCSI device.
3. Disable termination for all other external SCSI devices.
1. The passthrough terminator provides termination at one
end of the bus.
2. Terminate the last external SCSI device.
3. Change the channel termination setting to Disabled.
4. Disable termination for all other SCSI devices.
Termination Procedure
6
1
1
1
1
1