Compaq TruCluster Server AA-RHGWB-TE, Manual

Page: 256 / 320

Whenever possible, connect devices to a shared bus so that they can be
isolated from the bus. This allows you to disconnect devices from the bus
for maintenance purposes without affecting bus termination and cluster
operation. You also can set up a shared SCSI bus so that you can connect
additional devices at a later time without affecting bus termination.
______________________ Notes ______________________
With the exception of the TZ885, TZ887, TL890, TL891, and
TL892, tape devices can only be installed at the end of a shared
SCSI bus. These tape devices are the only supported tape devices
that can be terminated externally.
We recommend that tape loaders be on a separate shared SCSI
bus to allow normal shared SCSI bus termination for those shared
SCSI buses without tape loaders.
Most devices have internal termination. For example, the KZPSA and
KZPBA host bus adapters, BA350 and BA356 storage shelves, and the
DWZZA and DWZZB SCSI bus signal converters have internal termination.
Depending on how you set up a shared bus, you may have to enable or
disable device termination.
Unless you are using an UltraSCSI hub, if you use a device’s internal
termination to terminate a shared bus, and you disconnect the bus cable
from the device, the bus will not be terminated and cluster operation will
be impaired. Therefore, unless you use an UltraSCSI hub, you must use
external termination, enabling you to detach the device without affecting bus
termination. The use of UltraSCSI hubs with UltraSCSI devices is discussed
in Section 3.5 and Section 3.6. The use of a DS-DWZZH-03 UltraSCSI hub
with externally terminated host bus adapters is discussed in Section 9.4.3.
To be able to externally terminate a bus and connect and disconnect devices
without affecting bus termination, remove the device termination and use Y
cables or trilink connectors to connect a device to a shared SCSI bus.
By attaching a Y cable or trilink connector to an unterminated device, you
can locate the device in the middle or at the end of the shared bus. If
the device is at the end of a bus, attach an H879-AA terminator to the
BN21W-0B Y cable or H885-AA trilink connector to terminate the bus. For
UltraSCSI devices, attach an H8863-AA terminator to the H8861 trilink
connector. If you disconnect the Y cable or trilink connector from the device,
the shared bus is still terminated and the shared SCSI bus is still operable.
In addition, you can attach a Y cable or a trilink connector to a properly
terminated shared bus without connecting the Y cable or trilink connector
to a device. If you do this, you can connect a device to the Y cable or trilink
9–6 Configurations Using External Termination or Radial Connections to Non-UltraSCSI Devices