Compaq AA-RHGWB-TE manual Terminating the Shared Scsi Bus when Using UltraSCSI Hubs

Table 3–2: SCSI Bus Segment Length

SCSI Bus	Bus Speed		Maximum Cable Length
Narrow, single-ended	5 MB/sec		6	meters
Narrow, single-ended fast	10	MB/sec	3	meters
Wide differential, fast	20	MB/sec	25		meters
Differential UltraSCSI	40	MB/sec	25		metersa

aThe maximum separation between a host and the storage in a TruCluster Server configuration is 50 meters: 25 meters between any host and the UltraSCSI hub and 25 meters between the UltraSCSI hub and the RAID array controller.

Because of the cable length limit, you must plan your hardware configuration carefully, and ensure that each SCSI bus meets the cable limit guidelines. In general, you must place systems and storage shelves as close together as possible and choose the shortest possible cables for the shared bus.

3.5Terminating the Shared SCSI Bus when Using UltraSCSI Hubs

You must properly connect devices to a shared SCSI bus. In addition, you can terminate only the beginning and end of each bus segment (either single-ended or differential).

There are two rules for SCSI bus termination:

•There are only two terminators for each SCSI bus segment. If you use an UltraSCSI hub, you only have to install one terminator.

•If you do not use an UltraSCSI hub, bus termination must be external. External termination is covered in Section 9.2.

______________________ 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.

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.

Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware 3–7