Compaq AA-RHGWC-TE manual Scsi Bus Length, Scsi Bus Segment Length

______________________ Note _______________________

If you are using a DS-DWZZH-05 UltraSCSI hub with fair arbitration enabled, SCSI ID numbering will change (see Section 3.6.1.2).

Use the following priority order to assign SCSI IDs to the SCSI bus adapters connected to a shared SCSI bus:

7-6-5-4-3-2-1-0-15-14-13-12-11-10-9-8

This order specifies that 7 is the highest priority, and 8 is the lowest priority. When assigning SCSI IDs, use the highest priority ID for member systems (starting at 7). Use lower priority IDs for disks.

Note that you will not follow this general rule when using the DS-DWZZH-05 UltraSCSI hub with fair arbitration enabled.

The SCSI ID for a disk in a BA350 storage shelf corresponds to its slot location. The SCSI ID for a disk in a BA356 or UltraSCSI BA356 depends upon its slot location and the personality module SCSI bus address switch settings.

3.4 SCSI Bus Length

There is a limit to the length of the cables in a shared SCSI bus. The total cable length for a SCSI bus segment is calculated from one terminated end to the other.

If you are using devices that have the same transmission method and data path (for example, wide differential), a shared bus will consist of only one bus segment. If you have devices with different transmission methods, you will have both single-ended and differential bus segments, each of which must be terminated only at both ends and must adhere to the rules on bus length.

Table 3–2 describes the maximum cable length for a physical SCSI bus segment.

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.

3–6Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware