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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 (MB/sec) Maximum Cable Length
Narrow, single-ended
5
6 meters (19.7 feet)
Narrow, single-ended fast
10
3 meters (9.8 feet)
Wide differential, fast
20
25 meters (82 feet)
Differential UltraSCSI
40 25 meters
a
a
The maximum separation between a host and storage in a TruCluster Server configuration is 50 meters
(164 feet): 25 meters (82 feet) between any host and the UltraSCSI hub and 25 meters (82 feet) 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.
3.5 Terminating 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 10.2.
Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware 3–7