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SCSI Cable Lengths and Device Speeds
For each SCSI bus, the type of bus and the data clock speed of the fastest device on that bus
determine its maximum length. Knowing this, you can determine the maximum cable length you
can use to connect external SCSI devices to the system.
The following table provides a guide to maximum SCSI bus length. Note that bus width (for
example, Narrow versus Wide) does not affect the maximum bus length.
Fastest device on the bus
Single-ended bus
Differential bus
LVDS bus
5 MHz (SCSI-1)
19.7 ft / 6 m
82 ft / 25 m
39.4 ft / 12 m
10 MHz (SCSI-2 or Fast)
9.8 ft / 3 m
82 ft / 25 m
39.4 ft / 12 m
20 MHz (Ultra or Fast20)
4.9 ft / 1.5 m
39.4 ft / 12 m
39.4 ft / 12 m
40 MHz (Ultra2 or Fast40) *
Not recommended
Not specified
39.4 ft / 12 m
* Bus used in the system
The total cable length on the system’s Wide Ultra2 SCSI bus is the sum of the following:
♦
Internal SCSI cable—40 inches (1.0 meter).
♦
SCSI cable between the system and the first external device
♦
SCSI cables between additional external devices
♦
SCSI cable inside each device connected to the cable
−
typically 8 inches (0.2 meters) or less
The maximum speed of a SCSI bus is limited to the speed of the slowest device on that bus.
SCSI Cable Quality
To ensure data integrity and optimum performance, do the following:
♦
Use the shortest cables possible to connect SCSI devices to the system and to each other.
♦
Use high-quality SCSI cables to ensure adequate shielding (impedance of 110 to 135 ohms).
CAUTION
When handling cables, flex them as little as possible. Ensure that cables do not
contact sharp metal surfaces or become excessively bent or twisted. In particular,
SCSI cables should not have any creased bends.
