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Cabling
Cables and devices must be chosen to maximize
performance and minimize the electrical noise
from the high-speed data transfers available with
the SCSI protocol. Cabling and termination
methods become important considerations for
proper performance. SCSI cables and devices are
subject to specific length and number limitations
to deal with electrical problems that arise at
increased operating speeds.
Cable types
Use high-quality cables rated for the type of SCSI
transfers required: well-insulated SCSI cables
ensure error free communications. Try to keep
cable lengths as short as possible to ensure higher
signal quality and performance.
Exhibit 1.5-1 Various types of SCSI operate at different speeds and require different bus lengths to support
a certain number of devices.
STA terms
MB/sec. max.
Fast SCSI
10
Fast/WIDE SCSI
20
UltraSCSI
20
Ultra/WIDE SCSI
40
Ultra/WIDE SCSI
40
Ultra/WIDE SCSI
40
Ultra2 SCSI
80
Ultra2/WIDE SCSI
80
Ultra3/WIDE SCSI
160
Bus speed
Bus width
bits
8
16
8
16
16
16
16
16
16
Examples
The SCSI specification limits total bus cable
length for single-ended SCSI in a non-UltraSCSI
environment to 3 meters (combined length of both
internal and external cable lengths).
In an UltraSCSI workgroup environment with a 7-
drive tower, you are limited to 1.5 meters between
the host and the tower, including the cabling for
the tower. If the 7-drive tower requires 1 meter of
cabling to connect all of its drives, the distance
from the tower to the host must be .5 meters.
Note
UltraSCSI is very sensitive to SCSI bus noise,
cable distances and the number of devices
connected on the SCSI bus. Carefully connect
your devices when working with UltraSCSI.
Max. bus lengths in meters
Single-ended
Differential
3
25
3
25
1.5
25
n/a
25
1.5
n/a
3
n/a
n/a
n/a
n/a
n/a
n/a
n/a
Max. device
support
LVD
n/a
8
n/a
16
n/a
8
n/a
16
n/a
8
n/a
4
12
8
12
16
12
16
Installation: SCSI connections
12
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