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3.1 Determining cabling and termination
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.
ATTO ExpressPCI Ultra3 SCSI host adapters will
operate with both Low Voltage Differential and
Single-Ended devices.
When connected to Ultra 3 devices, the ATTO
ExpressPCI Ultra3 SCSI host adapters will
negotiate to Ultra 3 transfer rates of 160 MB/sec.
per channel.
When connected to Single-Ended devices, the
ATTO ExpressPCI Ultra3 SCSI host adapters will
negotiate to Ultra3 transfer rates of 40 MB/sec.
per channel.
Cable types
Single-ended and differential SCSI host adapters
use the same cables. 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 3.1-1 Various types of SCSI require different bus lengths to support a certain number of devices.
STA terms
Fast SCSI
Fast/WIDE SCSI
UltraSCSI
UltraSCSI
Ultra/WIDE SCSI
Ultra/WIDE SCSI
Ultra/WIDE SCSI
Ultra2 SCSI
Ultra2/WIDE SCSI
Ultra3/WIDE SCSI
7 ATTO Technology ExpressPCI Ultra3 SCSI Host Adapters Installation and Operation Manual
Bus speed
Bus width
MB/sec. max.
bits
10
8
20
16
20
8
20
8
40
16
40
16
40
16
80
16
80
16
160
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.
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.
Ultra3SCSI and Ultra2 SCSI are less sensitive to
bus noise and offer you the ability to connect a full
load of devices up to 12 meters.
Max. bus lengths, meters
Single-ended
Differential
3
25
3
25
1.5
25
3
n/a
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
4
n/a
16
n/a
8
n/a
4
12
8
12
16
12
16
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