7 ATTO Technology ExpressPCI Ultra/WIDE SCSI Host Adapters Installation and Operation Manual
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.
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.
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.
Exhibit 3.1-1
Various types of SCSI require different bus lengths to support a certain number of devices.
Connectors
ATTO ExpressPCI PSC:
one industry standard 68-
pin “P” (16-bit) and one 50-pin (8-bit) cable
connector for internal device connections and one
68-pin “P” (16-bit) cable connector for external
device connections.
ATTO ExpressPCI DC:
two industry standard 68-
pin “P” (16-bit) cable connectors for internal
device connections and two 68-pin “P” (16-bit)
VHDCI (Very High Density Cabled Interconnect)
cable connectors for external device connections.
NOTE When installing SCSI devices on the ATTO
ExpressPCI PSC, use any two SCSI ports on your ATTO
ExpressPCI SCSI adapter, but not all three. Use all four
channels on the ATTO ExpressPCI DC.
STA terms
Bus speed
MB/sec. max.
Bus width
bits
Max. bus lengths, meters
Max. device
support
Single-ended
Differential
LVD
Fast SCSI
10
8
3
25
12
8
Fast/WIDE SCSI
20
16
3
25
12
16
UltraSCSI
20
8
1.5
25
12
8
UltraSCSI
20
8
3
n/a
n/a
4
Ultra/WIDE SCSI
40
16
n/a
25
12
16
Ultra/WIDE SCSI
40
16
1.5
n/a
n/a
8
Ultra/WIDE SCSI
40
16
3
n/a
n/a
4
Ultra2 SCSI
80
16
n/a
n/a
12
8
Ultra2/WIDE SCSI
80
16
n/a
n/a
12
16
