Instruction manual

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.
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.
Connection considerations
If using a combination of Wide 16-bit devices and
Narrow 8-bit devices on the same connector,
Wide devices must be connected first (closest to
the connector), followed by the Narrow devices.
Please refer to the documentation you received
with your SCSI devices to determine if your
device is Wide or Narrow.
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