Product guide

8
Cabling and termination
• If you combine Wide 16-bit and Narrow
8-bit devices on the same connector,
connect the Wide devices first (closest
to the connector).
• Please refer to the documentation for
your SCSI devices to determine if they
are Wide or Narrow, UltraWIDE SCSI,
Ultra2 SCSI, Ultra160 or Ultra 320.
3 Determine which terminator to use
• Use an LVD terminator if you are only using
LVD devices.
Although you can use a Single-Ended
terminator, all devices will be limited to
Ultra SCSI speeds. Single-Ended devices
require a Single-Ended terminator. If you
use an LVD terminator with Single-Ended
devices, the system may hang or devices
may not be seen on the SCSI bus. Some
termination manufacturers provide
automatically sensing terminators.
• External terminators should be attached to
the last external device in the SCSI chain.
Don’t use any other termination on the
external SCSI chain. The last device on an
internal SCSI chain should also be
terminated. This can be done in several
ways. Many Single-Ended Ultra SCSI and
earlier devices provide a jumper setting for
applying termination: place a jumper over
the pins designated for termination on the
last device on the internal cable. Check
with your drive manufacturer if you are not
sure which pins to use.
• LVD Ultra2 and Ultra160 SCSI devices
cannot supply their own termination.
Use an internal ribbon cable which has a
SCSI terminator attached to the end of it,
connect the unterminated end of the cable
to the host adapter card and the internal
drives to the subsequent connectors. The
terminator should be at the opposite end of
the cable from the host adapter card.
• Wide (16-bit) and Narrow (8-bit) devices can
be connected together on the same
connector of the host adapter card, but
wide devices must be attached first,
followed by narrow devices.
To terminate the SCSI bus, the cable or
adapter used to convert from a wide (68-
pin) connector to a narrow (50-pin)
connector provides partial termination,
allowing upper 8-bits (or byte) of the wide
SCSI bus to be properly terminated. A
narrow terminator should be used on the
last narrow device to terminate the rest of
the SCSI bus. A SCSI bus without partial
termination between the wide and narrow
devices may at first appear to work
correctly, but occasional I/O errors occur
without proper termination.
• If you use both internal and external
connectors and mix Single-Ended and LVD
devices on the same bus, even if using
different connectors, the host adapter card
will operate with Single-Ended signaling at
UltraSCSI speeds.
• Automatic termination: when both internal
and external connectors are used, the host
adapter card detects the presence of
devices and turns off termination. If devices
are removed from one connector of the
card, the host adapter automatically
detects the change, and enables its own
termination.
• Software controlled termination: You may
have to override the host adapter’s
automatic termination if only narrow
devices are attached to one connector and
wide devices are attached to the other
connector on the same bus. The host
adapter must supply partial termination to
the upper 8-bits (byte), but not
automatically. Please refer to your
ExpressPCI Utilities manual for instructions
on setting the host adapter’s termination to
Upper Byte.
• Termination power: host adapters supply
termination power to the bus at all times
and many SCSI devices are also able to
supply termination power. SCSI signal
quality, particularly with long or marginal
quality cables, may be improved if the
device supplies the termination power.
Contact your device manufacturer for more
information on your device’s ability to
supply termination power.