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Cabling Configuration
The cabling configuration is divided into two
subsections in this section, configurations pertaining to
Single-ended Ultra Wide SCSI and those for
Differential Ultra Wide SCSI. These two sections are
divided into groups describing the cabinet/cabling
scheme and provide the recommended cable and
termination configurations when operating the DEU in
a specific mode.
Cable length is a primary consideration when
configuring the subsystems. There are different
limitations for Single-ended Ultra Wide SCSI (Fast-20)
and Differential Ultra Wide SCSI.
Ultra Wide SCSI
According to SCSI-3 ANSI specifications (X3T10/1071D)
the total SCSI cable length for Single-ended Ultra Wide
SCSI is 3 meters (118 inches) with four SCSI devices
distributed evenly across the cable and 1.5 meters (59
inches) with eight drives distributed evenly across the
cable with each SCSI device having a 25 pico farad
load. By controlling the SCSI device loading and
impedance on the SCSI bus, SCSI device spacing and
cable lengths can vary and be improved. In a standard
configuration, the DEU is designed to accommodate a
total of nine SCSI devices in a single cabinet (eight
drives and one SAF-TE controller), all connected to a
host controller using a 1 meter cable. The DEU has an
internal equivalent SCSI bus cable length of just under
1 meter giving an overall cable length of 2 meters
(external 1 meter cable plus the internal 1 meter
equivalent cable length).
The number of SCSI devices and cable lengths on the
SCSI bus can also be increased and improved by adding
SCSI buffers that recondition the SCSI signals. With
the optional Ultra Extender card installed in the DEU,
SCSI signals are buffered and reconditioned allowing
the external cable length to be increased to a maximum
of 3 meters and still support nine SCSI devices in a
Single-ended Ultra SCSI environment. The Ultra
Extender Feature card also provides the DEU with the
ability to add a second cabinet on the same SCSI bus
supporting an additional six SCSI devices for a total of
sixteen SCSI devices. The maximum number of SCSI
devices supported by this specification on a single bus
is sixteen SCSI devices.