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.
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