Installing System Options
4–11
SCSI Bus Length
The total SCSI bus length includes the length of the internal cables, wiring on the
motherboard, external cables, and the cables inside the external storage enclosures. The
total length must not exceed:
• 3 meters (9.8 feet) for fast SCSI (10 MB/second) operation.
• 4 meters (13.1 feet) for "slow" SCSI (5 MB/second) operation.
The Digital AlphaStation 255 system's internal SCSI length is 1.1 meters (3.6 feet). This
leaves 1.9 meters (6.2 feet) available externally for fast SCSI use. If slow SCSI is used,
then an external length of 2.9 meters (9.5 feet) is available.
SCSI Termination
The Digital AlphaStation 255 system contains an embedded SCSI controller. The bus is
used to connect the controller to both the internal and external drives. Both ends (and only
at the ends) of the SCSI bus must be terminated correctly.
External
Like the internal SCSI bus, the external end of the SCSI bus also must be terminated.
When no SCSI devices are present on the external SCSI bus, the bus is automatically
terminated on the motherboard.
*
When, however, an external SCSI cable is present, the
automatic external termination is disabled, and the last external SCSI device must be
properly terminated.
F
For information on disabling the automatic termination sensing feature, see
Appendix B, Technical Specifications.
Internal
The internal SCSI cable used for the Digital AlphaStation 255 system is a flat ribbon
cable. A terminator is plugged into the last connector on the cable. Therefore, no internal
storage devices should have termination enabled.
*
Initial shipments included motherboards without automatic termination. In these cases,
an external terminator was provided in the accessories kit shipped with the system.
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