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General Information on Installing Drives
This section covers general drive-installation information, including internal and external
SCSI drives, external SCSI, and cable layouts.
SCSI Bus Length
The total SCSI bus length, including internal cables and etch on the motherboard as well
as the external cables between storage enclosures and the cables inside the storage
enclosures, must not exceed 3 meters (9.8 feet) for fast SCSI operation or 4 meters (13.1
feet) for slow SCSI (5 MB/second) operation. The AlphaServer 300 system's internal
SCSI length is 1.2 meters (3.9 feet), leaving 1.8 meters (5.9 feet) available for external fast
SCSI use (2.8 meters [9.2 feet] for external slow SCSI use).
Internal SCSI Termination
SCSI drives are connected to a SCSI bus, which must be terminated at each end.
Internally (inside the system box), the termination is set correctly at the factory. However,
if you add or remove an internal SCSI drive, you may need to change the termination,
especially if the drive you add or remove requires moving the end connector of the SCSI
cable. For example, Figure 4-6 shows a typical configuration after a CD drive
was
added. Previously, the end connector of the SCSI cable
was plugged into the hard disk
and the SCSI bus was terminated by the hard disk. One of the daisy-chained
connectors on the SCSI cable
was unused. The SCSI cable originates on the
motherboard
and goes first to another SCSI device .
e
f
g
c
d
h
3
6
4
2
Figure 4-6 Internal SCSI Termination
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