Compaq Service Reference Guide
5-1
chapter
5
SCSI G
UIDELINES
When installing and operating SCSI devices, you must follow these guidelines:
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A narrow SCSI controller allows you to daisy-chain up to 7 additional SCSI devices.
Counting the controller, that amounts to 8 total SCSI devices.
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A single Ultra SCSI, Ultra-Wide SCSI, or Wide-Ultra SCSI controller allows you to daisy-
chain up to 15 additional SCSI devices. Counting the controller, that amounts to 16 total
SCSI devices.
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If two SCSI controllers are each connected to separate system board SCSI connectors,
each controller may have seven SCSI devices attached. This will give a total of 16 SCSI
devices on the system.
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Compaq does not recommend mixing different width SCSI devices on the same SCSI
chain or on the same SCSI controller. Mixing devices of different widths on the same
chain or controller will always result in a data transfer rate of the slowest machine in that
chain.
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If two controllers are used, each may use SCSI devices having widths and speeds different
from the other. If a 68-pin data cable is used on a controller having 50-pin SCSI devices,
use an internal cable adapter (Compaq number 199618-001) or an external cable adapter
(Compaq number 270187-B21).
CAUTION:
Do not route data cables near the air intake to the power supply. Cables routed in this manner
may block the airflow and cause the computer to overheat.
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The Ultra SCSI, Ultra-Wide SCSI, or Wide-Ultra SCSI controller requires a unique SCSI
ID (0-7 or 8-15) for each SCSI device installed. The controller identifies a SCSI device by
its SCSI ID number rather than its location. Moving a SCSI device from one position to
another on the SCSI chain does not affect communication between the controller and the
device. The reserved and available SCSI ID numbers for SCSI devices are:
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0 is reserved for the primary hard drive.
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7 is reserved for the SCSI conroller.
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1 through 6 and 8 through 15 are available for all other SCSI devices.
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Every SCSI chain or circuit must be terminated (closed) at both ends. Some system boards
have both ends of the SCSI cable connected to, and terminated by, the system board.
Termination can be accomplished in one of several ways:
❏
Use a cable with a built-in terminator.
❏
Use a cable with a terminating resistor plug in the last connector.
❏
Connect a SCSI device with its termination enabled into the last connector.
❏
Connect an external SCSI device with its termination enabled to the external SCSI
connector on the rear panel of the computer.
Summary of Contents for Deskpro 2000
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