Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 142

Optional drives are available from Compaq in kits that include a
special drive cable. The configuration of the drives employs a
cable-select feature that identifies the drives as device 0
(primary drive) or device 1 (secondary drive). The system board
determines which drive is device 0 or device 1, based on the
way the drives are connected to the special drive cable. The
device 0 drive is the drive connected to the short segment of the
drive cable (or that connector closest to the system board); the
device 1 drive is the drive connected to the long segment of the
drive cable. The device 0 and device 1 identities are used by the
system to distinguish between the two drives.
The installation requires no jumper setting changes on the
existing or optional drives. All Compaq drives have the jumpers
preset for cable-select installation.
If installing a second device on the primary controller,
you must use an 80-conductor Ultra ATA cable for
optimal performance. This cable is available as a Compaq
option.
The Compaq Deskpro 6000 Series of Personal Computers
features an embedded Ultra-SCSI host adapter that supports up
to seven internal and external SCSI devices. Deskpro 6000
models come with an internal SCSI hard drive preinstalled.
Additional high-performance SCSI devices can be added to all
Deskpro 6000 models.
When installing additional SCSI devices, you must adhere to the
following guidelines:
A SCSI host adapter supports up to 7 SCSI devices.
Every SCSI device must have a unique identification (ID)
number (0 - 7). The SCSI host adapter identifies signals to
and from 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 the communi-
cation between the controller and the SCSI device. The
reserved and available SCSI ID numbers for SCSI devices
are as follows: