Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 107

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:
0 is reserved for the hard drive.
1 through 4 are available.
5 is reserved for a CD-ROM drive.
6 is available.
7 is reserved for the SCSI host adapter.