Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 143

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.
Every SCSI chain or circuit must be terminated (closed) at
both ends. Termination can be accomplished by using the
terminating feature on the device or by using a terminated
cable.
4X and 8X devices may not have terminating jumpers on
the device. Termination on these devices must be
achieved with terminated cable.
Turn on an external SCSI device before turning on power
to the computer. This enables the system board controller
to recognize the external SCSI device and automatically
reset. When an external SCSI device is connected to the
external SCSI connector on the rear panel of the
computer, that device becomes the end of the SCSI chain
and must be terminated.