IBM PS/2 90 XP 486 Hardware Maintenance Reference page 38

3.3.1 Selecting a SCSI ID Priority
The SCSI ID (6, 5, 4, 3, 2, 1, or 0) available for a device depends on
which IDs are already assigned to devices in the same SCSI chain.
example, if there is already one device in the SCSI chain assigned SCSI ID
6, no other SCSI devices (in that chain) can be set to SCSI ID 6.
The ID you choose determines the priority of the device.
should be set as follows:
The SCSI adapter is shipped assigned SCSI ID 7 (highest priority).
A fixed-media read and write device, such as a hard disk drive, should
be assigned a high-priority ID such as 6 or 5.
Note: The default drive with the IML code loaded is usually SCSI ID
6.
Removable-media devices (CD-ROM, optical drive, or a tape drive),
should be assigned a priority ID of 4, 3, 2, 1, or 0 (an ID below that
of the fixed-media devices).
In general, a low-priority or mid-priority device should not be assigned a
SCSI ID higher than a high-priority device.
IBM PS/2 Model 90 XP 486 and 95 XP 486 HMR
Selecting a SCSI ID Priority
¦ Copyright IBM Corp. 1990, 1992
3.3.1 - 1
For
SCSI Device IDs