Xerox 1200 User Manual page 168

For example, the internal hard disk drive is, by convention, always assigned
SCSI ID 0 (internal). The system software shows the hard disk at that
location.
You can connect a total of seven internal and external SCSI devices to the
Macintosh. Each device must have its own unique SCSI ID number, from 0 to
6 (or from 1 to 6 if you have an internal hard disk drive installed).
About ID numbers: Do not assign numbers 0 or 7 to the scanner. The default
setting for the internal hard disk drive is 0, and for the computer, 7. Make sure
you use a number that is not assigned to any other SCSI device because each
ID number must be unique.
Factory setting
The SCSI ID number for the Apple Color OneScanner is set in the factory at
2, with termination on (see the next section), but you can change the setting
if necessary.
About SCSI termination
To ensure accurate transmission of information, a terminator is used at the
end of a SCSI chain, even if there is only one device in the chain.
Any Macintosh computer with an internal hard disk drive contains its own
terminator. This means that you need only one other terminator (internal or
external) attached to the last device in the chain.
The Apple Color OneScanner has an internal (built-in) terminator. This
control switch can be set to various SCSI ID numbers. Each number can be
set with termination on or off.
m With termination on, information stops at the scanner.
m With termination off, information can pass through the scanner and travel
onward down a chain to another SCSI device.
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Appendix C