Apple Power Macintosh 8500/120 Series - Manual (Page 73)

IMPORTANT The total length of the cables in a SCSI chain should not exceed
6 meters (20 feet). SCSI cables must have a 110-ohm impedance. For best
results, use SCSI cables manufactured by Apple Computer.
m Make sure that the last (or only) device in the SCSI chain has a terminator.
Make sure that no other external SCSI device has a terminator.
To ensure accurate transmission of information, a terminator must be at
each end of a SCSI chain. Your internal hard disk, which is the first device
in the chain, has a built-in terminator.
Some external SCSI devices from manufacturers other than Apple have
built-in terminators. (Check the information that came with the device.) If
the device at the end of the SCSI chain does not have a built-in terminator,
you need to attach an external terminator.
If your SCSI device has a built-in terminator, you may choose to use it as
your first or last device in the chain, or you may have your Apple-authorized
service provider remove any extra built-in terminators. You can attach or
remove external terminators yourself.
SCSI terminator
61 Connecting Additional Equipment