Apple Macintosh Performa 6400 Series - Manual (Page 55)

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 computer’s main logic board, 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, use it as your last device in the
chain or have your Apple-authorized service provider remove the extra built-
in terminator. You can attach or remove external terminators yourself.
SCSI terminator
55 Connecting Equipment and Expanding Your Computer