We found this article in the Redgate Buyer's Guide Library on AppleLink:
"The AT-Tester is a LocalTalk network cable tester that can test both LocalTalk
cables (Mini DIN 3) or PhoneNET cables (RJ-11).
"A user can ensure total circuit integrity in Mini DIN 3 cables by plugging the
cable into the power unit and plugging the remote end to the test unit. The
tester will then show a user what the problem is. The telephone cable is
tested the same way, but using the RJ-11 ports mounted beside the Mini DIN 3
port on either side of the unit."
For more details, search the Technical Info library under "AESP, Inc."