When setup the LocoNet® at a meeting, a test device is often required. It is sufficient to check the levels on the lines. Such a simple test device can be found here.
The device was developed by FREMO.
The PCB fits into housing TEKO 10007
Original came from here:
http://dcc-mueller.de/loconet/ln_tst_d.htm
This page contains also a brief description for the usage.