The module tests three plugs at a time, allowing all four output pairs to be tested in turn. I took a piece of plywood, screwed three Märklin plugs into it and connected them up to two LEDs and a 1.5K Ohm resistor.
This allows me to insert the tester into a k83 and when the ports are activated the red and green LEDs flash accordingly providing a clear indication that the module and address selection etc. is working!
Update 2017This little tester has proven to be invaluable over the years, and I sometimes still use it 10 years later if I need to determine if some accessory has a wiring problem or if the k83 type module has a problem.
I also have k83/k84 testing capabilities through the main program and my browser on my phone