Since I have now built a pile of k83 modules (actually Viessmann 5211) I need to test them. Each one has 12 outputs to be tested, I decided that a small plug-in module was in order.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.