I've developed a simple tool for testing/debugging custom display/controllers which use the KSPSerialIO mod by @zitronen
This allows you to monitor controls from the Arduino, as well as send control values to the Arduino. Reloading KSP multiple times, and the lack of real time onscreen data make this tool a bit more efficient in finding what isn't working.
It also allows you to test/play with your display or controller without loading KSP.
The project is open source and can be fou