According to the Helms, the bus between the nav unit and the touch screen is GA-NET II. I've been googling a bunch for it, and haven't come up with anything too solid. I've taken some data captures with an osciloscope, and it isn't obvious how the protocol works. I plan on taking some more indepth data captures with a high speed data logger soon to see if it makes more sense when I have several different messages to compare.

If anyone has any info about it, I would really appreciate it.

Thanks,
Dave