I have been struggling to connect garmin GPS to Raymarine 2000 pilot.
In Raymarine 2000 plus user manual, the connecting chart shows only two NMEA wires; my portable Garmin GPS has a cable with 3 wires. One is called IN one is caled OUT and the third is common. If I connect IN-IN OUT-OUT and leave common free the pilot senses nothing. So it is likely that your suggestion might work. My only question is how can the pilot perceive a voltage with
NO ground? Amazing! I agree that nothing should go from pilot to GPS; but the signal from GPS to pilot should be proportional to the ERROR; no voltage if set course=actual course; positive or negative voltage otherwise. But to detect a (say) positive voltage a ground is needed: positive respect to WHAT??