I did some playing around and discovered that:
1) The source of the GPS data is of course the GPS receiver head, or "mushroom." Mine was circa 1995 (the year Reboot was built.) This particular "mushroom" came in both a Seatalk (tm) and NMEA version. Mine is the Seatalk (tm) version. It happens that it was the only instrument on this particular Seatalk (tm) bus connected directly to the autopilot. The GPS display head was on a second Seatalk (tm) bus that also communicates to the Raymarine autopilot control head.
|ST 50 era GPS display|
3) The autopilot control display immediately started displaying the same information that appeared on the Garmin Multifunction displays (5212 & 5012.) They had always been slightly out of sync. I had just written it off to a network interfacing issue. Now I know that the autopilot computer was getting the wrong data from the GPS mushroom. Obviously it was using that data as the "preferred" data. Since the Seatalk (tm) bus for the mushroom was powered by the autopilot computer I never saw the condition without the GPS mushroom providing data until I actually disconnected ti electrically.
4) I have posted this on the Raymarine web site. We will see what they say.
5) Sort of a post script: I realized yesterday that the last time I swung the compass I was using the wrong GPS data. I was considering re calibrating yesterday (there was no wind and the water was flat) but I was in a hurry to get to Norfolk (yup, that worked, nope) so I passed on the opportunity. It will be interesting to see what happens when I do the re calibration.
Fair winds and following seas :)