Join Date: Feb 2003
Location: New England USA
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You will be pretty much stuck with a Raymarine, as their proprietary SeaTalk interface does not follow NEMA very closely. It would make everything easier to interface.
You can do it, but you will have to get the listing of what NEMA "words" the autopilot recognizes, and match them up with the "words" the gps transmits to the data bus.
Think about what the system has to accomplish, the autopilot needs to maintain a heading that it receives from the gps. Because the AP does not know where it is actually going, just a reference point on a fluxgate compass, the gps needs to feed deviation information to the gps, ie, "go +2deg". then the AP needs to respond. When interfacing such a system, it is usually best to have a server involved, which is a device that looks at all available data, (AP Wind, True Wind, Boat Speed, compass course, GPS course, etc.) then resolve it to make a judgment call on course corrections. There is usually a feedback device involved telling the server rudder position, so it can know if a suggested course correction is even feasible.
Pretty complex stuff! Not for the faint of heart, or technically challenged, and best left for the pro's. But if you are a techy hobbiest who loves a challenge, go for it! Be advised, it took months of fiddling with data settings, NEMA version settings, bus speeds etc. for me to integrate my electronics, Wind, Boatspeed, Dual Fluxgate Compass, GPS, VHF, PC Mapping Software. Some from different vendors. The server saved the day, making sure everything worked and played well together. And I have left the AP out of "The Loop" because of the added complexity the wiring, rudder position sensor, and tuning would have caused. That, and I like to steer myself!