##(A good deal of related and helpful information for noobs like myself learning how NMEA devices are connected surfaced at this thread on making a gps puck feed data to an old Standard Horizon vhf radio: http://forum.openmarine.net/showthread.php?tid=1815 )
My Tartan 37 has a working Raymarine 3000 which used to be driven through a Raymarine Z075 Radio Navigation Interface by a very old external GPS, now dead. This autohelm works fine with its internal compass in the controller unit.
I've recently built OpenPlotter into an RPi3B+ with a dAISy hat. It's set up to run headless via wifi with VNC, or by sharing data over Kplex. Works great with OpenCPN, and iNavx. Now I'd like to try using OpenCPN to drive the autohelm through that Z075 radio nav interface to track a course.
It's difficult to find tech data about that interface unit. I've read that it uses either or both of NMEA0180 and NMEA0183 sentences, and I've also read (but not confirmed) that it uses the XTR sentence (not XTE) to command a left or right turn to the helm motor. The interface has a gain selector control which is set according to boat speed. I've not used this unit, but presume it would work if it receives the x-track error information in a form it can understand.
I understand that XTR x-track is a "dead reckoning" measure, while XTE is a "measured" x-track error. I'm guessing that XTR has been deprecated in favor of XTE, but I don't know if one would be the working equivalent of the other.
I've read extensively about the Pypilot project, and eventually would like to build or buy a TinyPilot unit to use to run my helm motor. But first, I'd like to know if it is possible to make OpenCPN running in my Rpi unit under OpenPlotter generate the correct NMEA sentences to drive the Raymarine Z075 radio interface, to track a course. I'm thinking that my existing 3000 motor controller (and the spare that came with the boat) would do fine service if I could figure out how to feed the turn commands to it through the Raymarine interface.
Has anyone tried this? I'd deeply appreciate any insights to using my existing hardware, at least for the time being. Thanks in advance for whatever anyone shares on this topic!