Hi Nick: What I have to say applies to my 2001 vintage B & G 2000 FFD (full function display), B & G Hydra 2000 processor, and Raymarine ST7000+ autopilot. I am going from my Cadd drawings from 18 years ago. NOTE: There evidently are NMEA FFDs and non-NMEA FFDs
There is a nine conductor cable that runs from the NMEA FFD to the Hydra processor. In my case the NMEA wires were NOT connected to anything at the Hydra processor. Amel had hooked up the supplied Furuno GPS to the Hydra NMEA processor Inputs, but there were no other NMEA inputs connected.
In that 9 conductor cable from the NMEA FFD the violet (NMEA Output Signal) and black (Ground) wires are the NMEA OUT from the FFD, and the brown (NMEA Input Signal) and blue (NMEA Input Return) are the NMEA input TO the FFD. My confusion at the time was because there are NMEA terminals on the Hydra processor but they are hooked up to the Amel installed Furuno GPS only. I spoke to Pochon (Amel's electronics sub-contractor) shortly after the the time we took delivery of the boat and they instructed me to hook things up the way I did, that being, splicing wires to the violet, black, brown and blue wires and running the NMEA out and NMEA in (from the FFD) to the auto-pilot computer. I do have a note that the NMEA shields should ONLY BE GROUNDED AT THE TRANSMITTING END OF THE CABLE.
Based on aviation ARINC 429 data bus (similar to NMEA data bus) NMEA data wires can be paralleled, so I think in retrospect it should have been possible to have those NMEA wires from the FFD go to the pins 21/22 and 26/27 of the Hydra, I am not sure why Amel didn't hook it up that way.
In my case I ran two, 2-conductor twisted pair shielded aviation grade tefzel wires from the Hydra and ran them to the autopilot computer NMEA input and output lugs. It has been 18 years since I did this but I vaguely remember that there was some sort of setting in the FFD programming or perhaps it was at the autopilot, that had to be set (something like a node designation) or something.
Joel at the time told me, and my experience has born this out, that winds can change too dramatically to allow the auto-pilot to steer to the wind (a squall comes, the boat jibes and damage is done). So I only display the relative wind at the helm and use the heading mode of the Auto Pilot to actually do the steering.
I hope this gives you some clues and doesn't further muddy the waters.
Gary S. Silver
Amel SM 2000 #335 (2001)