connecting other namufacturers NMEA 2000 compliant devices to the Seatalk ng backbone

[Bill B]

I have the EV-1 autopilot with the P70R controller. I have connected a Garmin GPSMap 527xs to the Seatalk ng backbone. This GPS is NMEA 2000 compliant.
The Garmin GPS chartplotter recognizes all the Raymarine devices but the Raymarine P70R controller does not recognize the Garmin GPS.
I have read everything I can find about this problem but with no help. The configuration settings in the P70R for which data source to choose is not explained well in the instructions. It doesn’t matter which data source I choose. The error message appears that says “source selection not possible. Consult manual for more information”. But the manual does not help.