Here's one for you network geeks:

My Standard Horizon (Yaesu) marine VHF radio communicates with my stand-alone GNSS unit via an NMEA 0183 network. Or, at least it used to until I swapped over to a more modern radio a few months back. Now it mostly doesn’t. There is power to the unit and the data bus is easily accessible for testing but I:

1) Don’t know what test equipment I’d need and

2) Probably wouldn’t know what I was looking for anyway.

NMEA 0183 is a simple serial bus that uses the EIA-422 electrical standard and all the data is sent in ASCII characters. Here's the Wikipedia article on the networking protocol: https://en.wikipedia.org/wiki/NMEA_0183

Any suggestions would be appreciated.