2021-04-05, 04:53 PM
First thing to try is to open a terminal and see if the device sending readable data. The command is dependent on what you named the USB device, for example if your USB device is named "ttyOP_garmin", then you would type the command "cat /dev/ttyOP_garmin". You should see NMEA strings running down your screen. If not, wiring and polarity is the first thing to check.
The other thing is baud rate of the interface. Most of the time, NMEA0183 are 4800 or perhaps 38400 bps, but it is worth checking your Garmin manual to see what the rate is on that interface. Since you are already bringing that interface to your VHF, which is probably 4800bps, then you need to make sure that is the interface speed you have set in the OpenPlotter serial app. It sounds like you are on top of that one, but it's worth a check anyway.
One thing I've done to troubleshoot NMEA connections is to bring them into OpenCPN and use the NMEA debug tool there just to make sure there is readable data coming in.
The other thing is baud rate of the interface. Most of the time, NMEA0183 are 4800 or perhaps 38400 bps, but it is worth checking your Garmin manual to see what the rate is on that interface. Since you are already bringing that interface to your VHF, which is probably 4800bps, then you need to make sure that is the interface speed you have set in the OpenPlotter serial app. It sounds like you are on top of that one, but it's worth a check anyway.
One thing I've done to troubleshoot NMEA connections is to bring them into OpenCPN and use the NMEA debug tool there just to make sure there is readable data coming in.