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Connect GPSMap 520s NMEA output to OpenPlotter

I know this is quite a newbie question but i've tried everything and it's driving me mad.

I have a chart plotter (Gamin GPSMap 520s) with depth transducer, which is working just fine. Also, i have the NMEA0183 output connected to the VHF radio, which is working fine too (VHF radio gets gps coordinates).

On the other side, i have Rpi4+Moitessier HAT 2 with OpenPlotter, and i bought this RS-422/RS-485 to USB adapter:

From the VHF instruction manual, i can tell that red wire is NMEA+ and blue one is NMEA-. But i have no clue how to connect those to the USB adapter. I know that if i connect the red wire to pin 1 (RS485 A+) or 3 (RS422 RXD+), the RXD LED starts flashing, making me think that is "receiving something". I've tried connecting the blue wire to RXD- and GND. Now that i think about it, i might have not tried connecting to B-, but i don't know if i should be using 422 or 485 for this 2-wire connection.

So problem 1: Correct wiring

In OP's "Serial" app, the device is properly detected. So i have assigned an alias, and set the data as NMEA0183. Moved to Connections tab, add to SignalK, selected "Auto", and one.

Then in SignalK the device appears, but isn't sending any data. I've tried several baud rate options (though i'm not sure which one should i use to be honest) without success.

So problem 2: Correct configuration

Any help will be greatly appreciated, and please let me know if i can provide more details, pictures, or anything else. The end goal, as you can imagine is to have the same (and more) information in OP as I have in the chart plotter, while keeping it there because for now is the most stable solution, and the depth is the last "mandatory" data missing.

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.
Whoa that was fast, thanks! I will try the "cat" command tomorrow when back on the boat. Didn't know that was possible, and indeed will allow me to diagnose where the issue is.

As for wiring and polarity, i can check all the possible permutations, but i just found this table that might be useful (Never occured to me that + => B- and - => A+  Huh  ):
[Image: connections.PNG]

Regarding the baud rate, i'm not sure where to find it, as i can't find anything on Garmin's or VHF instruction manuals, but i will try with the most usual ones.

[Image: garmin_screenshot.PNG]

Icom VHF:

[Image: icom_screenshot.PNG]

Quote: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.

Nice tip, didn't think of it, will definitely try again tomorrow.

Thanks again, @abarrow
@abarrow Thanks to your tips I managed to make it work ^^
Watching at the stream with the "cat" command, i tried all possible wiring options (note: the good one was NMEA+ to A+ and NMEA- to GND), and adjusted the baud rate until i could see properly encoded NMEA messages (it was 4800 baud rate).
So all good! Now i have depth information distributed to all devices.

Outstanding! Great news that it worked!

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