It would be a way of getting data from your PI to your Chartplotter. If you have another NMEA input, that's what you'd use, with an RS-422 or RS-485 interface converter plugged into your PI, just wired in to the receive on your Chartplotter.
A few things you can try:
If you don't have a spare NMEA input on your C80, You could buy an NMEA/Seatalk converter and plug the NMEA side into your PI, and the Seatalk into your Seatalk bus.
Another thing worth trying: A lot of the old Raymarine remote displays, Autohelm controls, etc. , like ST-50 / ST-60 etc have an NMEA0183 output (only). On your C80 it is both input and output, but the displays are output only. You may be able to free up an NMEA input/output on your Chartplotter by moving your NMEA feed into your PI to one of those displays. Then you'll have a free NMEA input to your chartplotter from the PI. It's worth a look and it won't cost you anything. If you look at the back of the display, you'll see wires in and out, and then you'll see another two-pin plug at the top of the little triangle. That's NMEA output.
Lastly, you might try is to use your existing RS-422/USB converter as both input and output. Just bring both RX and TX (and ground) from your C80, and reconfigure the port in OpenPlotter as BOTH. You can set up filters in OpenPlotter so that only the strings you want are received by the C80. I've never tried this, but I don't see why it wouldn't work. Others who have configured BOTH ports with NMEA might wish to give their advice on this option.
A few things you can try:
If you don't have a spare NMEA input on your C80, You could buy an NMEA/Seatalk converter and plug the NMEA side into your PI, and the Seatalk into your Seatalk bus.
Another thing worth trying: A lot of the old Raymarine remote displays, Autohelm controls, etc. , like ST-50 / ST-60 etc have an NMEA0183 output (only). On your C80 it is both input and output, but the displays are output only. You may be able to free up an NMEA input/output on your Chartplotter by moving your NMEA feed into your PI to one of those displays. Then you'll have a free NMEA input to your chartplotter from the PI. It's worth a look and it won't cost you anything. If you look at the back of the display, you'll see wires in and out, and then you'll see another two-pin plug at the top of the little triangle. That's NMEA output.
Lastly, you might try is to use your existing RS-422/USB converter as both input and output. Just bring both RX and TX (and ground) from your C80, and reconfigure the port in OpenPlotter as BOTH. You can set up filters in OpenPlotter so that only the strings you want are received by the C80. I've never tried this, but I don't see why it wouldn't work. Others who have configured BOTH ports with NMEA might wish to give their advice on this option.