How to connect a Standard Horizon GX2100 AIS VHF to a Raymarine RL-70C plotter

[GNU]

I've spent quite some time troubleshooting this issue together with a very helpful chap from Horizon's support department (Devin) and thought I share our findings with RL70 and RL80 owners.

What I wanted to do:
Get an NMEA0183 signal from my RL70's NMEA out port to feed GPS info to the Standard Horizon AIS capable VHF.

What I tried to do:
Connect the NMEA cables to the VHF following the instructions in the VHF manual.

What was the result?
Well not much would be an overstatement. The VHF didn't even indicate a fault signal (it would do that by flashing a graphical representation of a satellite in the upper right corner of the display)

After days of research and troubleshooting it turns out that my plotter had an outdated firmware (2.09) and only supports NMEA0183 version 1.0 (The VHF requires 2.0 or later) This by itself was quite tricky to find out since Raymarine's "support" couldn't tell me what's been updated in later versions (V4.x is the latest version). All they could tell me was that they no longer support memory card updating and I had to disconnect my unit and send it to them for an update that might fix the issue (this would take weeks according to them (sigh))

So, rather than dealing with Raymarine I read up on the difference between NMEA 1.0 and 2.0.
Turns out the differences includes changes to the voltage levels and tge way the signal ground work.
After hours of thinking and testing we found that there were two potential fixes.
1. Buy a Raymarine Raytech NMEA box for $150+tax+shipping.
2. Alter the NMEA output signal levels slightly and disconnect the signal ground.

Summary:
We went with option 2.
Here's what we did:

1. Connect NMEA+ from the plotter to NMEA in (blue cable) on the VHF.
2. Connect NMEA- from the plotter to the NMEA in on the VFH via an 8k Ohm resistor ($1 part at the closest Electronic part store / Radio shack etc).
3. Isolate the NMEA ground (green cable) from the VHF and leave it disconnected.
4. On the "NMEA out" menu option in the RL70, select RMC (you can de-select the other options if you don't have another NMEA consumer.

That's it. Your GX2100 will now be able to read the GPS location from your plotter!

PS. The RL-70 doesn't support an AIS overlay so you'll have to connect the VHF NMEA out to another device (Seaclear on a PC?) if you want that.