I am also facing issues to get my Garmin 18 HD running with OpenCPN
Here are the steps I completed:
1. Install the Amphenol switch by soldering pin 5 with chassis ground (see picture below)
2. Configure laptop
IP address to 172.16.2.1, with subnet mask 255.255.240.0
3. Add in OpenCPN a TCP connection to address: 172.16.2.0, port 24
4. Enable Master Mode
In the gradar_pi.log, I get the message that scanner at 172.16.2.0 is accessible. However, the GRadar icon is inactive and I can't see anything on the screen.
In a first place, I would need a confirmation that I did the right connections on the Amphenol component: The picture posted in this post does not match the Amphenol document in regard to the position of pin 5 (see pdf file).
Thanks in advance for your help