Check the version of NMEA that the
Garmin is using. You should be able to chose between 2.0 and 1.5. If you look at the "Words" that the
Garmin uses, they have better communications with the older level (1.5) than the newer level (2.0) You also need to look at the "Words" that the ICOM 502 can read.
By switching the NMEA on my
Garmin to 1.5, I was able to get my
GPS 128 to talk to my NEXUS network and my ICOM 502.