Mission Planner not detecting Garmin SONAR

Hello everyone! I’ve built an RC boat for use in bathymetry studies, and while things are mostly working correctly, I can’t seem to get Mission Planner to detect our sonar or log depths. For reference, we’re using a Cube Orange, and the sonar is a Garmin Intelliducer NMEA 0183.

Using the instructions in this thread and the Intelliducer manual (PDF), I connected the Intelliducer to the Cube’s TELEM2 port using an RS422 to TTL converter.

The Intelliducer’s gray wire (Tx+) is connected to the converter’s “B” input, and its white/red wire (Tx-) is connected to the converter’s “A” input. Its red wire (power) is connected to a positive terminal on my PDB, and its black & orange wires (ground & accessory) are connected to the associated negative terminal. The converter’s TTL side is connected to the TELEM2 pins as described in the linked post.

I did at one point try swapping the Tx+ and Tx- connections because Garmin labels them in the PDF as “A” and “B” respectively, which is the opposite of the labels on the RS422 side of the converter. But this didn’t get me anywhere.

I set my parameters as described in the linked post, with the exception of SERIAL_2_BAUD, which I set to 4. As far as I can tell, this is the rate used by the Intelliducer. I did try 38 as well, but there was no change.

When I check the Status panel in Mission Planner’s Data tab, “rangefinder1”, “sonarrange”, and “sonarvoltage” all have a value of 0. We’re also not seeing anything in our logs that looks like depth readings. Finally, in “Setup > Optional Hardware > Range Finder”, there is a photo of a rangefinder module and a dropdown menu, but it’s disabled and I can’t interact with anything.

I’d really appreciate any troubleshooting tips, as this is the final blocker preventing us from starting bathymetry missions. Thank you!

Have any photos or videos of the boat in action?