I have been having a tough time with my telemetry system in recent months. To be clear, this USED to work. I used to be able to connect from my mission planner to the APM over both USB cable and over telemetry radio. At some point, during the rest of my build out, the APM stopped arming.
I have troubleshot the problem down to a single circumstance, the connection of the 3dr radio using the 3dr telemetry Y splitter. Please see the attached video for full explanation.
Any suggestions or clues would be very welcome!