When everything is connected and we start QGC, we get the video from the companion without issues, but QGC keeps saying “Disconnected” in the top left corner and the Pixhawk also can’t be reached via mavlink or QGC’s Firmware Setup page.
As a troubleshooting step, we tried to just connect the Pixhawk to the power to start it up without the periphery and see if anything else might be causing the problem, but it behaved exactly the same. The voltage coming out of the power connector is 5.13 V.
Here is a video of the behaviour on start-up:
Note the big LED is quickly changing between amber and a light purple colour, but FMU and IO seem to be fine.
And this is the sound it makes, a bit like a fire alarm:
In the documentation for ArduPilot/Copter I found a page on the meaning of several LED combinations and sounds. However, none of these seem to apply to what we are observing. What could be wrong?
Please try going through our troubleshooting steps, and if they don’t manage to fix the connection issue then please let us know the results from each step
ArduSub by default expects a Leak Sensor to be connected to AUX port 6. That leak sensor is effectively a ‘normally closed’ switch, which becomes ‘open’ when one of the probes comes into contact with water, so if it’s not plugged in ArduSub thinks there’s a leak and engages the Leak Failsafe.
If you don’t have a leak sensor then you should be able to disable the leak sensor failsafe in QGC on the Safety Setup Page. If it’s annoying while you’re trying to get the connection working via Companion, plug the Pixhawk directly into the topside computer and turn off the failsafe before reconnecting it to Companion and continuing the troubleshooting steps
While the ArduPilot docs match our behaviour for the most part, we don’t yet have proper documentation for how the various LEDs/sounds work with ArduSub. It is something that’s in the pipeline, but we unfortunately haven’t yet had the time to complete it.
Thank you very much for your answer, helpful as always
The issue was indeed the Leak Sensor setting - so that was what the LED and sound were trying to tell us! With the help of a colleague, I eventually managed to disable this failsafe via QGC. The trick here was to first start QGC and then connect the Pixhawk directly via USB and it seems I had always done it the other way round. Our Pixhawk also still had an old version of ArduSub (3.5.4) installed and I was able to update this through QGC without any issues.