Here are some things I encountered while setting up my Pixhawk 4 board with PX4 and QGroundControl that led me to suggest improvements/additions to the documentation. I’ll continue to add to this post as I continue to use PX4 and the Pixhawk 4 (I’ve only had everything for about 2 days as of writing this post on 10/20/2018).
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I couldn’t find any information about what the CAP_ADC ports are on the main board and the PDB. Judging from the name and the headers on the PDB, I though it was to connect analog sensors, which I still believe it’s partly for. Therefore, since I had no analog sensors, I didn’t connect it (even though it says to connect it; I’ll get into this later). After all the hardware was mounted and connected, I tried too many times to calibrate and recalibrate the sensors. However, I continually got all kinds of errors, including self-check fails for the accelerometer and gyro, and inconsistencies in the accelerometers, gyros, and magnetometers. I tried everything, and before looking up GetFPV’s return policy, I decided to connect that one wire. And all my problems were solved. Knowing the importance of this one connection would have saved me 14 hours of troubleshooting, and can save others’ time as well.
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This includes why I didn’t connect the wire mentioned above. This one puzzles me much more than the problem described in the above suggestion - despite the rather large oversight, I can kind of understand why only little information was provided about the connection mentioned above. I am running my Pixhawk on a flying wing, and while wiring the PWM from the main board to the PDB I looked at the Airframe Reference. I found that the signals for the servos and the ESC would connect to the MAIN outputs (those small circular pads that seem to be much more appropriate for multicopters). This seemed ridiculous to me, as I would have to cut off the 3-pin servo connectors on the ESC and both servos and then solder their signal wires to the MAIN outputs, while also having to solder the servos’ power and ground wires to either a 5V output on the PDB or to the ESC’s BEC, which would have made for very messy wiring. I then noticed that if I connected IO_PWM_OUT on the Pixhawk to the FMU_PWM_IN on the PDB, I could use that 3-pin connector preinstalled on the PDB, where the ESC’s servo connector could plug in, getting its signal, while also powering the two servos with its BEC (and the servos would also get their PWM singal through their pins). So I connected it this way and everything worked perfectly. Incidentally, I connected the wire mentioned in the first suggestion after all of this, so with the newly acquired mindset that “I could wire it how I want,” I decided not to connect it because, as mentioned above, I thought I didn’t need to. I firstly have no idea why flying wing PWM outputs are mapped that way, and secondly don’t understand why the documentation seems so strict about connecting everything exactly like it says. If the mapping doesn’t change, at lest the documentation should say that you can swap the FMU and IO PWM wires for convenience.