In Mission Planner, I select Load Custom Firmware, and choose ArduCopter-v1-coax.px4. It then scans the comports and reports that it can't find the board. I then tried the Force PX4 Bootloader option beforehand, with the same result.
I read somewhere that QGroundControl should be used for PX4 files, but this doesn't find the board either. I've tried different USB cables - also no result.
Mission Planner seems very happy with the standard firmware options - I can load any of the standard options.
Is there another binary format that I should be using, or am I doing something wrong ?