APM 2.6 USB Port as Serial Connection

I have a APM 2.6 and a Linux board. I would like to have them communicate using a serial connection over USB. I can communicate from APM 2.6 to Linux, but I cannot communicate from Linux to APM 2.6. Is this even possible? If so, what do I need to do to get the second direction working.


You can use MAVlink to communicate between them

Is the MAVlink protocol required?

I figured out what the issue was.
The APM 2.6 was missing crucial data packets while it was starting up.
The solution is to have the linux board wait for a startup complete message from the APM 2.6.