My setup is:
uart - external GPS 38400 baud
USB0 - mavlink telemetry via 3DR radio 57400 baud
USB1 - mavlink telemetry via USB-TTL adaptor 57400 baud
my startup line is:
/home/pi/APMrover2.elf -A udp:192.168.1.182:14550 -C /dev/ttyUSB0 -D /dev/ttyUSB1 -E /dev/ttyAMA0
The GPS works fine but it comes in MP as the second GPS so it must be serial4 ???
Based on the forum -C is serial1 57600 default & protocol in MP set to mavlink
and -D is serial 2 57600 default & protocol in MP set to mavlink
If I plug the 3DR radio into any USB socket then on boot is works. So I assume that the first device is sees is assigned USB0.
If I only plug the USB-TTL adaptor into any socket it works after boot.
BUT !!! If I have both plugged in and then boot - only one works (random). I can see that ttyUSB0 and USB1 exist and they are assigned to the devices.
What is going on ??