I made some test with OEM serial modem that are MAVlink unaware. Everything was fine except when I enable joystick in Mission Planner it starts constantly drop packets. I checked modems media and protocol level and find no packet drops, so packets are rejected by mission planner. I had 115200 baud on serial ports on both sides and 200+kbps air rate. Mission planner shown continuous 1.8 KBps download and 450 Bps upload. Hardware flow control had no effect.
What am I doing wrong? Where to dig?