Serial Passthrough

“Serial Passthrough” allows the flight controller to route serial protocol messages between serial ports on the flight controller including the USB port. This can be used to configure a sensor (which has a serial interface) without disconnecting it from the flight controller.


This feature is available in Copter-3.7 (and higher), Plane-3.10, Rover-3.6 (and higher)


  • The sensor (with a serial interface) should already be connected to one of the flight controller’s serial ports. Ideally it should already be working
  • Connect the flight controller to a PC using a USB cable and connect with a Ground Station (i.e. Mission Planner, QGC, etc)
  • Set SERIAL_PASSTIMO to a lenght of time (in seconds) that gives you enough time to connect with the sensor’s configuration software. 30 to 60 seconds is a good choice
  • Set SERIAL_PASS2 to the number of the serial port connected to the sensor. I.e. “2” if the sensor is connected to Telem2/Serial2
  • Press the “Disconnect” button on the ground station but leave the USB cable from the PC to the flight controller connected.
  • Open the sensor’s configuration software and connect to the flight controller’s COM port. If all goes well the configuration software should work as it does when the PC is directly connected to the sensor

If the configuration fails to connect there are some things to try:

  • Some configuration software will not allow connecting to the flight controller’s COM port by default but may have a option to display all available COM ports
  • If no serial messages are received from the PC the timeout will expire and SERIAL_PASS2 will revert to -1