Ardupilot configuration

The two main wiring configurations are

ArduPilot sends native frsky passthrough telemetry data

The flight controller is configured to send native frsky passthrough telemetry data either with an inverting cable or without (pixracer). To enable this feature the SERIALn_PROTOCOL of the uart connected to the receiver has to be set to 10, check the ardupilot wiki for details.

This configuration requires a "special" cable that acts as logic level converter and inverter, an example of such a cable is here

For the pixracer an inverting cable is not needed but the wiring requires that the TX and RX pin of the frs port be connected together, check this image for further details.

ArduPilot sends mavlink telemetry data

The flight controller is configured to send mavlink messages and an external board (Teensy, Blue Pill,etc) is used to convert mavlink to frsky using Eric Stockenstrom MavlinkToPassthru firmware.

This is the default configuration for long range systems (Dragonlink, TBS Crossfire, ULRS to name a few) unable to carry native frsky telemetry but compatible with mavlink.