After the Telemetry mod of the Turnigy 9x, the air-side has to be able to communicate, too.
Because the APM is not able to speak the FrSky-Protocol itself, a converter is needed to translate the data. There are several projects out there, mostly using a teensy. I decided to use rotcehdnih’s version, which is also cabable of running on an arduino nano or micro.
This is quite simple, just burn that softwre to an arduino (I’m going with an arduino pro micro), connect RX to the TX of the telemetry port and pin 9 to the S.Port of the FrSky receiver. Then just add some power (from the APM Telemetry port for example) and there you go!