The receiver sends back two analog inputs and any other rs232 input from the model to the transmitter, On the back of the transmitter module there is a connector with the rs232 output, positive and ground. The battery pack in the transmitter is 9,6 Volts.
Thanks for the tip, I will check the voltage output from the transmitter. I found PD0 and PD1 to be the Rx and Tx.
Still no clue where to start, any links are welcome.