There are 3 different ways to connect your receiver to the Pixhawk, depending on your hardware.

SBUS or PPM receiver: If you have a SBUS or PPM receiver, as "X8R" or "D4R-II" from FrSky, you must plug it on the RC input of the Pixhawk.

Classic Receiver: If you have a conventional receiver you must go through a PPM encoder (up to 8 CHANNELS) and connect it to RC of the Pixhawk. Remember to activate the solder bridge on the encoder if you want to power your receiver from the Pixhawk.