Keep in mind that you`ll need the particular receiver to allow the mode to work....
ie: to run HRS you`ll need the HRS receiver with digital servos
.....to run PCM you`ll need the PCM receiver but theres no need for the digital servos......same again if you run a PPM receiver.
If youre running a Spektrum module, you`ll only be able to run it in PPM mode, unless of course you`ve managed to get one of the NEW HRS Spektrum sets.
Cheers