In order to save pins on the other modules i decided to do a popular modification to my RC receiver. My receiver only has 4 channels (well, kind of - there are 6 wired channels but the TX broadcasts only 4).
So the theory of operation is that the receiver decodes an FM stream into PPM (pulse position modulation) which is basically a way to multiplex several PWM (pulse width modulation, this is what servos act upon) channels (aileron, elevators, etc).
In order to save pins, chose the easier path of finding the PPM trace on the receiver PCB that goes into the PPM->PWM demultiplexer. Then i would cut the trace and solder one wire on each end. This way the new setup is:
RX->PPM START ->WIRE1 | WIRE2 -> PPM to PWM -> servo connectors
This enables me to route WIRE1 into a flightdirector GPIO to decode the code in software and store the channel values internally and also to route WIRE2 into another flightdirector GPIO to send a software encoded PPM into the demultiplexer so i don't need one GPIO per servo. The setup also has the advantage of allowing me to control (via groundstation) which signal i want to route to the WIRE2, either the RC Tx stream (WIRE1 in RC Manual Mode) or a control packet converted to PPM (flightdirector in either one of the assisted/auto modes).
The most challenging part of this mod is finding the correct wire on your receiver. I have some tips. First of all, a list of requirements:
1 - Oscilloscope
2 - RC Transmitter turned on (necessary for me. the rx would not create ppm until the tx was on)
3 - Patience
Unlike what you may think, the first requirement is already part of your setup. I downloaded a Mac sound analyser with oscilloscope and spectrum analyser (PPM leaves a signature after a FFT) and simply connected an audio cable (minijack) to the line-in port (note that some machines do not have line-inputs, only high impedance microphone inputs. If that's the case then keep the recording volume low) and voila.
Now some Rx units have 2 mcus others only one. Mine had two. The easiest way of figuring out which is the PPM decoder simply follow the servo connector traces until they connect to one of the mcus. Then you need to figure out which pin has the PPM input. If there are markings on the mcu, try and find a datasheet on the net for a pinout. Again, mine didn't have markings, so i just ruled out the servo traces and tried all others. And at last, voila :)
Now that's a clear PPM signal. And what a surprise too. Looks like my transmitter isn't a 4 channel after all :) Maybe i can mod it and add a manual/auto switch on the extra channel but that's a whole new entry.
So now that i figured out which pin i wanted, i used an x-acto knife to cut the trace before the mcu and the rest you already know.
You may want to check the level of the PPM signal before you send it to a microcontroller. On my setup there's no problem as the entire electronics (except servo voltage) is 3.3v which is the same as the flightdirector.