So today, I have got the updated PPM encoder s/ware working to do RPM measurement on one input channel.
For an RPM range of about 300 - 2000, the output pulse to the AP CPU is ranging from 2mS to 0.5mS. The ACM code might need modding to accept this, without thinking its a sync pulse, but thats the next task. All this will need further calibration when I test it with the motor running.
The RPM sensor is an IR LED and IR photodiode, glued into holes drilled into the frame either side of the heli. One of the holes was used to mark the rotor shaft, which I then drilled through (offset about 2mm from center). This gives a very clean pulse, hopefully long enough to be detected at full RPM.
Above you can see the wires leading to the photodiode, just above the main gear.