As discussed in my last post, I installed a new APM2 PPM encoder firmware and intended to set up the failsafes on my APM2 and my Spektrum Dx8 transmitter.
Following the excellent guide you can find at DIY Drones (link here), I configured my Dx8 / AR8000 in exactly the same way.
Anyway - everything works, exactly as desired! When I turn off the radio (ie, the AR8000 failsafe triggers) the PWM signal to the APM2 drops to ~ 911us, which is short enough to trip the failsafe. When I pull the throttle line out of the APM2 (simulating a cable failure, which is what the new PPM encoder firmware protects against), the PWM throttle signal drops to ~900us, which is also low enough to trip the failsafe.
Thinking about it, there’s other failure modes possible - namely that you might lose other lines besides the throttle line. I guess the solution there would be to either flip your mode switch to RTL, or simply turn off your radio thus triggering the normal failsafe and an appropriate response.