Operation of Failsafe using a PPM Sum Receiver
If the APM is connected to a PPM Sum receiver in pass-through mode (APM input pins 2&3 shorted) the APM’s PPM encoder operates transparently.
There is no independent channel analysis and there is no channel manipulation.
If the receiver is disconnected or the PPM signal is cut off, after a 250 ms timeout the PPM encoder watchdog interrupt will start an 8 channel PPM-PWM generator with the following fail-safe values :
CH1, CH2, CH4, CH6, CH7, CH8 = 1500
CH3 = 900
CH5 = 1555 (flight mode 4)
If an input level change is again detected from the PPM receiver the fail-safe output is terminated and normal PPM pass-through operation is restored.
Alternatively, there is a provision on the APM 2.5 board to fully bypass the PPM encoder when using a PPM-Sum receiver..
The PPM signal is connected to APM 2.5 board’s input CH5 (instead of CH1 for pass through).
Also, a pad needs to be modified on the APM 2.5 board and soldering is required.