Hello
(i’m french)
I have a Flysky fs-i6 with de i6ab receiver (with PPM output)
the receiver PPM output is enabled but not visible in PX4 or APM firmware :’(
uncompatible ?
The flysky 16AB in PPM is fully Pixhawk compatible
You have turned PPM on in the receiver setup on the transmitter, its turned off by default
Yes , PPM is turned on 
Resolved !
My pcb is too old.
So ppm work only with arducopter <= 3.1.5