Navio2 PPM Receiver Input with 6 channel PPM Receiver

I am building a rover with Navio2 (Ardurover 3.2) with RasPi 3 (Stretch) (using the 2017/09/22 Navio2 RasPi image)

I am trying to get RC input working with a Spektrum DX6i transmitter and OrangeRX R618XL 6Ch 2.4GHz DSM2/DSMX receiver. I have been able to bind them and control servos directly from the PWM channels on the receiver, so I know the transmitter/receiver pair work normally. Now I have connected the PPM from the receiver to the Navio2 PPM/SB, but I don’t read any RCInputs. The power module is connected, the receiver turns on with solid orange (binded, good signal) and even runs servos off PWM, but nothing is received on PPM. I don’t have an oscilloscope, but the PPM line reads about 2.9V, which changes slightly sending different stick values, so I have no reason to believe the receiver isn’t working properly. The servo rail is powered by one external 5V BEC.

cat /sys/kernel/rcio/status returns 1
cat /sys/kernel/rcio/init_ok returns 0
cat /sys/kernel/rcin/connected returns 0
Mission Planner shows all grey bars in Mandatory Hardware -> Radio Calibration (no signals on any channel)

Are there any parameters I need to change to only use 6 channels on PPM?

Hello!

Could you please cat /sys/kernel/rcin/ch2 when throttle stick is up and the pair is bound?

All /sys/kernel/rcio/rcin channels return 0, no matter where the sticks are. I know the receiver is working because I can run servos off it and they are controlled normally. Only PPM doesn’t seem to come through. I’m wondering if there’s a configuration or setting I might have missed, or something special for 6 channels vs. 8, or for my receiver vs. FrSky brand (are the number of channels and frequency detected automatically?)

My best guess for now is that there’s some timing incompatibility between the PPM decoder library used by Navio and the Spektrum/OrangeRX output PPM. Without buying a whole new transmitter, I’m hoping to find another DSMX receiver that is tested to work with Navio2. In the meantime, without an oscilloscope, I’ll try debugging with an Arduino decoder.

My problem may be related to: Spektrum DX6i -> Lemon RX DSMX -> PPM - Problems

UPDATE
The problem is not solved but I can confirm that my receiver is outputting a PPM signal, which I can decode using an arduino and the code from here.
I am reading 6 channels, with pulses ranging from ~1052us (min) to ~1900us (max) with ~1480us as midway (0-point.)
Now the question remains: why can’t the Navio2 decode this? It seems like if this is an issue with timing or # channels, I may need to alter APM and recompile from scratch…

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