AP 3.4: Pixhawk only accepts 12 channels from CPPM but 16 from SUMD via Spektrum satellite port

[marcmerlin]

With openlrs, if I output CPPM, pixhawk only receives 12 of the 16 channels (as seen in chanxx_raw in status)
If I output SUMD and wire it to go into the satellite receiver port, I see all 16 channels from my openlrs receiver in chanxx_raw.

Only problem is that using SUMD prevents me from using RX/TX on my receiver (i.e. I can have SUMD or RX/TX, while I can have RX/TX and CPPM), so I'd like to go back to CPPM.

Can AP be improved to receive all 16 channels from a CPPM stream, and not just 12?

[marcmerlin]

I've been reading the code
void RCInput::_process_ppmsum_pulse(uint16_t width_usec)
in ./libraries/AP_HAL_Linux/RCInput.cpp
The PPM-sum code looks generic enough, it'll read channels until it hits a pulse of 2700 (not possible as a servo reading), or
ppm_state._channel_counter >= LINUX_RC_INPUT_NUM_CHANNELS
and that's
./libraries/AP_HAL_Linux/RCInput.h:#define LINUX_RC_INPUT_NUM_CHANNELS 16

So I find no obvious reason why it would be limited to 12 channels.

[magicrub]

@marc,
You're looking at ./libraries/AP_HAL_Linux/RCInput.cpp so does that mean
you're running on linux? I thought you were on Pixhawk.

-TomP

On Mon, Dec 14, 2015 at 8:47 AM, Marc MERLIN notifications@github.com
wrote:

I've been reading the code
void RCInput::_process_ppmsum_pulse(uint16_t width_usec)
in ./libraries/AP_HAL_Linux/RCInput.cpp
The PPM-sum code looks generic enough, it'll read channels until it hits a
pulse of 2700 (not possible as a servo reading), or
ppm_state._channel_counter >= LINUX_RC_INPUT_NUM_CHANNELS
and that's
./libraries/AP_HAL_Linux/RCInput.h:#define LINUX_RC_INPUT_NUM_CHANNELS 16

So I find no obvious reason why it would be limited to 12 channels.

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[marcmerlin]

@magicrub oops, you are correct, I was grepping in the wrong place.
./libraries/AP_HAL_PX4/RCInput.h:#define RC_INPUT_MAX_CHANNELS 18

Is PX4 using ./Tools/ArduPPM/ (even though it's not arduino based?)

./libraries/AP_HAL_PX4/RCInput.cpp doesn't have anything relating to PPM.

[marcmerlin]

@tridge this is the 12 channel limit for PPMSum I noticed (but that isn't there for SBUS or SUMD)

[marcmerlin]

Another person with the same problem:
http://discuss.ardupilot.org/t/how-many-channels-supported-on-ppm-rc-input/18133

[marcmerlin]

https://www.rcgroups.com/forums/showpost.php?p=37662843&postcount=10861
claims that 16 channel CPPM works on Rover, but everyone agrees that no more than 12 channels work on AP.