PPM SUM with other than 8 channels

I have tried to put a PPM-SUM signal with less than 8 channels directly to an APM 2.5.

The method chosen was that described in  the WIKI (http://code.google.com/p/arducopter/wiki/PPMsumRC):

  • Shorten input pins 2 & 3
  • Put PPM-SUM signal to input 1

The PPM-SUM signal is created by the HK-TR6A-V2 and has 6 channels.

The signal can now be measured on the PPM pad located in front of the ATMEGA 2560:

3690970190?profile=originalSeen on the oscilloscope:


Unfortunately the radio code (used ArduCopter-2.8.1.zip) does not recognize any PPM-SUM signal.

Searching for a solution I found this comment from John Arne Birkeland who tells: "If there is less then eight channels, the decoder will update those channels and leave the unused left-over ones at the default initialization values"

So following this statement, the 6 channel PPM-SUM signal should be decoded successfully.

But looking at the decoder code I found this definition in the APM_RC library (APM_RC.h):

#define NUM_CHANNELS 8

So my questions are:

  • did anybody got a working PPM-SUM configuration with other than 8 channels signal?
  • has the APM_RC library to be modified when using such a signal?

You need to be a member of diydrones to add comments!

Join diydrones

Email me when people reply –


  • Just to report that I was successful with connecting JETI Rsat2 to APM 2.5 (no modifications to ppm encoder) - the "input 1" approach as per wiki.

    PPM output on RX is set to 8ch. I would like to use more, but I have no idea how to access them in APM. Has anyone done this?

    The nice thing about Rsat2 is that it can be programmed it the same way as the standard receivers, therefore I was able to re-map the channel assignment so that my JR radio now works nicely with APM (Futaba assignment) even through PPM.

  • David, what RX do you use?

    I am trying to use either a Futaba 6208SB or FRSky TFR4SB which have sbus outputs.


    I can't get the Channel 1 solution to work regardless of flashing PPM decoder firmware to latest.


    Hesitant to cut/solder the board though so wanted to confirm its possible with my setup.

  • You got further than me on ch 1. I had to do the PCB jumper.

    Tried outputting 6 channels of PPM and it didn't work; neither did 7 but 8 worked OK. These were all done at 20ms frame rate.

    In answer to your first question the answer is no using a Jeti RSat2 receiver.
  • I use 8ch PPM sum OK with my APM2.5 board but not tried programming the receiver to only pass out 6ch. I didn't jumper those pins though. I use channel 5 and did the solder jumper on the the underside of the board as per the wiki. I saw posts from people who said the 2, 3 pin jumper didn't work.

This reply was deleted.