APM2 and Rx All-in-One!


As most of you may or may not know, I've been a big proponent of using PPM Sum (aka CPPM) with the Ardupilot since I first figured out how to make it work.  The simplicity of just running a single wire from the Rx to the APM is very attractive.  It eliminates 7 possible failure modes (losing the wire connecting channels 2-8) which could result in loss of control.  There's only 1 wire to get right, but also one wire to get wrong.  Fortunately, the APM can handle complete loss of signal well, and if you set your failsafe to handle complete signal loss, all is good.

I was using this on my HK450GT FBL helicopter, along with a FrSky D8R-SP Rx. Again, as anybody who knows me is aware, I'm a big fan of FrSky radio equipment.  It is affordable, and high performance offering true diversity on the Rx, easy to use CPPM (just requires a jumper on two pins), the Rx is very tolerant of low voltage (it'll keep going LONG after the APM would have shut down, down below 3V).  And rebooting and reacquiring the Tx signal takes less than 1 second.

All of these features are superior to the big name brands, Futaba, Spektrum, etc.

And the telemetry features offered by FrSky is just the icing on the cake.  RSSI (signal strength), Rx voltage, external voltage (flight battery), and even the option for a telemetry hub with all kinds of goodies This is somewhat negated by using an APM, but still, you can put one of these on your Tx and have all the info right at your fingertips.


Anyway, back to my little 450 heli with a HUGE Rx mounted on it, the D8R-SP.


I discovered that FrSky offer a new Rx, the D4Fr which is a small, yet full-range Rx.  And while it only has 4 channels out, it can be configured to do 8-channel CPPM with the simple addition of a jumper! Brilliant, and there's no reason why any modern compact receiver shouldn't be able to do this.  This Rx does not have as many telemetry features as it's big brother.  It can only report RSSI and Rx Voltage, it does not have connectors for the telemetry hub or external voltage.  But that's fine, as the APM can be used to report voltage anyway.  So I ordered one and it arrived Friday.


I started working on setting it up, when the wheels started turning in my brain.  The "case" is just a flimsy plastic thing, almost useless anyway.  And I'm only going to have a single cable going from the Rx to the APM, and it's so small.  And the APM2 has those stand-off pins onto which the daughterboard is soldered.  And those pins are kinda long...  And I've got this piece of empty prototype board sitting around...

Pretty soon I had de-cased the Rx and stripped all the input pins off the APM2.


I soldered the proto-board to the daughter board standoffs, and then glued the Rx board onto the proto-board.  I soldered jumpers on both the Rx and APM2 in order to set both into CPPM mode.  And then I soldered a jumper wire for the CPPM signal, power and ground.  Done.

The complete unit is compact.  There's no wires to get in the way of whatever vibration-damping mount you are using.  There are no wires to come disconnected.  I suspect this setup is at least an order of magnitude more reliable than a standard 8-channel APM2 setup.

Even better, it's compact, and easy to move between vehicles.

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  • Hi,
    Is my APM 2.5 broke? This blog seems knowledgeable on the CPPM, maybe you can help me out.
    I have an APM 2.0 which works fine including CPPM. The only way I could get it to work with CPPM was using the "APM 1.0 method". That is, jumper signal on Ch2-3 and input on Ch 1.
    Now, the problem I need some help on. I just received the APM 2.5 I ordered and applied the same method as described above but it doesn't work. So, I tried the APM 2 method described in the Wiki of input on Ch 5 with a "solder jumper" across the backside of the board ->PPM/<-Ch5. STILL DOESN'T WORK.
    Please keep in mind, EVERYTHING on the two APMs is IDENTICAL, same firmware and parameter load, same connections, etc., etc. It would seem my new APM 2.5 is defective or there was a small circuit change that affected the APM.

  • Note: CPPM channel can not handle all eight channels at the same time with all throws are maxed out, as it does not have enough frame gap. It is recommended to use at most six channels from CPPM channel while leaving off the rest two channels, otherwise improper performance might occur.


    Wow, that sounds bad.  But I haven't had any problem yet.  I assume the problem only occurs if you max out all 8 channels at once which is virtually impossible...

  • No, sorry, no problem and I've done this 3-4 times now.  What equipment are you using?

  • It should be noted that the FrSky now list the D4FR as discontinued and replaced by the D4FR-II. This Receiver and the two FASST compatible ones (TFR4 and TFR4-B), have the following warning about using CPPM


    Note: CPPM channel can not handle all eight channels at the same time with all throws are maxed out, as it does not have enough frame gap. It is recommended to use at most six channels from CPPM channel while leaving off the rest two channels, otherwise improper performance might occur.


  • Wow wonder if you can make it more compact,I would by one.
  • 3692467229?profile=originalDoes anyone see anything wrong with this? I'm trying to get my APM2 to work with a FrSky D4FR in cppm, I think I've tried every way I can find and nothing is working.

  • @Jani, thanks, i love simple answers.

    Even if i dont like the content ;-)

    Looking forward to your solution, keep up your good work :-)

  • Developer


    Simple answers.

    1) All APM2 serials are based on UART and they cannot be inverted

    2) you cannot. We have been talking this over on our devel group several times and with current system it's rather hard or impossible to connect them

    I am personally working on another type solution for this same issue and we will announce that in week or two after software is ready.


  • Hi together,

    i want to use a D4R-II together with APM2, PPM and RSSI are no problem. What i want to connect as well is 

    the serial port of RX and APM2 to send compressed GPS information over the FrSky data downlink to my antenna tracker. in principle i have that working, currently with an arduino pro mini evaluating an attached GPS sensor, compressing the GPS data and sending it over the downlink. So tweaking arducopters software to do that as well will be no big deal. I use NewSoftSerial with the arduino as it provides the inverted TTL signal, that is expected from the FrSky RX.

    Question now is:

    1) Can FastSerial in Arducopter 2.6 somehow be used for generating an INVERTED TTL signal?

    2) Which Pins/Port should i use on the APM2 to connect the FrSky RX serial port?



  • Hi Mohammed.

    Its a FR4 PCB board a base of Glassfibre with a 1oz (35micron) copper clading. Its available in copper on one side or both. The one in the picture is a double sided board. The thing worked out a little bigger than I would have wanted but it solved a few issues i.e. unsightly wiring, and the jumper wires seen on the legacy AP board are run as copper tracks on the board. thanks for taking an interest.


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