FrSky inverted TTL APM2

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?



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  • This is how I am getting CPPM. Make sure you bind the receiver the normal way first then connect the jumpers and cables.


  • Ok, now i ran into a problem it thought would not happen....

    The CPPM Mode of neither D4R-II nor D8R-SP works with the Arducopter 2.6 Firmware on APM2....ARGH

    Tried both variants:

    1) Bridged signal input pins 2 + 3 and connected sum signal to chanel 1 input: Result NOTHING

    2) soldered the liddle bridge ppm->-->Ch5 and connected sum signal to Channel 5 Input on APM2: Result NOTHING.

    Any Ideas on that?

  • Im going to try this with my mini.. slightly different setup tho. I have been planning using the mini as an i2c interface port to the AMP1 for the sonar digital port. ie. sonar ->digital out -> Mini -> i2c -> APM.

    would be nice to have just the GPS data going to the RX - as its not fast enough for the whole kit and kaboodle in the mavlink data on the uart port.

    I wonder what the best way to get the data would be? listening to the GPS when it sends data to the APM would be the least intrusive and wouldnt involve any changes on the apm side. but is that possible with i2c? or would it tie up the mini too much waiting for data like that?

  • Very interesting topic as I wanted to get this working as well.

    Problem is I believe we don't have enough processing power left on the APM to run NewSoftSerial.

    I have heard however, that it can simply be inverted using hardware?  Simply use a transistor?  But I stopped there as I have too many project on the go and this started getting too complicated.

  • You don't want to be using oddball signals.  It's a bad idea.  

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