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?
If by level you mean voltage, they are not important in this case.
If you mean logic, i.e. reversal of high - low voltage values for logical 0/1, then these are already reversed in jD-IO.
So, you don't need a max !
Frankly, I dont know exactly what they mean by level. But I do know you can't talk to an RS232 port by connecting TTL Tx/Rx pins to it. I don't know the exact details, but my understanding is that voltage is different and LOW/HIGH in RS232 are negative and positive voltage, where TTL is 0 and either +3.3 or +5v.
For what its worth, I connected up an Arduino UNO, with its pin 0/1 (tx/rx) to the telemetry tx/rx pins coming from my APM 2.5 and pin 5 up to my FRSky D8R-XP's 'RX' port, along with its GND ports connected from APM telemetry, to UNO and to D8R-XP GND, and powered the Uno from the APM telem +5v. I then loaded the jD_IOBoard_FrSkyMAVLink sketch onto the Uno (pulled from jdrones google code svn repo).
I have a FLD-02 LCD connected to the back of my DJT module and never got any telemetry other than the stock RSSI that shows up from the receiver.
I tried switching Uno<->APM tx/rx pins just in case something was backwards. I did enable some of the debugging #defines in the code and opened serial monitor at speed 57600 to make sure the sketch was actually running, and it was, because I saw all the EEPROM debug messages.
I was pretty sure this setup would have replicated what the jD-IOBoard was doing, but perhaps I missed something. I am hoping my real jD-IOBoard actually works when it gets here :)