After several though I decided to change the old APM 2.5 for a brand new PIXHAWK. The telemetry set I had was a pair of Xbee pro S1 , Sparkfun xbee explored regulated V14 (https://www.sparkfun.com/products/11373), and the usb adapter also from sparkfun.
However, when I changed to PIXHAK the telemetry set up did not work at all, but I had an old sparkfun xbee explored regulated v13 (https://www.sparkfun.com/products/retired/9132) and did the trick. I bought another PIXHAWK and unfortunately, I am out of xbee explored regulated v13.
I did a search to get the shchematics to find out the difference between V14 and V13, it seem that the old version connected directly the DIN and DOU pin to the xbee corresponding pinout, while the V14 release uses a transistor in the middle, I guess is to warrant 3.3v at the xbee in .
Another difference I find is that the UART from the APM Megaseem to be a 5V TTL, while the UART in the pixhawk is be 3.3v. I believe this is the reason why the V13 work and the V14 does not. It seems that a voltage lower than necessary arrives to the Xbee in/rx pin in the V14. Sadly I have not an oscilloscope to confirm this.
Have any of you had this situation?
I also read that 3DR suggest to use the Xbee pro S2b with the sparkfun xbee explorer V14.
I really need help. I am thinking on hack the sparkfun xbee explorer V14 to be similar to the V13.
If I got u right , when you use Xbee s1 with sparkfun regulated board v13, it worked like a charm?
To solve this issue I ended up using this adapters
You can also try to get the sparkfun V1, is is something with the working voltage between Pixhawk and the module fro xbees.