For everyone who wants to build a drone with a 3DR Pixhawk flight control:
The RX704 (devention, 12 channel) woks fine with the BUS-Output to a pixhawk.(You do not need the converter to PPM!)
I tested the RX703, but it did not work. It seems to have a different protocol.
(I had to wait about 50 days for delivery of my RX704, thanks to German customs/ German Post! So I'm finally glad to have it here!)
Have fun with Walkera!