PixHawk and Mission Planner

I just swtiched from APM2.6 to pixhawk .
I managed to upload the firmware, did all the calibrations. It arms fine and will test fly later.
However … I can not connect to the PixHawk via USB or Radio when it is connected to the flight battery (through 3dr power module) once it is armed.
I can not connect via radio, as long as it is connected to the battery (armed or not).
Connecting via radio only works when it is powered via USB.

Did I miss something ?

Ok, answering myself.
Radio link not working when connected to battery is due a Y cable that I did for radio and OSD that is wrongly wired (It seems that I inverted Rx/Tx once too many time).
But the usb issue remains : I need to connect via USB BEFORE arming otherwise I can not connect. If I connect via USB before arming , then I stay connected after arming.