Can not get a char from apm telemitry port,using arduino nan

I am trying to modify the minimOSD extra code to show some mav data on a arduino LCD,
Minimosd is based on arduino nano so i just block all
the code for max7456 hardware.
But i tried days but can not read 1 char from the apm telemitry port, should be serial
0 right, to find the problem, i even wrote lines code setup arduino nano serial0 to 57600 ,for hardware i connect arduino rx0 to apm telemitry port tx0,then i can see arduino rx led flash once 1 second which i believe is apm heartbeat msg, but why my code can not read 1 char available from Serial.available()???
I wrote simple code for another piece of arduino nano to send char from serial0 57600 and connect the 2 , first nano can immediatelly receive char…
can someone know what would wrong with the code or hardware wiring?