I bought the 915MHz 3DR radios and I am having the same problem. The ground and air radios link together and I can run mission planner 1.0 with the the APM2 using the USB cable but when I disconnect the USB cable and connect the telemetry cable to the telemetry port and try to connect to the APM2 with the radio link through mission planner the countdown timer goes the whole way to o and times out with no heartbeat packet received. I have checked the serial port baud rate in APM2 and it matches the radios. Don't know what else to do. Have re-flashed both radio units and the settings match but no luck getting the telemetry info through the radios back to the PC. Everything works OK through the USB cable. I will have to send the radios and possibly the APM2 back to 3d robotics if I cannot resolve this issue. I am running windows 7 professional 64 bit software . Running the radios at 57k and APM2 serial baud rate at 57k.
Check the serial communication between the two.
3DR really needs to put together a basic diagnostic guide. Seems like there's several posts a day from people having problems and wanting help, but who haven't even checked if the units are working.
I have found the problem. It relies with the APM2 board. No matter what baud rate I set the APM2 for telemetry using the usb cable and mission planner even when I verify that the eeprom setting has been updated to the value and then when I disconnect the usb cable from the APM2 and connect the 3d robotics radio into the uart the APM2 keeps the baud rate at 115k which is the default for the APM2 when using the USB cable for communications. I can program the APM2 baud rate to any other setting...verify that it went into eeprom...but when the usb cable is unconnected and the raidio is then connected it doesn't switch to the baud rate programmed into eeprom...therefore it won't communicate with the radio which is set at 57k. My temporary solution was to program the radios to 115k and it will connect and display telemetry info in mission planner. I will have to send the defective APM2 back to 3d robotics.