I have the APM2.5 board with the power module and GPS connected. I am awaiting delivery of my telemetry system. I have Mission Planner 1.2.74 mav 1.0 installed.
I followed the instructions to install the firmware (ArduCopter V3.0.1 Quad) onto the APM and this is fine. Once the firmware was installed I tried to connect to the APM through USB. The connection times out and the error message says "only 1 heartbeat received". Please see the image for details of the error.
I have tried a few things to solve the problem after reading some other posts with similar issues but nothing works. I have tried:
1. Uninstalled the driver from the COM port and reinstalled
2. Used different USB ports
3. Used a different computer
4. Connected a battery to provide power (in case of brownout)
5. Changed USB cable
6. I have checked the COM port selected is correct and used the 115200 baud rate.
When I go into terminal and connect to APM it tells me:
load_all took 1848us
Arducopter V3.0.1] ?
When I type reboot I have a few system stats and then the line:
3Calibrating barometer Q///+?PANIC: AP_Baro::read unsuccessful for more than 500ms in AP_Baro::calibrate 
Any help will be much appreciated. Pulling my hair out at the minute!!