I use Dronekit Python program to connect to the Pixhawk, and I have downloaded the latest Ardusub in the Pixhawk. But the connection can be successful only once. And then I must unplug the micro-USB line and the battery and plug them again for another successful connection.
After one successful connection, the later connection will have problems:
>>> Link timeout, no heartbeat in last 5 seconds
>>> No heartbeat in 30 seconds, aborting.