There could be different reason as to why this might be happening:

  1. This could be caused if you haven’t installed the firmware to the autopilot. To install the firmware to the autopilot use a USB cable and follow the next procedure:
  2. Confirm you are using the proper cable according to your autopilot:
    • APM uses a 6 to 5 pins cable
    • Pixhawk uses 6 to 6 pins cable
  3. Verify you are selecting the serial port assigned to the telemetry radio and 57600 as is shown in the next pictures and hit connect:

Widows detect the port as COM# if using Mission Planner:
Captionless Image
Note: Mission Planner only works with Windows not with MAC.
 
Windows detect the port as COM# if using APM Planner: