How to free your Raspberry Pi serial port via raspberrypi-spy.co.uk:
By default Raspbian configures the Raspberry Pi serial port (GPIO14-GPIO15) to provide boot-up information. It also allows you to login via a connected device. If you need to use the Pi’s serial port for something else (i.e. a specific add-on board) you will need to disable this default functionality.
It’s something I needed to do when I played with the Ciseco Pi-Lite and luckily isn’t too difficult as the instructions below will hopefully show.
Read more and learn how to do it yourself here!