The Raspberry Pi uses the serial port as a monitor, by default, so you will need to disable this so that you can use the serial port for the base station receiver.
There are a couple ways to do this and it differs a bit depending on what model Raspberry Pi you own.
Try the easiest method first:
1. Run the Raspberry Pi configuration tool to disable the serial port:
Select the Advanced Options menu option, followed by Serial menu option, then select Disable. Exit save and reboot.
2. Manually edit the inittab and cmdline.txt files. Follow the steps provided below, which include steps to make a backup of the files we will be editing in case you make a mistake or you want to revert back to the original files.
Note for Raspberry Pi Model 3:
The raspberry pi 3 has changed things around a bit: ttyAMA0 now refers to the serial port that is connected to the bluetooth. The old serial port is now called ttyS0. So if you have an RPI3, everywhere you see "ttyAMA0" below, you should read "ttyS0".
At the command prompt type: