In this tutorial we will see how to use the serial port on Raspberry Pi. We will use the serial port available on Raspberry with a RS232/TTL 3-5,5V adapter and a USB-serial adapter.
By default the Raspberry Pi’s serial port is configured to be used for console input/output. This can help to fix problems during boot, or to log in to the Pi if the video and network are not available. 

To be able to use the serial port to connect and talk to other devices (e.g. a modem a printer.. ), the serial port console login needs to be disabled.