The Processing serial library allows for easily reading and writing data to and from external machines. It allows two computers to send and receive data and gives you the flexibility to communicate with custom microcontroller devices, using them as the input or output to Processing programs.

The serial port is a nine pin I/O port that exists on many PCs and can be emulated through USB with the Keyspan USB Serial Adaptor and other compliant devices.

Note: Issues with the Serial Library and specific platforms are documented on the Processing Wiki.