http://processing.org/reference/libraries/serial/Serial_write_.html


You will probably find problems if you try to send data to a printer and expect the printer to print out an image for you. You can, however, use machines that work with the http://nl.wikipedia.org/wiki/RS-232 protocol. It's quite easy to control these machines by sending commands of the serial port, using the serial library that is allready inside Processing.

I'm using Processing to control an old plotter. Works perfect because it has a simple protocol that's easy to understand.

Also there is Arduino. A very close to processing microcontroler. For much more than printers. Processing talks to Arduino and Arduino talks to almost anything :-)