As I understand it the light pen detects the electron beam on the CRT display as it passes the end of the pen and can decode from the timing where the pen is located. Somewhere I have a light pen kit that was used to emulate certain types of IBM terminals. Many hospitals used light pens to move through the menus for patient registration and order entry. Pointer to some more information http://www.ftgdata.com/products/lightpennav.html Mike mmcfadden(a)cmh.edu