You might not believe it, but I actually have a light pen on my Altair 8800. There was an article in one of the old hobbyist magazines how to do it, and we did it. (my little brother and I) We have a Processor Technology VDM-1 "video display module" in the Altair. This was a very popular add-on module. The VDM-1 allows software to read the current horizontal and vertical sync registers. To make a light pen, we simply put a phototransistor in the end of an empty pen and strung some wires from it to a prototyping card in the Altair. We have a simple circuit that triggers an interrupt whenever a certain threshold is hit The interupt processing software then reads the current horizontal and vertical scan positions from the VDM-1 card. The VDM-1 only has character-level black&white graphics, but we could still draw on the screen by reversing the background of the character blocks. We had never heard of the "mouse" when we did this, so it seemed really high-tech and was quite fun. The mouse is better though :-) -Rob -----Original Message----- From: McFadden, Mike [SMTP:mmcfadden@cmh.edu] Sent: Monday, July 16, 2001 11:40 AM To: 'classiccmp(a)classiccmp.org' Subject: RE: Subject: Light Pen for PCjr or PC? 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