Light pens?

I have a "home made" light pen on my Altair. There was an article in one of the early BYTE magazines on how to do it. It is just a phototransistor stuck into a fat ballpoint pen body on a long wire. The circuitry is very simple -- you adjust a threshold at which the transistor triggers an processor interrupt. When you get the interrupt, you software reads the scan position of the monitor from a register in the video card. The Cromemco VDM-1 allows this. We had a very simple drawing program that read the pen position and toggled the background color of characters underneath. -Rob -----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]On Behalf Of Tony Duell Sent: Saturday, March 19, 2005 6:09 PM To: cctalk at classiccmp.org Subject: Re: Light pens? A light pen is beaically a small light detector (normally a phototransistor) that detects the ligth from the screen. Since the image on the screen is scanned (either as a raster, or as vectors), you get a pulse from the light pne output as the part of the image that the light pen is aimed at is being refreshed. Typcially, on raster scan systems (like almsot all microcomputers), the CRT controller takes the light pen signal and uses it to latch the current video RAM address, or the X/Y coordinates, or something like that. The latched values can then be read out by the processor. It can't. Period. Any comptuer/light pen that claims to is doing one of 2 things. Either displaying, say, a 3*3 array of pixels under the light pen, and the computer moves said pattern to keep the middle pixel under the light pen (this is how a lot of vector displays did it -- if the light pen posiition is 'lost' for any reason, it was typical to sweep a line across the screen, then down the screen, to find it again) Or more likely, the 'black' screen isn't totally black, but a very dark grey or something. Dark enough that _you_ don't notice it, but the light pen does -tony