Light pens?

I can tell you how the C64 does it, and I'm sure the principle is broadly applicable. The VIC-II knows at any given point in time which pixel coordinate the raster beam is currently at. Within the light pen is a sensor which can pick up the presence of the beam. The screen is being updated 60 times per second (NTSC), so unless you get the pen off within that 1/60th of a second, at some point the raster beam will be within view of the light pen's sensor. When this happens, a triggering signal is sent to the VIC-II, which latches the current X/Y position of the beam (and therefore the pen) into its registers. Once read, the registers reset for another read. Since this all happens in literally a jiffy or two, there is a lot of jitter and so the C64 PRG recommends sampling and averaging over a few frames before using the generated coordinates. There can be additional switches present for more functions. The Inkwell Systems 170C that I use with my C128 has a simple single switch in the pen nib which is triggered when depressed, but the pen location can still be tracked with the pen held sufficiently close to the screen to see the beam, even without the switch activated. -- ---------------------------------- personal: http://www.armory.com/~spectre/ -- Cameron Kaiser, Floodgap Systems Ltd * So. Calif., USA * ckaiser at floodgap.com -- Whatever it is, I'm against it. -- Groucho Marx ----------------------------