On Sat, 14 Jul 2001, Michael Brutman wrote:
> I've never seen a light pen in use on any machine ... I've
> only seen them in pictures. How do these things work?
> Does anybody know how to obtain one or have information
> on how to build on?
Where were you during VCF? I sold and gave away a bunch of them,
including some Atari ones.
An over-simplified description (Tony can elaborate in more accurate
detail):
The lightpen itself is little more than a photocell with just enough
circuitry to clean up the signal and a switch to notice when you are
pushing the tip against something.
Although it LOOKS like the lightpen "writes on the screen", it is only
sensing what is happening on the screen.
The computer watches to see when the photocell "sees" light, either from
software turning on and off pixels (which is what we did on the TRS-80
when we made lightpens), or watching for the actual scanning beam.
When the computer hears from the lightpen that it sees the light, it knows
(I promised over-simplification!) where the light is at that instant, and
therefore, now knows where the lightpen is.
Once the computer knows where the pen is, it can use the switch and
context to do things such as make a menu selection, or light up pixels in
a drawing mode.
For example: for making menu selections, we put a group of boxes on the
screen, and flashed them on and off. It wasn't visibly obvious that they
were actually NOT flashing at the same time. When the software saw the
light, it would look up which box had been lit at that time.
For a drawing program (our TRS-80 software was too slow), whenever the
computer sees the light, AND the switch is on, it sets a pixel and leaves
it on.
We also did a bunch of other stupid stuff with it, such as sticking the
lightpen to the screen with a clear suction cup, and lighting a ring of
pixels around it, so that tilting the lightpen made a VERY crude joystick.
We taped the lightpen to the printhead of a printer and attempted to
scan. Later on, "Thunderscan"? did that much more successfully for
Apples.
The IBM CGA and MDA boards both have a 6 pin berg connector for
lightpen! With that, it isn't necessary to do the crude setting and
resetting of pixels, since it can provide a usable signal based on the
scan position.
Lightpens won't work with very long persistence phosphors. The IBM
monochrome monitor killed the market for lightpens, and the market never
came back, to such an extreme level that now people ask, "what was a
lightpen?"
--
Grumpy Ol' Fred cisin_at_xenosoft.com
Received on Sat Jul 14 2001 - 23:44:56 BST