• Please review our updated Terms and Rules here

Light pens for IBM PC?

SomeGuy

Veteran Member
Joined
Jan 2, 2013
Messages
4,434
Location
Marietta, GA
As you know, IBM CGA cards and many clones have a port to attach a light pen. However, I think I have never seen an IBM/CGA specific light pen on eBay.

While they were obviously not popular, it makes me wonder if they were really that rare?

I've also heard of people building their own. Is there a general howto with parts suggestions anwhere?

Could these be rewired for CGA?
http://www.ebay.com/itm/Light-Pen-f...faultDomain_0&hash=item35e9c1c0aa#ht_86wt_864
Does any one know the pinouts for this 3278 type of light pen?

My CDP 1600 has a CGA light pen port, and found one program that can make use of light pens. It would be interesting to try this out.
 
Rare and expensive. I have seen ads where the manufacturer crowed about selling 7,500 light pens.

The May 14, 1984 issue of InfoWorld has an article covering light pens for IBM PC on page 43-44. Also covers some of the problems with IBM's implementation which makes light pens less than useful.

I know there were a lot of build your own light pen articles for systems other than the IBM PC and related. I expect similar articles for the IBM PC might be out there.
 
I have an almost working light pen on my PCjr. It was professionally made for another system and then modified. There were plenty of "make your own lightpen" articles in the magazines back 20 years ago.

My understanding of light pens is that a phosphor has to be lit for the light pen to register a hit. So it is useful for selecting an area of the screen that is lit up at the moment, but it is not useful for drawing on a part of the screen where the screen is not lit. You could work around this by having a light background to draw on.

I've also found the accuracy to be sorely lacking, although that might be a problem with my specific pen.
 
I tried building my own light pen for my PCjr, but I couldn't get it to work at all. Probably found plans that were better suited to a different system or didn't have the right type of sensor or something. I also tried hooking up a Sega Light Phaser to the light pen port with no good results, haha.
 
I have an almost working light pen on my PCjr. It was professionally made for another system and then modified. There were plenty of "make your own lightpen" articles in the magazines back 20 years ago.

My understanding of light pens is that a phosphor has to be lit for the light pen to register a hit. So it is useful for selecting an area of the screen that is lit up at the moment, but it is not useful for drawing on a part of the screen where the screen is not lit. You could work around this by having a light background to draw on.

I've also found the accuracy to be sorely lacking, although that might be a problem with my specific pen.

I've never used a light pen, but I thought they timed the CRT retrace from the start until it hit the pen and got the position from that, like a console/arcade light gun.

Edit: Nevermind I think you're saying the same thing :p
 
I bought a ZX Spectrum lightpen a while back with the intention of making a CGA adaptor for it, but I haven't had time to play with it yet. Previously I had tried to build my own - I tried a bunch of different photodiodes and phototransistors, and one of them kind of worked but wasn't really accurate enough. By reprogramming the CRTC it should be possible to make a nice drawing program with a resolution of 80x100.
 
I still have a light pen, complete with spring-loaded fiber-optic tip, that I salvaged from something years ago. It requires a regular CRT, but works okay--and accuracy is nothing to crow about.
 
I bought a ZX Spectrum lightpen a while back with the intention of making a CGA adaptor for it, but I haven't had time to play with it yet. Previously I had tried to build my own - I tried a bunch of different photodiodes and phototransistors, and one of them kind of worked but wasn't really accurate enough. By reprogramming the CRTC it should be possible to make a nice drawing program with a resolution of 80x100.

I had a Spectrum light pen, and it was virtually useless. It was nowhere near sensitive enough to pick up the CRT trace, so instead it worked by having the software flash different parts of the screen between black and white. The different pen software either flashed rows and columns sequentially, or set half the screen black and half white, then quarters, eighths, sixteenths, etc to narrow down where the pen was. Both these methods meant the screen flashing black and white for 20-50 frames every time it tried to read the pen position.
 
Last edited:
Both these methods meant the screen flashing black and white for 20-50 frames every it tried to read the pen position.
That's basically my experience with them too. They were so utterly and completely useless it's no wonder it was a dead end technology.

Had one place that tried to use them instead of a keyboard at PoS; the employees hated it so much and it was so unreliable that they ended up going back to having a keyboard there.

Needless to say when touch-screens came along, the employees were less than enthusiastic. Of course as someone who can't stand fingerprints on glass, I don't blame them.
 
I had a Spectrum light pen, and it was virtually useless. It was nowhere near sensitive enough to pick up the CRT trace, so instead it worked by having the software flash different parts of the screen between black and white. The different pen software either flashed rows and columns sequentially, or set half the screen black and half white, then quarters, eighths, sixteenths, etc to narrow down where the pen was. Both these methods meant the screen flashing black and white for 20-50 frames every it tried to read the pen position.

I suspect that's because there's no direct way for ZX Spectrum software to know what the current raster position is, rather than lack of sensitivity per se. In fact, that's an interesting idea for a way to increase the effective resolution - find the character position using the CRTC and then subdivide down by flashing small sets of pixels within that area.
 
I had a light pen on my Fairlight CMI synth back in the 80's. I liked it much better than alternatives. So much that when buying my system, I bought the older model with the light pen and had it upgraded instead of buying their latest which had a touchpad. Now this could be a tainted experience. The Fairlight was a purpose built synthesizer with multiple CPUs so that it could do true concurrent tasks so the light pen worked awesome, that may be why I loved it.

Cheers,
Corey
 
Last edited:
That's basically my experience with them too. They were so utterly and completely useless it's no wonder it was a dead end technology.
Selecta pens were an extremely popular accessory on 3270 terminals and worked very well - that is to say flawlessly - and so they were certainly not a "dead end technology". Maybe other products did not work so well.

However I agree that they were replaced by touch-screens, early versions of which were nowhere near as reliable as Selecta pens.
 
The May 14, 1984 issue of InfoWorld has an article covering light pens for IBM PC on page 43-44. Also covers some of the problems with IBM's implementation which makes light pens less than useful.

Found that PC Mag Jul-Aug 1990, page 246, echos this statement. The light pens in that article that were reviewed favorably had additional add-in cards that allowed for pixel-level accuracy. The ones that simply connected to the EGA's built-in connector had only column-level resolution (a "column" is an entire byte, so while this works for text-mode interfaces, it also has text-mode X/Y resolution in graphics mode, no individual pixels).

I know there were a lot of build your own light pen articles for systems other than the IBM PC and related. I expect similar articles for the IBM PC might be out there.

I keep thinking I want to do this, but I've looked over the 6845 tech info and I can't figure out a way (through any sort of software method) to get better than column-level resolution out of a pen directly connected to CGA or EGA, so I think this dream of mine will die on the vine.
 
I wondered about the light pen port on my Compaq's CGA card. Besides the hardware what software do you use? Does it end up working like a mouse?

-J
 
My guess is that in order to operate like a mouse in any fashion you need a mouse emulating light pen driver. Where exactly such a thing might come from I have no idea. My ATI Small Wonder has the light pen port as well. I have two light pens here but I've been procrastinating on making the little adapter to go form the five pins on the card to the DB9 connector, of which five pins are actually used. There seems to be some form of standard on pin arrangement on the MDA/CGA/EGA cards.

Code:
    Pin 1 is light pen input
    Pin 2 is not present
    Pin 3 is the light pen switch input
    Pin 4 is logic ground
    Pin 5 is +5V
    Pin 6 is +12V

I've always wondered if AutoCAD could be configured to use a light pen
 
I have Micrografx PC-Draw 1.4 and a couple of other programs that said they support light pens, but they don't say what kind of light pen. Just assumed that was the CGA light pen.

Still interested in obtaining whatever kind they worked with.
 
Code:
    Pin 1 is light pen input
    Pin 2 is not present
    Pin 3 is the light pen switch input
    Pin 4 is logic ground
    Pin 5 is +5V
    Pin 6 is +12V

I've always wondered if AutoCAD could be configured to use a light pen

The 6845 data sheet warns about the inaccuracy of the light pen and they're not kidding, from my own experience. The "light pen switch" is usually operated by depressing the tip of the pen on the screen. You could easily rig up an ersatz one with a phototransistor and a pushbutton switch--it's not rocket science.
 
All you get from the 6845 is the 14-bit address that the lightpen strobe was detected at. The BIOS interface that provides lightpen support takes this address and converts it to x and y coordinates. In graphics mode, that gives you a "resolution" of 40x100, which you can think of as a 40x100 grid across the entire screen. You're not going to be drawing with it. PC-Draw will let you drag objects around and resize them, but it's coarse. The only exception to this behavior is 80-column text mode, where the faster dot clock allows the lightpen to register every character on an 80x25 screen.

My experimenting with a light pen and the 5151 monochrome showed that long persistence phosphors are not your friend.

The Lite-Pen Company (LPC) marketed a light pen that could deal with the 5151 phosphors. I've never tried it, but reviews of it were mixed to positive.
 
Back
Top