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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 3rd Nov 2019, 10:46 am   #1
Argus25
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Default S-100 Light Pen System

I had never actually seen a working light pen system on a computer before.

I read about them in vintage computer magazines. Recently I restored a SOL-20 computer and also experimented with a very early Matrox graphics card.

I realized the card could be deployed to make a light pen system. So I designed a prototype S-100 card for use in the SOL-20 computer along with the Matrox card.

Basically the light pen system uses a photo-transistor to detect a CRT pixel, loads the coordinates to a graphics card. It does sound simple enough, but as always there were some interesting technical challenges along the way, as explained in the article.

Here is the article on it which includes some construction techniques for prototype logic cards to avoid the Bird's nest of wires that occurs when you are dealing with 100's of interconnects, it might be of interest to those making prototype cards:


http://worldphaco.com/uploads/LIGHTPEN.pdf

Also there is a very short youtube video here:

https://www.youtube.com/watch?v=awPFZCJFguM
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Old 3rd Nov 2019, 1:36 pm   #2
dglcomp
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Default Re: S-100 Light Pen System

One of the more famous uses of the light pen were in the Fairlight CMI Series I/II, which were VERY expensive (OMD's cost 30,000 in 1983) sampling/sequencing devices with the light pen being extensively not only for navigating the system but for drawing in waveforms.

https://en.wikipedia.org/wiki/Fairlight_CMI
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Old 3rd Nov 2019, 2:41 pm   #3
cmjones01
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Default Re: S-100 Light Pen System

I remember playing with a home made light pen on my BBC Micro. It worked quite well, and was almost embarrassingly simple. The 6845 CRT controller chip had a 'light pen strobe' input which simply latched the values of the horizontal and vertical counters. That input was brought out to the computer's analogue input socket. Connecting a phototransistor stuck in the end of an old biro to this signal made a very low cost light pen!

Figuring out the light pen position was just a matter of reading a couple of registers in the 6845. I can't remember if there was any operating system support, but it was hardly necessary.

There was some software which took advantage of it, but not much. It was soon overtaken by conventional mice, which serendipitously also had native hardware support in the 6522 VIA chips, but that's another story!

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Old 3rd Nov 2019, 4:33 pm   #4
electronicskip
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Default Re: S-100 Light Pen System

I have a light pen for a Sinclair ZX Spectrum 48k although ive never used it along with one for an Apple IIe system.
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Old 4th Nov 2019, 1:26 am   #5
SiriusHardware
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Default Re: S-100 Light Pen System

I'm not sure if anyone else will remember this but one of the BBC 'Micro Education' series of programs, possibly 'Micro Live', took to offering downloads of the software listings featured during the program by using a flashing dot or block in the corner of the picture and a light-pen like device to pick it up.

Presumably 'light spot' support software for each of the featured machines was available as a type-in listing or could be requested on cassette or disc, since this was all in the pre-internet age.

While not a true light pen application in the sense of using the known position of the scanning 'dot' to determine the x-y position of the tip of the 'pen', it is a different example of using a pen-like device to read the light coming off a CRT screen.

Most of those old BBC computer programmes were made available to view online on the BBC web site a while back, it might be fun to find those programmes and try to 'download' the software programmes offscreen from the online versions - it should work as long as the framerate is reasonably high, since the 'baud rate' of the flashing dot data is unlikely to have been more than about 1200 or 2400.
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Old 4th Nov 2019, 7:04 pm   #6
duncanlowe
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Default Re: S-100 Light Pen System

We had a light pen to use (I think) with our PCW8256. It was a commercial unit but even so was clearly a phototransistor in a berol felt tip pen. It is vaguely possible and it was in fact for my VIC20, but I'm sure I'd have seen it around if it was mine.
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Old 4th Nov 2019, 11:32 pm   #7
cmjones01
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Default Re: S-100 Light Pen System

I think the Amstrad 8-bit machines used the 6845 CRT controller, so they would have had the light pen facility.

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Old 9th Nov 2019, 2:46 pm   #8
electronicskip
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Default Re: S-100 Light Pen System

The Vectrex Games unit also had a light pen game although it was pretty rare i believe.
The Vectrex has a CRT but uses vector graphics so would be interesting to see one in action.
I have a couple of Vectrex machines but alas no light pen ever came my way.
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