Light pens?

Randy McLaughlin cctalk at
Mon Mar 21 14:53:02 CST 2005

From: "Kapteyn, Rob" <kapteynr at>
Sent: Monday, March 21, 2005 2:05 PM
>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

I am in the process of setting up a cromemco "museum" webpage.  I have never 
heard of a Cromemco VDM-1, can you give details of the card?

I am interested in:

Date of manufacture.

Display characteristics (how many pixels etc).

What major chips were used.

Was it DMA based like the Cromemco Dazzler or did it have RAM or RAM bus.

Was there OS support.


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