Light pens?

[GManuel (GMC)]

> 
> On Sat, 19 Mar 2005 00:01:31 -0600, Jim Leonard 
> <trixter at oldskool.org> wrote:
> > Okay, now my age and lack of experience is going to show:  Can 
> someone explain
> > to me exactly *how* a light pen works?  What is the feedback 
> mechanism if
> > drawing on, say, a completely black screen?  How does the computer get
> > coordinates for where the pen is touching?
> 
> On the machines I'm familiar with (VLSI video chip like a 6545 or C=
> VIC-II), the video chip has a light pen input which looks for a pulse
> from a phototransistor which may or may not have a
> press-to-the-screen-to-read or a squeeze switch on the pen body (to
> keep it from firing when it's not meant to).  When the chip sees the
> pulse, it latches its internal row/column counters to a register that
> can be read after the fact by user software.  Some chips (like the
> VIC-II) can generate an IRQ when that occurs (the initial version of
> the VIC-II chip, on the C= proto run had a flaw that when they warmed
> up, they would generate spurious IRQs and hose the machine).
> 
> The only requirement is that the beam produce a bright enough flash to
> cause the phototransistor to fire deterministically.  From memory,
> that typically required the application to put something visible to
> humans as well on the screen.  I've seen check boxes, etc., when used
> to capture simple user input (YES/NO...)  I do not know if it's
> possible to have a dark area of the screen fire the phototransistor or
> not.
> 
> -ethan
> 

It must be able to somehow. I have used a light pen on a Commodore 64 with a
Graphics Program called Picasso's Revenge and have drawn on the screen with
it even on a completely black background. Not sure how it does it though.

Greg




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