That feature was always a problem to use because the 6845 only had byte-level
information about when the light pen strobe occurred. Most people wanted the
level of pixel-by-pixel resolution that we all have, by now, come to expect as
we get it from our mouse or whatever, but the lightpen required something to
generate a pixel count to indicate where, within the byte, the desired location
lies. This requires LOTS of effort in software and support in hardware. There
were a few fairly effective implementations, but I didn't personally get
familiar enough to cook them up from memory.
One of the challenges was that there was frequently loss of relative
synchronization between the lightpen strobe and the software that would read the
pixel counter, which was external to the 6845. This came about because
characters (bytes of pixels) were promoted through the external
"high-speed-video-timing" logic from the graphics memory or character generator
at a fixed rate, with which the system software was not so precisely
synchronized that the light pen register could necessarily be relied upon for
precise timing. Moreover, since the processor might take variable a bit of time
getting to the interrupt that got its attention, the amount of time involved
could be off by a byte-time or two, and the sweep logic could be off on the next
scan line by then. Additionally, the light pen often was somewhat variable in
its response, depending on things like sweep rate, display intensity, etc.
These things were pretty fussy!
My hat's off to anybody who could put together an accurate and precise light-pen
interface hardware and code set.
Dick
----- Original Message -----
From: "Cameron Kaiser" <spectre_at_stockholm.ptloma.edu>
To: <classiccmp_at_classiccmp.org>
Sent: Monday, July 16, 2001 12:32 PM
Subject: Re: Light Pens ...
> > > I hadn't realized that on a PC w/ CGA that the light pen was getting
> > > help from the CRT controller. That explains how a relatively simple
> >
> > Yes. On most machines designed to have light pens, the CRT controller
> > does some of the work. The popular 6845, as I mentioned, has a lightpen
> > input pin. Incidentally, both the BBC Micro and the Sirius/Victor9000
>
> The Commodore VIC-II also supports light pens in hardware. When the switch is
> closed, the light pen registers in the chip latch at the current raster
> position if light is present and an IRQ is generated. (In simplistic terms.
> *grin*)
>
> --
> ----------------------------- personal page:
http://www.armory.com/~spectre/ --
> Cameron Kaiser, Point Loma Nazarene University * ckaiser_at_stockholm.ptloma.edu
> -- This message will self-destruct in five seconds. Good luck, Jim. --
M:I ----
>
>
Received on Mon Jul 16 2001 - 14:28:09 BST