----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, July 17, 2001 3:45 PM
Subject: Re: Light Pens ...
> >
> > The light pen on the CDC system had a button that had to be pressed when one
> > wanted its attention. Likewise, the MOT system that I mentioned earlier had
a
> > button also. Consequently, I'd conclude that one normally pressed the
button
> > when it was in contact with the face of the tube and released it when it was
> > still there, so ambient light was not so big an issue. With a switch, of
> > course, the response time issues you mentioned before became very real,
since
> > switch bounce can be quite long, and, more than one scan line might go by
while
> > the switch is debounced.
>
> The contact bounce time is irrelevant.
>
Yeah ... I thought about that, and the only real purpose the button would serve,
aside from gating the lightpen input, perhaps in the manner you suggest, is to
tell the system to flash the screen for one frame so it can find the pen.
>
> When you press the switch you tell the system to start looking for light
> pen events. This might just be a signal to the software to accept
> interrupts from the video system that indicate that the light pen has
> detected something. The light pen connects to the video hardware as
> usual, and latches the video address so that the software can read it.
>
If it does work that way, then ambient light becomes a factor, doesn't it? I
was thinking the button would gate the lightpen input. Apparently that's not
necessary.
>
> The video hardware can latch that address every time the light pen
> outputs a signal, no matter what the switch is doing. OK, it may not mean
> anything, particularly if the light pen is not pointing at the screen,
> but then the software will only read out that latch when the switch is
> pressed, so it doesn't matter.
>
seems reasonable ...
>
> On most micros that I am familiar with, the light pen switch signal
> doesn't go anywhere near the video circuitry (6845, etc). It goes to a
> general input port. The software reads that port and starts checking the
> 6845's registers if the switch is pressed.
>
> -tony
>
>
Received on Tue Jul 17 2001 - 17:45:42 BST