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.
Dick
----- Original Message -----
From: "Pete Turnbull" <pete_at_dunnington.u-net.com>
To: <classiccmp_at_classiccmp.org>
Sent: Monday, July 16, 2001 6:27 PM
Subject: Re: Light Pens ...
> On Jul 16, 14:01, Sellam Ismail wrote:
>
> > Ok, my mind is still blown. How does a light-pen drawing program work
> > then? (Or is a light-pen drawing program not possible?)
>
> It works by doing what Chuck described (flashing a frame or a line) or as
> Megan described (using a target). I've seen it done both ways. I've never
> seen a commercial lightpen do what I did, using the low light from "black"
> (which isn't quite black) to trigger a sensitive detector -- a detector
> that sensitive is just too prone to being swamped by ambient light, I
> discovered. It was fine when the pen was pressed against the tube, but
> goes haywire when lifted away. I fitted a touch switch to mine to prevent
> that, but it still took the amplifier a moment to settle.
>
> --
> Pete Peter Turnbull
> Network Manager
> University of York
>
>
Received on Tue Jul 17 2001 - 11:49:06 BST