17 Jul
2001
17 Jul
'01
11:49 a.m.
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(a)dunnington.u-net.com>
To: <classiccmp(a)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