Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!sunkisd!maxwell!jean
From: jean@maxwell.Concordia.CA ( JEAN GOULET | DCKS004 |              | )
Newsgroups: comp.sys.atari.8bit
Subject: Finding the HSYNC signal
Summary: Getting a TTL signal for a light pen
Message-ID: <773@sunkisd.CS.Concordia.CA>
Date: 17 Apr 89 22:11:36 GMT
Sender: usenet@Sunkisd.CS.Concordia.CA
Reply-To: jean@maxwell.Concordia.Ca ( JEAN GOULET | DCKS004 |              | )
Organization: Concordia University, Montreal, Canada
Lines: 24

Hello.  I've been looking into making a light pen for my computer, and I think
the design is complete, except for one important detail.  My circuit needs
to be restarted every time the electron beam starts a new horizontal line, so
I figure the HSYNC signal is what I need.  Unfortunately I don't have the
schematics for my computer, so I'm wondering if someone knows if this signal
is availble on the 130XE motherboard.  I looked at the schematic for the
400/800, but it looks like color, brightness, and timing info are mixed
together inside one of the chips and sent out as the video signal.

Would it be easier to extract HSYNC from the RF plug (or the composite output?
Or the luminance output?).  If so, what kind of circuit would I need
(comparator, etc.)?

By the way, I realize the computer has hardware for decoding the position of
the light pen, but my experiments have shown that the x-input is so inaccurate
that it's unusable.  My circuit will have a resolution of 640 on the x-axis
(overdesign is better than underdesign).  I'll use the y-coordinate output
from the computer, though.

                                            Jean Goulet
                                            Electrical Engineering
                                            Class of '89
                                            Concordia University
                                            Montreal, Canada