[Coco] New thread, cga-rgb->vga convertor GBS-8220
Mark Marlette
mmarlette at frontiernet.net
Wed Jul 27 18:28:29 EDT 2011
Gene,
There is an adjustment pot that will allow your display to be cleaned up with Roy's.
Just not sure which one it is ATM....Roy?
Quartus 11.0 is currently working overtime........ :)
Regards,
Mark
Cloud-9
----- Original Message -----
From: "gene heskett" <gheskett at wdtv.com>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Wednesday, July 27, 2011 3:40:56 PM
Subject: [Coco] New thread, cga-rgb->vga convertor GBS-8220
Hi folks;
I pinged Tim again, and got an answer this time, fairly quickly.
He said that one user had used a resistor pair, of 100 and 1000 ohms as an
attenuator, and that this user had tied the H&V syncs together, then the 1k
from that point to the S input on the left hand 5 post input, with the 100
ohm resistor from the S pin to ground.
But that could be pretty hard on the driving gates, having 2 outputs which
may be pulling in opposite directions shorted together.
So I made a combiner with a resistor of 1800 ohms, connected to the near
end of R6 on the coco's motherboard via a small drop of silver bearing
solder and butting the side of a shortened end lead against R6 to pick up a
5 volt source.
I connected the other end of this 1800 ohm resistor via a wire to the S pin
of the P3 header. This then is the pullup resistor because the next piece
blocks the logic 1's in between the inverted sync pulses.
Then I had already added a piggy-backed 74LS04 onto IC15, with its pins
bent out flat, hooking the HSync output of IC15 (see the schematic) to the
input pin of the same gate in the added chip, doing the same for the VSync
signal also. So I had right side up Vsync on the added chips pin 13, and
right side up HSync on the added chips pin 11.
Then the outputs of those two gates were connected via a 1n914 for each
signal, to the 1800 ohm resistor, with the cathode end of the diode facing
to the added chips output pin 11 and 13, and the anode end of both diodes
connected to this pullup resistor and the S pin. Each sync signal can then
pull the line feeding the S pin of P3, down to the total of the gates VSat,
about 80 to 100 millivolts, plus the forward conduction voltage of the
1n914, somewhere in the .65 volts range. Schotkey diodes would help, but
this worked a buck a diode cheaper.
So I now have a well combined sync signal, sitting at about 1.5 volts in
between sync pulses, and is pulled down to about .78 volts during the
active time of either sync signal.
This works very well but did take some fiddling with the menus to optimize
the display, which I found the 640x480 setting the easiest to read on my
aging touch screen Elo vga monitor. Its tube is getting a bit soft with
old age & hours as it sat face up in our weathermans composition desk,
running at enough brightness to be usable when 300 ft/candles worth of
studio lights were on. For several years.
Booting from the HD, even multivue ran albeit on its somewhat smaller area
screen. And only recognized one mouse (left) button on my seriel mouse. I
can recall when all 3 were usable for the icon editor.
I received the rest of the chips I had ordered from Jameco today, after
doing this of course (there is a Murphy's Law about such things) and its
possible that a single chip adder solution can also be done. Future
project, would only need one gate of a 74LS00. ;-)
The image I am seeing is not quite as noisy as Roy's adapter gives, but if
you have one of those, I do not see a $50 improvement in the picture from
my chair. I do not know what Roy's situation is now, but frankly I'd
druther see the money go to someone 'in' our community if you need
something like this.
I more than likely made more diff in my picture by putting a real, honest
to goodness ferrite bead across the 2.2 ohm resistor my motherboard has in
the "FB5" position, which gives my gime nearly 200 millivolts more Vcc
voltage to play with, and my rise and fall times are quite noticeably
improved now.
Cheers, gene
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
millihelen, n.:
The amount of beauty required to launch one ship.
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