[Coco] New thread, cga-rgb->vga convertor GBS-8220

[gene heskett]

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
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