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A question for any IBM PC & PC/XT Motherboard experts

My question is what is the purpose of the C1 trimmer capacitor
near the crystal at the back of the XT motherboard near slot 6 & 7?
Just curious.

Frank G.


0
Frank
7/4/2009 3:19:53 AM
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On Jul 3, 8:19=A0pm, "Frank Galphin" <chromedom...@comcast.net> wrote:
> My question is what is the purpose of the C1 trimmer capacitor
> near the crystal at the back of the XT motherboard near slot 6 & 7?
> Just curious.
>
> Frank G.

It's connected to the clock generator chip, so it adjusts the crystal
frequency.

Of course, you may wonder why a crystal would need adjusting. The
schematic says "color adjust."  Again, this leads to a question: Why
would the motherboard, which has no video output, need "color
adjust"?  I was wondering that too.  My guess is that the color
graphics card uses the clock signal from the bus for generating
composite video output, and won't get the right colors unless the
frequency is just right.  That's just a guess, though.

--Paul Vojta, vojta@math.berkeley.edu
0
Paul
7/4/2009 5:24:16 AM
"Paul" <vojta@math.berkeley.edu> wrote in message 
news:b6f33df0-c4cc-475b-8306-04a17ea21206@g7g2000prg.googlegroups.com...
On Jul 3, 8:19 pm, "Frank Galphin" <chromedom...@comcast.net> wrote:
> My question is what is the purpose of the C1 trimmer capacitor
> near the crystal at the back of the XT motherboard near slot 6 & 7?
> Just curious.
>
> Frank G.

It's connected to the clock generator chip, so it adjusts the crystal
frequency.

Of course, you may wonder why a crystal would need adjusting. The
schematic says "color adjust."  Again, this leads to a question: Why
would the motherboard, which has no video output, need "color
adjust"?  I was wondering that too.  My guess is that the color
graphics card uses the clock signal from the bus for generating
composite video output, and won't get the right colors unless the
frequency is just right.  That's just a guess, though.

--Paul Vojta, vojta@math.berkeley.edu

Thanks Paul,
I wasn't sure what it was but was guessing that the close location
to the CPU might be some type of clock adjust. Thanks again
for the info.

One more dumb question though. Can you hook up a monchrome
9 pin TTL monitor to a CGA card? I think you can but can't remember
if it was possible.

Frank G.


0
Frank
7/5/2009 2:29:51 PM
In article <ufGdnU2koIV_Jc3XnZ2dnUVZ_jKdnZ2d@giganews.com>,
Frank Galphin <chromedome45@comcast.net> wrote:
>
>One more dumb question though. Can you hook up a monchrome
>9 pin TTL monitor to a CGA card? I think you can but can't remember
>if it was possible.

I'm not sure what a 9 pin TTL monitor was.  I do recall, though, that
you couldn't connect a monitor made for the MDA up to a CGA card, and
vice versa.

--Paul Vojta, vojta@math.berkeley.edu
0
vojta
7/16/2009 7:55:26 PM
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