CONSTRUCT PC STYLE SVGA/VGA INTERLINK CABLE/ADAPTER'S FOR ST

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jimmysword
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Re: CONSTRUCT PC STYLE SVGA/VGA INTERLINK CABLE/ADAPTER'S FO

Postby jimmysword » Wed Apr 28, 2010 8:39 am

I'm intending to build a vga cable for hooking up my Atari in hi-res mono and have consulted a few different schematics around the web including this one. However there are some conflicts regards what resistors should be used on the RGB wires, if any.

For instance the information in this thread suggests 47 ohm resistors while the cable on logicsays website (http://www.logicsays.com/atari/mono/docs/atari_vga.htm) suggests the use of 150 ohm resistors (possibly trimmers as suggested in this article). I've seen 75 ohm suggested somewhere else and atari-forum's wiki (wiki/index.php/Atari_ST_to_VGA_adaptor) doesn't mention them at all!

So what is the reason for adding resistors to these lines and do I actually need to do it? I'm not interested in being able to alter values on RGB.

Many thanks in advance.

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1024MAK
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Re: CONSTRUCT PC STYLE SVGA/VGA INTERLINK CABLE/ADAPTER'S FO

Postby 1024MAK » Fri Apr 30, 2010 12:29 am

The reasons that the resistors are included is because the output from the Atari is at a higher voltage level than the input the monitor or TV is expecting.
Some can cope or have controls that you can adjust. But with some it "over drives" the monitor or TV circuits and you end up with too bright an image.
There is an additional reason if you are using the monochrome high-res output to drive a colour monitor, in that for the colour you want, the signal has to be split in different amounts to go to the three colour input channels (RGB) to give you the correct colour.

Through experimenting with different values people have found the "right" values for their monitor or TV. That is why there are different values used.

Hope this helps :)
Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (nearly forgot the Psion's)

jimmysword
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Re: CONSTRUCT PC STYLE SVGA/VGA INTERLINK CABLE/ADAPTER'S FO

Postby jimmysword » Fri Apr 30, 2010 8:03 am

Yes this helps thank you. I was thinking I could get away without using any resistors but I'll see if I can find 3 between 40 and 150 ohms from my bag of random resistors and put them in for good measure.


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