Author Topic: Can I use VSYNC with a cap to get +1V ish too?  (Read 1151 times)

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Offline khaz

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Can I use VSYNC with a cap to get +1V ish too?
« on: 02:56, 21 December 17 »
The Scart cable page on the wiki describes how to make a steady current from CSYNC with a 100µF capacitor, in order to get a blanking signal for Scart pin16. I did it for the CPC and it works great, but I was wondering if I could do the same with the much slower VSYNC signal with the proper capacitor and still be in the 1-3V range needed?

I'm using https://gglabs.us/node/2022 to get an RGB picture with CSYNC out of CGA. For some mysterious reason VGA pin9 (+5V) isn't connected so I can't use that. I have two separate pins for CSYNC, and VSYNC which was left untouched from CGA. I could use CSYNC to drive both SYNC and BLANKING, but I want to avoid that, as soldering both to the same VGA pin will be a mess.

So, yeah. Can I use a cap to smooth VSYNC to a steady current above +1V, and what cap value should I need?
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Offline Bryce

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Re: Can I use VSYNC with a cap to get +1V ish too?
« Reply #1 on: 14:22, 26 December 17 »
Yeah, it would probably work, but I've never tried it. Use a 100µF cap and give it a go. It won't damage anything even if it doesn't work.

Bryce.
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Offline khaz

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Re: Can I use VSYNC with a cap to get +1V ish too?
« Reply #2 on: 18:31, 27 December 17 »
100µF would be enough? What dictates the size of the capacitor?

That's cool, as I still have 19 caps left from when I did the mod on the CPC.
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Offline Bryce

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Re: Can I use VSYNC with a cap to get +1V ish too?
« Reply #3 on: 14:12, 31 December 17 »
100µF would be enough? What dictates the size of the capacitor?

That's cool, as I still have 19 caps left from when I did the mod on the CPC.

The capacitor needs to be large enough to hold a charge for the length of time that the VSync signal is at 0V. You can calculate from the RC time constant: https://en.wikipedia.org/wiki/RC_time_constant

Bryce.
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Offline khaz

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Re: Can I use VSYNC with a cap to get +1V ish too?
« Reply #4 on: 02:27, 01 February 18 »
Right, so it seems 100µF isn't enough, I couldn't get an RGB image with it. I tried with the only other capacitor I had, which is 1200µF. It gave me a very faint, rolling picture. I'm not sure why. I temporarily replaced the cap with a 1.5V battery and that gave me a very clean picture, so at least I know the cable works fine otherwise.

Any idea as to why it didn't work? Do I need an even bigger capacitor? or Did I go overboard, if that's even possible?
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Offline Audronic

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Re: Can I use VSYNC with a cap to get +1V ish too?
« Reply #5 on: 05:23, 01 February 18 »
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Offline Bryce

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Re: Can I use VSYNC with a cap to get +1V ish too?
« Reply #6 on: 09:38, 01 February 18 »
A bigger capacitor won't make any difference. From your experiments, it looks like the averaged voltage on the VSYNC line is lower than 1V. The bigger capacitor may have worked because it already had some residual charge on it?

Bryce.
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