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Odd Colours With a Cirrus GD5428

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First post, by jami3rez

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I’ve just been trying out a 486 system with a Cyrus Logic GD5428 VLB display adapter and have had some issues with weird colours. A few examples -

Doom 2 seems the worst, it sometimes flashes to completely incorrect colours. Reliably this happens when the screen flashes on taking damage and it goes very blue for a while before snapping back to normal. Sometimes does it on the main menu too.

IMG-0219.jpg

Day of the Tentacle - The colours go wrong on the Lucasarts screen, the blue swirl turns green and the logo takes on a pinkish hue. Otherwise seems ok in-game.

Beneath a Steel Sky - On the Revolution splash screen, the brightest part of the background is purple or blue.

I have installed the utility software but it doesn’t seem to matter what I set it to. I have a more modern Samsung CRT.

I have switched to a Trident TVGA9000C 16 Bit ISA 512KB VGA Video Graphics Card and all the above issues go away.

What could be the problem with the Cirrus card? Is there something I can change or is it likely faulty?

Am I suffering at all my switching to the Trident? I’m only wanting to play old games that my Win98 system is too fast for.

Reply 1 of 4, by MattRocks

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There are utilities to test the VRAM.
https://sourceforge.net/projects/vmtce/

Desktop timeline [ MOS 7501 → 68030 → x86(P5/MMX) → x86(K6-2) → x86(K7*) → PPC(G3*) → x86-64(K8) → x86-64(Xeon) → x86-64(i5) → x86-64(i7) ] * lost

Reply 2 of 4, by MikeSG

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Also check capacitors on the card

Reply 3 of 4, by st31276a

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It sounds as if the palette gets corrupted somehow.

Reply 4 of 4, by mkarcher

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Yeah, that's clearly a RAMDAC issue, not a Video RAM issue. The CL-GD5428 has an integrated RAMDAC, so the part can not be changed on its own. Do you run VL at elevated clocks (above 33MHz)? Possibly you need to somehow add a wait state for palette updates to be correctly received by the CL-GD5428 chip. I don't think it's an electrical contact issue, because it seems video RAM writes are working perfectly, and only palette I/O cycles get corrupted, although a bad M/IO line on the VL connector might cause a very low fraction of port 3Cxh writes as writes to memory address 3Cx h (which the Cirrus chip does not respond to). Note that palette updates might generate 768 I/O port writes in a row, and if one of them is not correctly registered by the card, you will get funny colors.

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