Are there any jumpers on the VLB card that you can play with? If yes I'd try toggling anything related to IRQs, etc.

Are you using Vanilla Doom? Or a another DOS port like MBF or FastDoom?

I've tried both with Doom vanilla and FastDoom. @badmojo unfortunately that video card does not have any jumpers for IRQs. For now, I've settled with a PCI video card, but I'd really like to use the VLB for nostalgic reasons 😁

One thing I've noticed is that it seems that the LFB for the VLB card is up in the 62MB range, whereas the PCI one stays at a lower 4MB; I wonder if that, together with a DOS extender, could be causing some sort of memory overlapping

Do the colors shift while digital audio is playing, or do they stay the same constantly wrong colors?

That is palette corruption for sure. Do other games have the same issue? The palette is copied to the video card via OUT instructions (ports), so memory overlapping shouldn't be a problem. FastDoom has a command line parameter to fix issues on palette copies, try the option "-fixDAC" to see if it makes any difference.

Edit: I had a card with a similar problem, some times it worked fine an others didn't. Turned out to be an issue on the DAC chip, I replaced it and the problem was fixed.

ViTi95 wrote on 2023-05-23, 14:21:

The palette is copied to the video card via OUT instructions (ports), so memory overlapping shouldn't be a problem.

I would suspect an issue with the AEN signal getting to the video card, causing it to misinterpret the DMA transfers to the sound card as port writes.

Maybe the thread below can bring some light:

Crackling sound with several sound cards on 486 VLB motherboard.

I would change the video card, because it appears to be a broken VBE implementation.

If so, get the utils here:

VESA Fix Utility Listing (for old video cards)

jmarsh wrote on 2023-05-23, 14:05:

Do the colors shift while digital audio is playing, or do they stay the same constantly wrong colors?

They don't change when digital audio is played, but they change depending on what's going on during the game.

ViTi95 wrote on 2023-05-23, 14:21:

That is palette corruption for sure. Do other games have the same issue? The palette is copied to the video card via OUT instructions (ports), so memory overlapping shouldn't be a problem. FastDoom has a command line parameter to fix issues on palette copies, try the option "-fixDAC" to see if it makes any difference.

Edit: I had a card with a similar problem, some times it worked fine an others didn't. Turned out to be an issue on the DAC chip, I replaced it and the problem was fixed.

Thanks for that tip! -fixdac yielded the same results

jmarsh wrote on 2023-05-23, 14:44:

ViTi95 wrote on 2023-05-23, 14:21:

The palette is copied to the video card via OUT instructions (ports), so memory overlapping shouldn't be a problem.

I would suspect an issue with the AEN signal getting to the video card, causing it to misinterpret the DMA transfers to the sound card as port writes.

I tried looking at the pinout for the VLB and didn't find that- can you give me more details on that please?

Luzemario wrote on 2023-05-23, 16:28:

Maybe the thread below can bring some light: […]
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Maybe the thread below can bring some light:

Crackling sound with several sound cards on 486 VLB motherboard.

I would change the video card, because it appears to be a broken VBE implementation.

If so, get the utils here:

VESA Fix Utility Listing (for old video cards)

Very useful tools indeed, thank you! Doom uses mode 13H, so VBE is not the problem

Also tried using a SCSI controller/hardrive and disabling the onboard PCI IDE, same thing. I've given up for now and resorted to a PCI video card, but would be good to know if it's the card or the motherboard's bus implementation at fault

I've tested the Vibra together with the VLB card on another motherboard with the same chipset but no PCI or onboard IDE and the problem isn't present. This one has FPM memory whereas the one with the issue has a single 16MB EDO stick, I might try using FPM on the one with the issue but I doubt that's the problem, I'm thinking it's the PCI implementation that could be buggy.

bregolin wrote on 2023-05-31, 14:16:

I'm thinking it's the PCI implementation that could be buggy.

Do you have VGA Palette Snoop option in BIOS? Make sure it is set to Disabled. It shouldn't really affect the (S)VGA but I've seen some DACs that are sensitive to write timings, so depending how the snoop is done it might just affect something. In general though mobos that have both PCI and VLB tend to have problems with both. VLB needs low latency and direct CPU bus access to be fast and PCI needs to be able to bus-master to work well.

Deunan wrote on 2023-05-31, 16:59:

bregolin wrote on 2023-05-31, 14:16:

I'm thinking it's the PCI implementation that could be buggy.

Do you have VGA Palette Snoop option in BIOS? Make sure it is set to Disabled. It shouldn't really affect the (S)VGA but I've seen some DACs that are sensitive to write timings, so depending how the snoop is done it might just affect something. In general though mobos that have both PCI and VLB tend to have problems with both. VLB needs low latency and direct CPU bus access to be fast and PCI needs to be able to bus-master to work well.

It is disabled, and enabling it doesn't make any difference. For now I've decided to use a PCI VGA card, the VLB one will go to another system that's VLB+ISA only and doesn't produce the same issue.