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Weird ISA issue on 486 motherboard

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First post, by paulisthebest3uk

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Hello been a while since I've posted in this forum! Anyway got a 486 system here some ISA cards won't work that are confirmed working in my pentium 1.
ISA VGA paradise is one such card, so is my soundblaster value 16. However the same cards work in my pentium 1.

However I've got a ISA VLB cirrus logic card which does work. It seems like the bigger part of the ISA slot might be the issue. Maybe a dead line there somewhere or something?

Recently I had to erase the EPROM and reflash it as it corrupted somehow. I used a bios I found on theretroweb website, maybe it has slight differences in its code?

If anyone has experienced something similar and knows a few steps to try would appreciate it. Many thanks Paul

Reply 1 of 4, by waterbeesje

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Sa after the reflash the graphics card still works in the pentium but not in the 486. That means the graphics card is probably ok.

Did you clean the contracts of the 486? With some non static cloth you should be able to clean the Isa slot on the board and the connector in the card. Just use IPA, and you'll be amazed by the amount of dirt you'll find.

Next, the 486 will boot with the vlb card so the Isa bus may be troublesome. You may want to check if the Isa bus speed is set to around 8MHz.
It the FSB is 33MHz you may find an option to set the Isa divider to 4. 33 / 4 =8.3MHz, should be ok. Bus 40, divider 5. Bus 25, divider 3.

With a lower isa divider the isa speed wil run out of spec, say, 12MHz. Some vga cards like that, some don't. Some sound cards won't like it either and scsi is prone to data corruption. Since the vlb card runs on the vlb bus with just a few days lines connected to ISA, it's not too sensitive.

Stuck at 10MHz...

Reply 2 of 4, by paulisthebest3uk

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I'm not sure how to change the bus divider, I did notice different results by lowering the CPU speed by jumpers. Still wasn't working propelery but the sound card was detected sort of. Diagnose detected it successfully but when I tried testing duke 3d setup utility system instantly hanged at digital sound effects part. Adlib weirdly worked

Could this be a bad crystal? It's weird how reducing the CPU clock speed changed things. If the cpu is at full speed sound card doesn't work at all

Reply 3 of 4, by Horun

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Can you boot off a floppy using a ISA controller ?
Which motherboard do you have ? A Link to theretroweb page might help 😀

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 4 of 4, by Thermalwrong

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What's the model name of the creative sound card?
There are some that use the OSC pin of the ISA slot to generate the operating clocks and if that signal isn't there the card wouldn't work. I've got a couple of ISA VGA cards that do the same including a Cirrus logic one.

paulisthebest3uk wrote on 2024-09-01, 16:18:

I'm not sure how to change the bus divider, I did notice different results by lowering the CPU speed by jumpers. Still wasn't working propelery but the sound card was detected sort of. Diagnose detected it successfully but when I tried testing duke 3d setup utility system instantly hanged at digital sound effects part. Adlib weirdly worked

Could this be a bad crystal? It's weird how reducing the CPU clock speed changed things. If the cpu is at full speed sound card doesn't work at all

Hmm, that does kind of fit what I saw with a soundcard that had a broken crystal. The digital side mostly worked so it could be detected and install drivers, but sound would not play.
But then why does Adlib work? 😒

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