VOGONS


First post, by lepidotós

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I know PCI Express to AGP and vice versa does, and that makes sense. I'm curious if wiring a PCI chipset up to an AGP card edge specifically (i.e. Voodoo 3) needs any internal reworking or bridge chips to work. I would suppose not, if AGP is a superset of PCI, it should just act like a 66MHz, isolated PCI bus in that situation (or the card look like a lazy AGP card that doesn't really do many fancy things to the bus) based on how I understand it, but I'm just double-checking for confirmation.

Thanks.

Reply 1 of 6, by sdz

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Older cards will work just fine, and all 3dfx AGP cards will work with just the PCI part of the AGP connected (as you said, basically PCI 66MHZ)
Here's an adapter, if you want to check the schematic:
https://sdz-mods.com/index.php/2020/06/22/pci … to-agp-adapter/
However, such an adapter may not work correctly on older motherboards, as all AGP cards strap internally IDSEL to AD17, while the slot the card+adapter is plugged in might have IDSEL strapped to something else. In that case a PCI to PCI bridge is needed.

Reply 2 of 6, by lepidotós

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Is that a concern going the other way around (so, a PCI card in the AGP port)? I see an adapter going that way on eBay, it seems to be passive to me, but unfortunately there's only one angle of it so I can't see the other side of the board to know for sure.

Reply 3 of 6, by sdz

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My bad, I thought you meant the other way.
A completely passive adapter for what you described can be made, but it will only support 3.3V PCI cards and possibly universal cards (if you found an adapter look at the PCI connector keying).
For 5V PCI cards, it would need level shifters, as AGP 1.0 is 3.3V.

Reply 4 of 6, by lepidotós

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That's alright, what I'm looking at is 3.3V, so just double checking. Hypothetically, if that card was based on dies that were meant for both AGP and PCI cards, would that be able to take advantage of the double/quad data rate of the bus? I would assume a pure PCI card wouldn't really know what to do with that but that silicon that's aware of that could, but then maybe that depends on pins that aren't there on the PCI side of things that are there on the AGP one.

Thanks for the reassurance.

Reply 5 of 6, by sdz

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I doubt the MB chipset would switch from SDR, when it doesn't actually see an AGP card.
Maybe the cards would support more than 66MHz PCI if you OC the AGP bus. At least V4/V5 cards run fine at 90-95MHz.

Reply 6 of 6, by lepidotós

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Ah, yeah, good point. I'll keep experimenting with this, thanks for the help.