This looks like a close match: http://www.win3x.org/uh19/motherboard/show/4179

Isn't it meant to be P24 or something like that ?

Robin4 wrote on 2020-09-06, 01:29:

*DX *P23 *P23N […]
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*DX
*P23
*P23N

Somebody know what those P23 are?

P23 and P23N are code names for 486SX and 487SX (https://en.wikipedia.org/wiki/Intel_80486)
No reason to go with an SX processor nowadays unless you're super nostalgic about one.

With the non-ZIF socket and no visible voltage regulator that board is almost certainly 5-volt only. A 486DX2/66 would be appropriate if you want to use a heatsink, or a 486DX/33 if you don't (not to open that can of worms again, but if you go strictly by the datasheets based on what ambient temperature is allowed). Both are readily available on eBay; just make sure it's 5-volt if you go non-Intel.

jakethompson1 wrote:

just make sure it's 5-volt if you go non-Intel.

I think there's a fair chance that the board will not work with anything non-intel. I have something almost similar (MB-495SX Rev1.1) that stays stone-dead with anything but Intel chips.

quicknick wrote on 2020-09-06, 21:47:

I think there's a fair chance that the board will not work with anything non-intel. I have something almost similar (MB-495SX Rev1.1) that stays stone-dead with anything but Intel chips.

If you have a dump of its BIOS, I'm curious. It wouldn't be a hardware problem, right? At least for AMD chips aren't they usually jumpered the same as Intel anyway?

Here's the BIOS:

And here's the elhvb page on it. VLB slot is indeed non-functional, tried with all my VLB VGAs, only after checking that the +5v and GND pins are conform to the VLB standard. Which they are, but nothing works in that slot. Sweet, I know.

quicknick wrote on 2020-09-06, 23:00:

Here's the BIOS:

VER1.1_495SX (486, OPTi495SX).rar

And here's the elhvb page on it. VLB slot is indeed non-functional, tried with all my VLB VGAs, only after checking that the +5v and GND pins are conform to the VLB standard. Which they are, but nothing works in that slot. Sweet, I know.

Interesting. This BIOS's CPU detection is indeed missing tests for 486 Cyrix CPUs (though 486SLC/DLC are supported). Nor is any 486 later than the 486DX2 detected. There is a catch-all "80486" case for unknown CPUs though. I'm surprised AMD CPUs don't work so long as they are plain 486DX or 486DX2 though. No surprise that 5x86 or Enhanced Am486 might not be supported.

Could this be a 386 board with the bare minimum changes to make a 486 work?

Yes, the board supports the 386 but only the PQFP variant. Too bad I don't have one to solder it down. I wonder if it would be possible to choose by jumpers the active CPU (386/486). Anyway, what puzzles me is the presence of a non-working VLB slot.

I made a bios dump from my board.. Dont know if it could be faulty or not.

40-0100-001266-00101111-040493-UMC491
It worked.

No problems with disk or floppy controller?

Are there people here that use deoxide to clean the ISA and vlb slots?

I had a pci and agp slots not work at all and then after a thorough cleaning with a electronics cleaner spray they worked and recognized all the cards I tried in them

PCPartner (Sapphire): nope. the 001266 part in your string indicates Modula Tech. PcPartner would have been xx8045. the elhvb page has 001107 in that area which indicates DataExpert. See if it has an EXPxxxx marking somewhere or even a simple four position number like 4407. For PcPartner it would need a 35-xxxx-xx marking somewhere.

Dont think there isnt any EXPxxxx mark or sticker.

Not even on the bottom ISA slot ?