I have never seen ATX socket3 or 4. I am quite certain it doesn't exist. Even ATX for socket7 isn't all that common.

If you get an ATX to AT motherboard power adapter cable then you can make an AT board work just fine in an ATX case (or an AT case with an ATX PSU), does that count? 😀

I've used these cables to get a 286 in an AT case with an ATX PSU (after the original AT PSU died), and a 486 in an ATX case with its original ATX PSU, and you'd never know the difference. Just had to swap out the momentary power button in the ATX case for a latching pushbutton and that was it.

ATX was introduced in 1995, well into Pentium era. It was not widely used until Pentium II era though, even Pentium Pro mainboards were mosly non-ATX.
486 ATX boards did not exist back then of course, but maybe some modern 486 powered boards fit ATX standard. Most likely they would use SoC instead of CPU socket though.

In a way they do exist but only as industrial units and often with the cpu integrated either onto the board or as part of a larger chip with the chipset. You may as well scrounge around for something that can emulate 486 performance and is atx.

Anonymous Coward wrote:

I am quite certain it doesn't exist. Even ATX for socket7 isn't all that common.

I find that ATX Socket 7 is quite common, for some time I originally thought there were only ATX Socket 7 boards due to lack of seeing an AT S7 mobo.

Dell (possibly others too) made ATX-like Socket 3 486 PCs in an LPX form factor for their Optiplex range. The power connector was identical to 20-pin ATX, but possibly wired differently and it had soft power on/off buttons on the case.

I have 1 Dell motherboard in storage with this connector but can't test if it's still working as the original Dell PSU is long gone and the case was destroyed. I kept the 20-pin plug from the old PSU and the wire colours appear identical to regular ATX, but other than getting it to half switch on (with no POST) with a PSU from a P2 era Dell, it mostly appears dead with any other ATX PSU. That could however be down to a faulty motherboard as opposed to different wiring though.

Some pictures:

https://i.imgur.com/XLAo4wj.jpg
https://i.imgur.com/3avH2xC.jpg
https://i.imgur.com/8xOI7Sq.jpg
https://i.imgur.com/3QPRC4t.jpg

That wire that's hanging off the motherboard was not a repair job done by myself btw and if I recall correctly - is not the reason why it's not working now, as it was previously functioning in that state back in the day.

For reference, the motherboard came from a Dell Optiplex 433/LE

No. The only ones even close were LPX, which is non ATX compatible.

nforce4max wrote:

In a way they do exist but only as industrial units and often with the cpu integrated either onto the board or as part of a larger chip with the chipset. You may as well scrounge around for something that can emulate 486 performance and is atx.

Actually that's a really good point. If you get a PICMG backplane and put a 286/386/486 PICMG board into it then you'll have a pre-Pentium machine in a kind of native ATX system! There seem to be enough of these on eBay although the prices vary quite a lot.

PTherapist wrote:

I have 1 Dell motherboard in storage with this connector but can't test if it's still working as the original Dell PSU is long gone and the case was destroyed. I kept the 20-pin plug from the old PSU and the wire colours appear identical to regular ATX, but other than getting it to half switch on (with no POST) with a PSU from a P2 era Dell, it mostly appears dead with any other ATX PSU.

There are a few different Dell PSU wirings, is yours this Optiplex 20-pin version? If so you might be able to just rewire a PSU plug, or buy an ATX motherboard power extension cable cheap from China and rewire that to make an ATX -> Dell converter cable. There's also this pre-built converter cable which includes a 6-pin Aux connector. However that loose wire originally connected somewhere so there's a good chance the board won't be reliable (if it works at all) without it. If you follow the kinks in the cable (lining them up at right angles like they would have been originally, passing through the second bit of sticky tape), you can probably figure out where it went originally, and there will be a little dot of extra wire at the original attachment point where it broke off.