I can't say 100% for sure from that picture, but the plastic housing for the connector looks to have the same layout as a standard ATX 20pin, as seen here:
http://ps-2.kev009.com/eprmhtml/eprma/h780.htm
(The shape of the plastic around each pin has to match.)

If you can find a pinout for this particular board online (could even be in a user manual) and it turns out to not be the standard layout then you should be able to move the pins around in the 20pin plug coming from your power supply to make it match the board. It's pretty easy to do that using an ATX or molex pin removal tool. If you do that, you obviously want to clearly mark that power supply's cable to indicate that it has been modified so that no one accidentally uses it on a standard ATX board some day.

First thing never to do is assume: you could well be right, but if not hooking up something with the wrong pinout will kill board and maybe PSU too. I don't trust late 1990s pinouts as far as I can throw the systems they're in.

Get a multimeter and measure which pin is connected to which line - the GND pins are easiest to map, but you can map all the rest via pins in the ISA bus. Then you know for sure and can look for a suitable PSU/adapter.

Get a multimeter , set it to continuity test and check if ground and powerlines between ISA and ATX match.
There is a chance that 3.3 line might not even be connected.

Thank you everyone for the responses I will certainly test continuity. I will also test the through holes on the back for the at headers and see if they are connected also for thoroughness.

dionb wrote on 2026-01-26, 21:17:

First thing never to do is assume: you could well be right, but if not hooking up something with the wrong pinout will kill board and maybe PSU too. I don't trust late 1990s pinouts as far as I can throw the systems they're in.

Get a multimeter and measure which pin is connected to which line - the GND pins are easiest to map, but you can map all the rest via pins in the ISA bus. Then you know for sure and can look for a suitable PSU/adapter.

Very true.

Sorry, I meant to say something about doing a continuity test on the ATX connector before proceeding, regardless of what information online says.

I apologize for the crudeness of the diagram

Definitely not standard atx even though it uses a standard plug still working out the other 3 contacts and whether the second plug is needed

I knew they were using non-standard ATX in PII era machines. I've never seen an FX running it though, thanks for the post.

Here's a link that reposts a .ru schematic that looks right from my memory. https://detailspin.com/dell/dell-non-standard … atx-pinout.html

As ever, check the pins though!

Inhibit wrote on 2026-01-27, 16:26:

I knew they were using non-standard ATX in PII era machines. I've never seen an FX running it though, thanks for the post.

Here's a link that reposts a .ru schematic that looks right from my memory. https://detailspin.com/dell/dell-non-standard … atx-pinout.html

As ever, check the pins though!

Holy cow, that page is painful. I really hope that is AI generated and some person didn't feel the need to use all of those words to say the same thing over and over.

Ozzuneoj wrote on 2026-01-27, 16:33:

Holy cow, that page is painful. I really hope that is AI generated and some person didn't feel the need to use all of those words to say the same thing over and over.

Had I looked at the page I may have dug up the original schematic. I think my mind filtered it out in an attempt to preserve my sanity...

That's 1995, before ATX introduction.
The board is - https://theretroweb.com/motherboards/s/dell-d … -p___t-minitowe
Pinout is in manual, for Slot 1 - https://dl.dell.com/manuals/all-products/esup … anual_en-us.pdf
I think it's the same.

Thanks everyone! After chasing the rabbit I believe I have solved my issue of PSU. It would seem that Startech made power supplies that specifically mention the dell model that includes this motherboard and that the PSU is not to be used with any other system or damage will occur
Model - atxpw300dell.
I made an offer and won on one that claims to be new in box. I guess we will see, though it looks spotless and the model on the PSU label matches the box.
I know it's Startech but it should be good enough for now

New problem, I got the power supply plugged it in to the fake atx and the 3.3v plug. I couldn't find out how to turn the board on as I'm guessing the original PSU would have had a switch built in which the new one doesn't . The motherboard has a "soft off" plug near the power header so I thought I would try that but all it does is cause the cpu fan to spin for a second and nothing else. No magic smoke so that makes me hopeful I can find a solution it would seem there must be a way as dell model was listed on the PSU box.

For a second you say? That sounds familiar to PSU short circuit protection.

You are right it does but I'm not sure how soft off works electrically on this board. If it expects latching or non latching.