One of the most annoying things in PC history is the change from ATX to AT standard. It was one of the few true compatibility breaks. But actually, if you are willing to do some hardware hacking, you can mod an ATX power supply to work with an AT board. First, of course, we have to know what the color codes of the cables mean:
| Black | Ground |
| Brown | +3.3V |
| Red | +5V |
| Yellow | +12V |
| White | –5V |
| Blue | –12V |
Note that the negative voltages (white and blue) were needed for the ISA bus only and might not be present on some newer PSUs, not that I've actually ever seen one without it.
| Green | Power on (PS_ON) |
| Gray | Power OK (PwrOk) |
| Purple | +5V StandBy |
Of these ATX-only cables, only the green one is of real importance. It must be grounded for the PSU to run. Through the gray cable, the PSU can send the mainboard a signal that the voltage is now stable and it can begin operation. The purple one has 5V even when the PSU is turned off (i.e. green not grounded). In a modding project, you could hook up some LEDs to this.
Luckily, the type of power connector AT PSUs had is still in use, though I never quite found out for what. It is known as the AUX6pin connector now. You will need two of them.
You can find the necessary tools, as well as single connectors, at AC Ryan. I suggest you get one of the complete sets, they cost less than the two pin removers they contain. Now all you need is the correct pinout, seen when the connectors are lying flat before you, extruding part to the top:
| Red | Red | Red | White | Black | Black |
| Black | Black | Blue | Yellow | Red | Brown |
Connect one black and the green cable to the power switch of the AT case, or to some sort of switch if you are using an ATX case. Or you can ground the green cable directly and use an external power switch. Isolate the unused cables well.
There are also ATX/AT adapters, but they tend to be rather expensive. Still it might be a viable solution if you don't feel comfortable messing around with cables, or if it's a one-time thing.
As of writing this, I have not actually tried this, though I intend to do so. Main purpose of this page is to document the color codes for my own reference. I will not be held liable for anything you do.