Different revisions of ATX specifications call for different power connectors. What's worse is some computer brands use in their motherboards standard ATX headers with proprietary non-standard pin-outs. In some cases, of course a wrong PSU will not fit mechanically. However, in many cases it will, and you can fry your motherboard if you plug a generic ATX PSU into a branded board and vice versa. Here you will find information on the main power connector P1 of both ATX and some branded PCs, which will help you determine the right replacement power supply.
ATX and ATX12V CONNECTORS
When ATX form-factor was designed, it first employed a 20-pin dual-row P1 with 6A/pin rating. Relative to the old AT-style, it had three new buses: +3.3V, +5VSB and PS_ON# line for remote ON/OFF. Intel® later on introduced so-called ATX12V that differed by an additional 2x2 +12V connector (you can find information on the auxiliary cables here
In the revision 2.0 of the ATX PSU specification, P1 changed to a 24-pin part for higher power. The designations of the original 20 signals left unchanged for backwards compatibility (see the ATX power connector pinout diagram to the right and also see our guide on interchangeability between ATX versions
). The rev.2.0 spec also called for a separate current limit on 2x2 connector, which was referred to as +12V2. In reality however, most manufacturers ignored this requirement and wired both +12V1 and +12V2 to the same output with combined overcurrent protection. In apparent acknowledgment of this fact, Intel's power supply design guide rev.1.2 made this requirement recommended rather than mandatory. Note that the revision numbers of the guides and the power supplies do not coincide. For example, the latest combined PSU Guide rev.1.31 specifies ATX 2.4. Also note that here and everywhere on this page we provide the front view, i.e. the view from the pin side rather than from the wire side. The colors are shown just for reference. Some manufacturers divert from the recommended wires, so don't trust colors too much.
For years, Dell used the same connectors as in standard ATX, but wired differently (see the diagram to the right for the pinouts of their Pentium® II and III PCs, Precision 410 and Dimension 8100). As far as I know, except for Dimension 8100, beginning with Pentium® IV their systems use the standard pin designations.
Apple's Power Mac G3 and some G4 (APG and PCI) power supplies likewise used standard 20-pin socket with custom pin designations. Although most G4 (such as QuickSilver and Gigabit) towers used 22 pins which are mechanically incompatible with any ATX, Mirrored Drive Doors had 24 pins. Note that TRKL refers to a so-called trickle output that is active whenever the computer is plugged in. Basically, it is just a different name of a standby supply that feeds the power-on circuit. By the way, recently Apple "conveniently" removed from their website pin assignments info on most older models.
Some older servers were based on GES specification developed by AMD for their processor. From a technical standpoint, GES pin-out makes more sense than any other one because they grouped together the signals of the same name. They also moved PWR_OK to 8-pin P2 that is used for processor power.
This guide of course is not exhaustive and does not cover all custom configurations. Particularly, Compaq and HP also used incompatible systems. As a rule of thumb, if you have a branded PC, you should suspect it may be incompatible with the industry standard.