AT motherboard power supply pinout signals @ PinoutGuide.com

12 pin 2x MOLEX 15-48-0106 connector layout

at the computer

AT PSU's were used before ATX become to be standard.

An AT power supply delivers +5 V, +12 V, -5 V and -12 V voltages using two six-pin connectors. A few newer boards used an aditional connector for +3.3 V. Note, that you must install these connectors to the motherboard in a way that the black wires are placed on the center, or your equipment will be damaged.

P8 AT power supply connector

Pin	Name	Color	Description
1	PG	Orange	Power Good, +5 VDC when all voltages have stabilized.
2	+5V	Red	+5 VDC (or n/c)
3	+12V	Yellow	+12 VDC
4	-12V	Blue	-12 VDC
5	GND	Black	Ground
6	GND	Black	Ground

P9 AT power supply connector

Pin	Name	Color	Description
1	GND	Black	Ground
2	GND	Black	Ground
3	-5V	White or Yellow	-5 VDC
4	+5V	Red	+5 VDC
5	+5V	Red	+5 VDC
6	+5V	Red	+5 VDC

Cable connector is Molex 90331.

The original PC debuted in 1981 and used two cables to connect the PSU (power supply) to the motherboard. The two cables plug side by side into the motherboard connectors. Sometimes they are keyed so they only plug in one way and sometimes they aren't. Even if they're keyed you can insert them the wrong way if you put a little effort into it. You always have to remember to plug them in so the black wires are next to each other.

In old PCs, almost all of the chips ran directly off of the 5 volt rail. As a result the PSU delivers most of its wattage at 5 volts. There are three or four lines dedicated to the 5 volt rail. The other main rail is 12 volts. That was used primarily to run disk drives, motors, and fans. The two negative rails are bias supplies which only have to provide small amounts of current.

P10 AT aux power supply connector (found on some Dell socket 5, 7 and 8 motherboards). The proprietary 3.3V connector is not only for Dell, but also IBM (especially Power Series) and Intel (e.g. Atlantis Socket 5 and similar Socket 7 mainboards). According to Intel documentation (e.g. Intel Advanced/AS mainboard), the pinout stays the same as shown in the document.
The 3.3V has been used exclusively to power PCI slots - while on-board internal regulator usually makes 3.3V for chipset and CPU, it may be too weak for some PCI boards so in early mainboards it is not routed to slots.