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
|1||PG||Orange||Power Good, +5 VDC when all voltages have stabilized.|
|2||+5V||Red||+5 VDC (or n/c)|
P9 AT power supply connector
|3||-5V||White or Yellow||-5 VDC|
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.