A power supply unit (PSU) is the component that supplies power to the other components in a computer. This device is designed to convert general-purpose alternating current (AC) to low-voltage DC power for the internal components of the computer. 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 additional connector for +3.3 V. There are three different AT Motherboard Power Supply Pinout found in market: P8 AT power supply connector, P9 AT power supply connector and P10 AT power supply connector. Below tables show each of AT Motherboard Power Supply Pinout mentioned above.
P8 AT power supply connector pinout
| PIN | Name | Color | Description |
| 1 | PG | Orange | Power Good, +5 VDC (when all voltage has 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 pinout
| 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 |
P10 AT power supply connector pinout
| PIN | Name | Color | Description |
| 1 | GND | Black | Ground |
| 2 | GND | Black | Ground |
| 3 | GND | Black | Ground |
| 4 | 3.3V | Green | +3.3 VDC |
| 5 | 3.3V | Green | +3.3 VDC |
| 6 | 3.3V | Green | +3.3 VDC |
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