12 pin 2x MOLEX 15-48-0106 connector at the computer motherboard.
6 pin 2x MOLEX 90331-0001 connector at the Powersupply cables.
The AT motherboard power connector is a split arrangement of two separate 6-pin Molex connectors, designated P8 and P9, introduced with the IBM PC AT (5170) platform in the mid-1980s and used through the Baby AT era into the mid-1990s. The power supply delivers +5 V, +12 V, -12 V, and -5 V rails along with a Power Good signal, but provides no soft power-off capability or standby voltage — the system can only be switched off by cutting mains power or using a mechanical switch. ATX superseded this arrangement in 1995–1996 with a single keyed 20-pin connector that added +5VSB standby, PS_ON# remote power control, and a +3.3 V rail.
P8
| Pin | Name | Color | Description |
|---|---|---|---|
| 1 | PG | Orange | Power Good, +5 VDC when all voltages 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
| 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 |
Notes
- When installing P8 and P9 onto the motherboard header, the two black ground wires must face each other at the centre of the combined 12-pin socket. Swapping P8 and P9 is catastrophically destructive to the motherboard despite each connector being individually keyed.
- Pins part number is 08-50-0276, Product specification is PS-90331.
- The AT power supply provides no standby voltage and no soft power-off signal; there is no equivalent of ATX PS_ON# or +5VSB — the PSU output is entirely controlled by the mains switch.
Signals
- PG (Power Good) — asserts +5 V when all output rails have stabilised within regulation; the motherboard holds the CPU in reset until this signal goes high.
- +12V — supplies +12 V for drive motors, fans, and serial-port level shifters.
- -12V — supplies -12 V, primarily for RS-232 serial-port transceivers.
- -5V — supplies -5 V, used by legacy ISA bus cards and early DRAM; absent or unused on most post-486 systems.
- +5V — primary logic supply for the CPU, memory, and ISA/PCI bus.
- GND — common ground reference for all rails.