PC Hardware in a Nutshell, 3rd Edition

AT Main Power Connector

Table 26-11 lists the standard AT Main Power Connector pinouts used by all AT power supply variants, including Desktop/AT, Tower/AT, Desktop/BAT, Tower/BAT, and LPX. PC/XT power supplies use the same pinouts, except that Pin P8-2 is unused. In addition to supplying various standard voltages, this connector includes a special-purpose pin. Pin P8-1, PG, carries the Power Good signal, which the power supply asserts once it has started and stabilized. The motherboard will not attempt to boot until the power supply asserts PG.

Table 26-11. Standard motherboard connector pinouts for an AT power supply

Pin	Color	Signal		Pin	Color	Signal
P8-1	orange (white)	PG		P9-1	black	ground
P8-2	red	+5V		P9-2	black	ground
P8-3	yellow	+12V		P9-3	white (blue)	-5V
P8-4	blue (brown)	-12V		P9-4	red	+5V
P8-5	black	ground		P9-5	red	+5V
P8-6	black	ground		P9-6	red	+5V

Note that P8 and P9 are separate connectors from the power supply, but mate to a combined connector on the motherboard.

Warning

The P8 and P9 connectors are individually keyed, but it is possible to connect P8 from the power supply to the P9 connector on the motherboard and vice versa, with potentially catastrophic results.

P8 and P9 were IBM’s original designation for these connectors. Not all power supplies or motherboards label these connectors, so you may have to determine which is P8 and which is P9 by examining wire colors. The colors shown are those most commonly used, but ...