The P1 power connector is the first ATX power supplies and motherboards used a single power connector. It also has 20 pins.
A P1 connector is a power connector found on ATX power supplies. It is used to provide main power to the motherboard, and to control ACPI functionality (tell the power suppy to turn off and back on).
P1 connector
P1
P1
20 pin P1 connector.
P1
p1
A regular ATX motherboard uses a single 20-pin P1 connector with optional 4-pin auxillary power connector
ATX version 2.2 allowed for 24-pin P1 connector, which is backward compatible with the 20-pin P1 connector.
24
12v, 3.3v 5v
You can identify a motherboard by the position of the CPU, along with the power connectors used. For instance, older baby ATX boards have the CPU positioned directly behind the PCI expansion slots. This can prevent proper seating of the expansion cards, and possibly over-heating to the CPU. The later full ATX (which is also the most common) now a days, but not the latest (BTX is the latest), has the CPU seated away from the expansion slots to provide more room and better ventilation within the computer case. Full ATX use a P1 power connector. P1 has a 20-pin connector that connects to a 24-pin P1 connector on a motherboard (ATX). While the older AT boards and Baby ATX both use P8 & P9 connectors. Sometimes these connections will be marked as "P1" or "P8" & "P9". The P8 connector provides 12volts of power, while the P9 provides 5volts. The later P1 connector added an additional 3.3volts to the already existing technology. This is a logical approach, as P8 & P9 are not combined, while the P1 was introduced combined.