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).
The P1 power connector is the first ATX power supplies and motherboards used a single power connector. It also has 20 pins.
P1 connector
ATX version 2.2 allowed for 24-pin P1 connector, which is backward compatible with the 20-pin P1 connector.
20 pin P1 connector.
P1
P1
A regular ATX motherboard uses a single 20-pin P1 connector with optional 4-pin auxillary power connector
P1
p1
24 pin
24 pin
24