Changes to ATX standard were made to support 75 watt PCI Express requirements. Most power is now provided on 12 V rails and the power on 3.3 V and 5 V rails was significantly reduced. The standard specifies that two independent 12 V rails (12 V2 for the 4 pin connector and 12 V1 for everything else) with independent overcurrent protection are needed to meet the power requirements.
New ATX v 2.2 uses new connector, but most motherboards nowdays allow to use an old ATX v 1.x power supply with ATX 20 pin connector - it connects to 24 pin motherboard receptacle.
|8||PWR_OK||Gray||Power Ok is a status signal generated by the power supply to notify the computer that the DC operating voltages are within the ranges required for proper computer operation (+5 VDC when power is Ok)|
|9||5VSB||Purple||+5 VDC Standby Voltage (max 10mA, max 2A in ATX 2.2 spec)|
|13||3.3V||Orange||+3.3 VDC. ATX V2.3 / EPS12V V2.92 both define that the PSU has to use remote sensing to compensate cable drops on the 3.3V line. Because of this there is an additional brown cable crimped together with the orange cable either to pin 13 (ATX) or pin 1 (EPS12V).|
|16||/PS_ON||Green||Power Supply On (active low). Short this pin to GND to switch power supply ON, disconnect from GND to switch OFF.|
|20||-5V||White||-5 VDC (this is optional on newer ATX-2 supplies, it is for use with older AT class expansion cards and can be omitted on newer units)|