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The generation of +3, +5, and +12 VDC is controlled digitally with the PS On signal. When the 
PS On signal is asserted, all DC voltages are produced. When PS On is de-asserted, only 
auxiliary power (+5 AUX) is generated. The +5 AUX voltage is always produced as long as the 
system is connected to a live AC source. 

The PS On signal is typically controlled through the Power Button which, when pressed and 
released, applies a negative (grounding) pulse to the power control logic on the system board. 
The resultant action of pressing the power button depends on the state and mode of the system at 
that time and is described as follows:

Off

Negative pulse, of which the falling edge results in power control logic 

asserting PS On signal to Power Supply Assembly, which then initializes. ACPI 

four-second counter is not active.

On, ACPI Disabled

Negative pulse, of which the falling edge causes power control logic to 

de-assert the PS On signal. ACPI four-second counter is not active.

On, ACPI Enabled

Pressed and Released Under Four Seconds: 
Negative pulse, of which the falling edge causes power control logic to 

generate SMI-, set a bit in the SMI source register, set a bit for button status, 

and start four-second counter. Software should clear the button status bit within 

four seconds and the Suspend state is entered.  If the status bit is not cleared by 

software in four seconds PS On is de-asserted and the power supply assembly 

shuts down (this operation is meant as a guard if the OS is hung). 
Pressed and Held At least Four Seconds Before Release:
If the button is held in for at least four seconds and then released,  PS On is 

negated, de-activating the power supply.