Before the power button is even pressed, the motherboard must establish baseline voltages to listen for a wake signal.
The SIO pulls the "Green Wire" on the ATX 24-pin connector to Ground, telling the PSU to turn on all main rails (+12V, +5V, +3.3V). Stage 3: Power Rails & DRAM (S0 State) desktop motherboard power sequence pdf exclusive
The SIO chip sends the Resume Reset (RSMRST#) signal (typically 3.3V) to the Southbridge (PCH) to indicate standby power is stable. Power Button Press: Before the power button is even pressed, the