The SIO acts as an AND gate. When it receives both PWR_OK (from the PSU) and ALL_SYS_PWRGD (from local rails), it outputs SYS_PWROK to the PCH.
Powers the Super I/O chip and the Southbridge/PCH (Platform Controller Hub). desktop motherboard power sequence pdf exclusive
The system is now sitting quietly in the S5 state, waiting for user input. The SIO acts as an AND gate
The user presses the front-panel power button. This temporarily shorts the high-signal line to the ground, creating a High-to-Low-to-High pulse sent directly to the SIO chip. desktop motherboard power sequence pdf exclusive