The user presses the power button. The SIO sends a signal to the PCH, which responds by pulling down the PSON# line to turn on the main 12V, 5V, and 3.3V rails from the power supply.
Attach a logic analyzer or oscilloscope to Pin 2 (Data Output / DO) or Pin 5 (Data Input / DI) . foxconn 115xdbp motherboard schematic
Steps I can think of: