Here is a curated list of search terms and sources (no direct links due to copyright, but easily findable):
The SIO pulls the green wire on the 24-pin ATX connector to 0V (ground), telling the PSU to turn on all main rails (12V, 5V, and 3.3V). desktop motherboard power sequence pdf
: For a summary of different voltage requirements (+12V, -12V, etc.), refer to the Motherboard Power Sequence Overview on Scribd . Here is a curated list of search terms
Does the green wire on the PSU drop to 0V when you hit the button? If not, the SIO isn't telling the PSU to start. desktop motherboard power sequence pdf