terryking228:
The Ardu' grounds the control pin of the ATX
Good idea for many applications!
Is the usual ATX power-on pin a 5V control signal? Did you drive it directly from an Arduino output or through a transistor / driver etc??
Just pull it low through a 1K and the whole thing comes to life. It reads +5V when on standby.
So yes, it's a normal 5V control and I just connect it directly to a digital pin throu' a 1K (I'm sure direct 0R connection would be fine but I'm nervous like that).
PS the ATX provides a "power OK" signal on the grey wire that goes to +5V when the supply considers the power to be stable. My next trick will be to wait and read that until it goes high so that I know I have good power before switching on my load. Not actually done that yet thou'.