[tjesh]

I just acquired an Asus board with a plug marked "PWR_FAN"
I just acquired an Antec Trio 650 PS that has a small matching socket with black & blue wires.
This socket has no markings on it that I can see.

Of course the Antec manual doesn't even acknowledge the existence of the latter, and the Asus manual doesn't specify any particular purpose for the former.

Since I don't fathom the logic where the PS fan takes back input from the mb that it is contemporaneously powering, my question is: Should I plug the damn line from the PS into the PWR_FAN pins on the board? tia

 

[crjdriver]

You should have a multiple fan connectors on the board. One will be labeled cpu fan or perhaps just by number ie fan1, etc. Check your manual as to which one you connect the cpu fan to.
The one you describe is most likely for a power supply fan to read rpm. You can connect it or not it really does not matter.

 

[gotrootdude]

Newer Antec power supplies are coming with this plug. It's so you can monitor and adjust the speed of the power supply's fan with software that either comes with the motherboard, or speedfan.

If your not interested in monitoring or adjusting the speed of the fan through the motherboard's fan sensors, then don't worry about plugging it in. Otherwise, any spare fan header on your motherboard will work.

Interesting that your Antec power supply manual doesn't mention it. The latest one I bought mentioned it in the manual. Go figure.

The one you describe is most likely for chassis fan or case fan.

Nope, it's for monitoring the power supply fan.