Question about Intel ASUS P4C800-E Deluxe 875P Socket 478 DDR 400 ATX Motherboard 3-in-1 Combo (MP4C800EDLXCOMBO)

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PS_ON revertsto high as soon as +12V starts to appear on 2+2connector

Can't start system. If 12V2+2 connector plugged into MB, PS_ON goes back high right after going low. If 12V 2+2 not plugged in, power stays up, fans run, voltages check OK. All true with peripherals removed, only CPU and a couple HDDs to load the PS.
What conditions cause PS_ON logic to reject power when 12V2+2 connector is connected?


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  • JAStarr Nov 20, 2007

    thanks, 24. Presumably, the overcurrent would be the result of short or excess peripherals on MB, and I think the 12V 4-pin connector only serves the CPU, so it might be implicated. Actually, I think the chip controlling the PS_On is more likely. But the big question is can the rise time of some PSUs be unacceptable to that PS_On logic? How many PSUs should I try to be sure the problem is the MB? or can I get the PS_On logic so I can watch the startup events with scope to see what's tripping it?

  • JAStarr Nov 23, 2007

    24, process you described would determine at what point the PSU trips. I need to know what causes the PS_On to trip on the MoBo--it is happening before the PSU ever gets up to 12V--it maybe gets to 2V before PS_On reverts to high (I don't have a storage scope, so can only watch the transient events as they happen). I've tried a third PSU now; same result--it's the MoBo. I need the logic of PS_On and what chip ANDs the required conditions together to pull PS_On low so I can see what's making it go back up, Know where I can find that? Thx.



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No need to get testy--no pun intended--I do appreciate your thoughts and help. What I want is to save the MoBo if I can. I'm sure it is deciding to turn off the PSU for a reason. I know the PSU will shut down if there is too much current draw, but I don't know what bad conditions the MoBo will use to shut down--probably voltage not reached in x msec, maybe overvoltage, and maybe in my case, the logic chip that ands the necessary conditions together and latches PS_On low is burnt. Today I'll try tracing the 12V from that connector on the board--probably doesn't serve much more than the CPU. Maybe I can find something.

Posted on Nov 24, 2007

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Usually it is over-current that will make PS_ON go high. However, the opposite is true: if not enough load draw, PS_ON goes high and never goes low. Not exactly certain about link to 12V2+2 connector, although same theory may apply.

Posted on Nov 20, 2007

  • Marty marty Nov 20, 2007

    set up a bench test for supply as follows:
    connect all but 12V 4-pin connector to mobo. now connect a load rheostat (a variable resistor to emulate various loading) from the 12V 4-pin connector to ground. Also connect scopt to PS_ON. Start with about 1 amp draw. Repeat process until you see a fault on PS_ON. Then determine if your total current draw exceeds supply nameplate. Alternatively, you can measure resistance from the 12V 4-pin connector on mobo to ground and calculate same load.

  • Marty marty Nov 24, 2007

    ok, just one thing...if you say "'s the MoBo. I need the logic of PS_On and what chip ANDs the
    required conditions together to pull PS_On low so I can see what's
    making it go back up, Know where I can find that?"...then focus on the mobo. If you want to force PS_On HIGH, then push that wire out of its socket and the mobo should continue to have power....may harm the mobo, but what is that you really want to accomplish...a new mobo?



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