motherboards ignoring power_good (grey wire) ?!
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Posted by jameshanley39@yahoo.co.uk on May 5, 2008, 10:42 am
Please log in for more thread options power_good(grey), and 5VSB(purple) I had read on a pcguide article, something along the lines of, the PSU sends 5V down power_good , and 5VSB. And when the power button is pushed, the motherboard then turns the power supply on (presumably connecting the green and black) Oddly though, I have noticed that for the few computers I have, if I cut the power_good wire on my atx psu , it still works fine. The motherboard is not checking the power_good. any thoughts why it isn`t checking it? TIA | ||||||||||||||||
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Posted by on May 5, 2008, 2:45 pm
Please log in for more thread options jameshanley39@yahoo.co.uk wrote: It's possible the Power_good input on the motherboard floats up to 5V when nothing is connected to it. I remember some very old motherboards, cheap knockoffs of the original vintage-1981 IBM PCs, that ignored the Power_good signal and instead generated their own power-on reset signal through an onboard resistor and capacitor. That may have been done because many power supplies from that era generated improper Power_good signals, using nothing but an R-C delay or even a direct connection to the +5V line, rather than the fairly elaborate circuit found in IBM brand supplies. | ||||||||||||||||
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Posted by Paul on May 5, 2008, 3:04 pm
Please log in for more thread options jameshanley39@yahoo.co.uk wrote:
> I was experimenting with the wires - power_on(green) ,
> power_good(grey), and 5VSB(purple) > > I had read on a pcguide article, something along the lines of, the > PSU sends 5V down power_good , and 5VSB. And when the power button is > pushed, the motherboard then turns the power supply on (presumably > connecting the green and black) > > > Oddly though, I have noticed that for the few computers I have, if I > cut the power_good wire on my atx psu , it still works fine. The > motherboard is not checking the power_good. > > any thoughts why it isn`t checking it? > > TIA Try this reference schematic from Intel. http://download.intel.com/design/chipsets/designex/BXDPDG10.PDF Flip to page 32. Look at "ATX Power Connector". On pin 8 of the connector, there is a signal called "PG1". Notice there is a 8.2K ohm pullup, resistor R216. If you were to cut the wire at the connector for pin 8, the pullup is still present on the motherboard side. With the wire disconnected, the pullup is saying "power still good". To see whether the motherboard pays attention to power_good, you'd have to do two things. Cut the wire feeding pin 8. Then, use a ground wire, to ground pin 8 on the motherboard side. That will send the "power no good" condition to the rest of the motherboard. What should happen, is the motherboard should not POST. The fans should spin (because all rails would be present), but the board shouldn't be able to come out of reset. That motherboard is a dual slot 1, and in addition to checking the state of PSU power_good, the logic on page 32 also checks that VRM #1 and VRM #2 are fully operational. On some of those old boards, the VRM was a plugin module with robust edge connector. On others, it might have been part of the motherboard. Paul | ||||||||||||||||
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Posted by jameshanley39@yahoo.co.uk on May 5, 2008, 6:28 pm
Please log in for more thread options > jameshanle...@yahoo.co.uk wrote:
> > I was experimenting with the wires - power_on(green) ,
> > power_good(grey), and 5VSB(purple) >
> > I had read on a pcguide article, something along the lines of, =A0the
> > PSU sends 5V down power_good , and 5VSB. And when the power button is > > pushed, the motherboard then turns the power supply on (presumably > > connecting the green and black) >
> > Oddly though, I have noticed that for the few computers I have, =A0if I
> > cut the power_good wire on my atx psu , it still works fine. =A0The > > motherboard is not checking the power_good. >
> > any thoughts why it isn`t checking it?
>
> > TIA
>
> Try this reference schematic from Intel. > > http://download.intel.com/design/chipsets/designex/BXDPDG10.PDF > > Flip to page 32. Look at "ATX Power Connector". On pin 8 of the > connector, there is a signal called "PG1". Notice there is a > 8.2K ohm pullup, resistor R216. > > If you were to cut the wire at the connector for pin 8, the pullup is > still present on the motherboard side. With the wire disconnected, > the pullup is saying "power still good". > > To see whether the motherboard pays attention to power_good, > you'd have to do two things. Cut the wire feeding pin 8. > Then, use a ground wire, to ground pin 8 on the motherboard > side. That will send the "power no good" condition to > the rest of the motherboard. > > What should happen, is the motherboard should not POST. The > fans should spin (because all rails would be present), but the > board shouldn't be able to come out of reset. > > That motherboard is a dual slot 1, and in addition to checking the > state of PSU power_good, the logic on page 32 also checks that > VRM #1 and VRM #2 are fully operational. On some of those > old boards, the VRM was a plugin module with robust edge > connector. On others, it might have been part of the motherboard. > I don't know about electronics beyond using a multimeter.. I can cut the grey wire - and have done.. In layman's terms, How would I ground it "at the motherboard side"- to use your terminology? if I look at the atx cable as 2 sides, above the connector is PSU side, below the connector, is MBRD side. I cut the wire above the connector.. But below the connector.. things are a bit cramped!! Considering that the connector is plugged into the MBRD! tx | ||||||||||||||||
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Posted by Paul on May 5, 2008, 9:03 pm
Please log in for more thread options jameshanley39@yahoo.co.uk wrote:
>> jameshanle...@yahoo.co.uk wrote:
>>> I was experimenting with the wires - power_on(green) ,
>>> power_good(grey), and 5VSB(purple) >>> I had read on a pcguide article, something along the lines of, the >>> PSU sends 5V down power_good , and 5VSB. And when the power button is >>> pushed, the motherboard then turns the power supply on (presumably >>> connecting the green and black) >>> Oddly though, I have noticed that for the few computers I have, if I >>> cut the power_good wire on my atx psu , it still works fine. The >>> motherboard is not checking the power_good. >>> any thoughts why it isn`t checking it? >>> TIA >> Try this reference schematic from Intel.
>> >> http://download.intel.com/design/chipsets/designex/BXDPDG10.PDF >> >> Flip to page 32. Look at "ATX Power Connector". On pin 8 of the >> connector, there is a signal called "PG1". Notice there is a >> 8.2K ohm pullup, resistor R216. >> >> If you were to cut the wire at the connector for pin 8, the pullup is >> still present on the motherboard side. With the wire disconnected, >> the pullup is saying "power still good". >> >> To see whether the motherboard pays attention to power_good, >> you'd have to do two things. Cut the wire feeding pin 8. >> Then, use a ground wire, to ground pin 8 on the motherboard >> side. That will send the "power no good" condition to >> the rest of the motherboard. >> >> What should happen, is the motherboard should not POST. The >> fans should spin (because all rails would be present), but the >> board shouldn't be able to come out of reset. >> >> That motherboard is a dual slot 1, and in addition to checking the >> state of PSU power_good, the logic on page 32 also checks that >> VRM #1 and VRM #2 are fully operational. On some of those >> old boards, the VRM was a plugin module with robust edge >> connector. On others, it might have been part of the motherboard. >> >
> I don't know about electronics beyond using a multimeter.. > I can cut the grey wire - and have done.. > > In layman's terms, How would I ground it "at the motherboard side"- to > use your terminology? > > if I look at the atx cable as 2 sides, above the connector is PSU > side, below the connector, is MBRD side. > I cut the wire above the connector.. But below the connector.. things > are a bit cramped!! Considering that the connector is plugged into > the MBRD! > > > tx There are a couple ways to think about it. If you look at the motherboard by itself, pin 8 would be exposed on the solder side. You could make contact there. But, you're missing another opportunity. Cut the gray wire in half. That disconnects the Power_Good signal from the PSU. Then, grab the half of the wire still connected to the motherboard. Ground that end, which should pull down the signal on the motherboard. ------+ gray pin 8 PSU |------X X---------------X Motherboard ------+ ^ Connector | connect | end to GND | Ground HTH, Paul | ||||||||||||||||
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> power_good(grey), and 5VSB(purple)
>
> I had read on a pcguide article, something along the lines of, the
> PSU sends 5V down power_good , and 5VSB. And when the power button is
> pushed, the motherboard then turns the power supply on (presumably
> connecting the green and black)
>
>
> Oddly though, I have noticed that for the few computers I have, if I
> cut the power_good wire on my atx psu , it still works fine. The
> motherboard is not checking the power_good.
>
> any thoughts why it isn`t checking it?