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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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motherboard pwr_on pins resistance?
What you should get between those pins of a good PC mobo when you
test with ohm-meter |
#2
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motherboard pwr_on pins resistance?
On 31/01/2011 15:16, mynick wrote:
What you should get between those pins of a good PC mobo when you test with ohm-meter Normally, something other than a dead short. -- Adrian C |
#3
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motherboard pwr_on pins resistance?
On Jan 31, 5:20*pm, Adrian C wrote:
On 31/01/2011 15:16, mynick wrote: What you should get between those pins of *a good PC mobo when you test with ohm-meter Normally, something other than a dead short. -- Adrian C are those directly connected to 'green and black wire' on atx power connector on motherboard |
#4
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motherboard pwr_on pins resistance?
On 31/01/2011 17:42, mynick wrote:
On Jan 31, 5:20 pm, Adrian wrote: On 31/01/2011 15:16, mynick wrote: What you should get between those pins of a good PC mobo when you test with ohm-meter Normally, something other than a dead short. -- Adrian C are those directly connected to 'green and black wire' on atx power connector on motherboard No, there normally is a transistor to switch that - and that is after a circuit powered by the standby 5V supply (for a typical ATX rig) that's involved in other power monitoring stuff. What you can measure as resistance across the contacts could be anything, and not really conclusive. What's the problem? Maybe a read of the following may help http://www.aitechsolutions.net/pchwtrblsht.html -- Adrian C |
#5
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motherboard pwr_on pins resistance?
mynick wrote:
On Jan 31, 5:20 pm, Adrian C wrote: On 31/01/2011 15:16, mynick wrote: What you should get between those pins of a good PC mobo when you test with ohm-meter Normally, something other than a dead short. -- Adrian C are those directly connected to 'green and black wire' on atx power connector on motherboard Not exactly. First, look at the power on button on your PC. It is a normally open, momentary contact switch. When you push the button, it creates a pulse. The logic input on the motherboard, has a pullup to +5VSB, and when you push the button, the logic signal is shorted to ground. A current of a milliamp or less may flow through the switch. (So the front panel switch can be a flimsy piece of crap, and still work. The front panel switch doesn't need a multi-amp current rating or anything.) The signal enters one of the motherboard ICs, and is conditioned. It is eventually converted into an active low signal called PS_ON#, driven by an open collector driver. The motherboard IC means there is no direct path, from front panel power on switch, to the PS_ON# signal. The motherboard IC doing the conditioning, is running off +5VSB at the time you push the button. If +5VSB is not available, then the signal from the switch can't be conditioned, and can't be acted upon. (Note - in the following, I'm illustrating the principle of cause and effect, not the timing. I didn't verify that the timing looks exactly like this. But it gets the idea across.) PWR -----+ +------ Momentary pulse, active low (Mobo | | Header) +---+ PS_ON# -----+ OFF (main | 20 pin) +---------- ON The decoupling is more apparent, if you attempt to turn off the PC, after the PC is booted. They have an option to check a timer, which validates the state change on the PWR switch. You have to press the front panel switch for at least 4 seconds, to get the PC to power off. And the switch can be set in the BIOS, to either do a controlled shutdown of the PC, or do a power off instead. In the following diagram, I'm showing the "immediate" power down option in action (it's how my PC is set up right now in the BIOS). So after the four seconds is up, the power just goes OFF, without warning the OS. This gives a "dirty" shutdown, and potentially needs a CHKDSK later, to fix the file system. PWR -----+ +------ (Mobo | | Header) +---------------+ PS_ON# - 4 sec - |--------- OFF (main | 20 pin) -----------------+ ON For a sample motherboard schematic, you can take a look at this old 440BX design. http://www.intel.com/design/chipsets...x/BXDPDG10.PDF On page 32, B_SUSC drives pin 14 on the ATX power connector. That is the green wire (PS_ON# signal) in the ATX standard. B_SUSC stands for "buffered SUSC signal". On page 18, you can see the creation of the B_SUSC (PS_ON#) signal. A 74F07 open collector driver is used. That is a beefy OC driver, with lots of current sink capability to ground. Modern PCs are probably using something a bit weaker than that. To operate PS_ON#, probably requires sinking a milliamp or two (I don't know the exact figure right off hand). It shouldn't need a lot of beef, but the beauty of the 74F07, is it is more likely to survive all insults. Occasionally, on modern motherboards, the equivalent to the 74F07 function, fails to sink properly to ground (logic 0). The SUSC# signal is coming from the Southbridge. So that is where the "conditioned" control signal, comes out of the motherboard chipset in this case. Now, still on page 18, you can see in the Power Management section of the Southbridge IC, they have a "PWRBT#" (Power Button) signal, which is active low. That is the signal the Southbridge is going to be looking for a pulse on. The power button circuit is back on page 32. And on page 32, they kinda ruined my explanation. They chose to use a momentary high pulse from the switch (switch pulls to 3VSB), plus a CMOS Schmitt trigger/inverter to clean up the edge of the signal. The 74LVC14 turns that signal upside-down again, so as the PWRBT# signal leaves page 32, it is an active low pulse. But as far as I know, modern motherboards don't have that additional step. The switch would be set up to pulse low, so the 74LVC14 would not be present. Using an ohmmeter, on the PWR/GND pair on the motherboard panel header, should have little to do with the PS_ON# signal on the main 20 or 24 pin cable, as they're separated by the logic in at least one chip. In the Intel schematic, that was the Southbridge. So ohming from PWR to PS_ON# wouldn't be telling you anything. What you want to do, is check the voltage level on PS_ON# (green wire), while you're fiddling with the front panel power button. If the motherboard open collector driver, pulls the PS_ON# signal towards ground (zero volts), then you should be seeing the power supply fan come on and the main rails pop up. On the input side, you'd monitor the voltage between PWR/GND pair, when you push the front panel button. PWR should drop to zero volts, for as long as the front power button is pushed. Alternately, you can connect the front panel PWR switch to your ohmmeter, and see if it reads zero ohms, when the button is pushed. Sometimes, the flimsy button breaks, and when you push the button, it no longer makes a proper momentary contact. HTH, Paul |
#6
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motherboard pwr_on pins resistance?
On Jan 31, 10:46*am, Paul wrote:
mynick wrote: On Jan 31, 5:20 pm, Adrian C wrote: On 31/01/2011 15:16, mynick wrote: What you should get between those pins of *a good PC mobo when you test with ohm-meter Normally, something other than a dead short. -- Adrian C are those directly connected to 'green and black wire' on atx power connector on motherboard Not exactly. First, look at the power on button on your PC. It is a normally open, momentary contact switch. When you push the button, it creates a pulse. The logic input on the motherboard, has a pullup to +5VSB, and when you push the button, the logic signal is shorted to ground. A current of a milliamp or less may flow through the switch. (So the front panel switch can be a flimsy piece of crap, and still work. The front panel switch doesn't need a multi-amp current rating or anything.) The signal enters one of the motherboard ICs, and is conditioned. It is eventually converted into an active low signal called PS_ON#, driven by an open collector driver. The motherboard IC means there is no direct path, from front panel power on switch, to the PS_ON# signal. The motherboard IC doing the conditioning, is running off +5VSB at the time you push the button. If +5VSB is not available, then the signal from the switch can't be conditioned, and can't be acted upon. (Note - in the following, I'm illustrating the principle of cause and effect, not the timing. I didn't verify that the timing looks exactly like this. But it gets the idea across.) PWR * * *-----+ * +------ *Momentary pulse, active low (Mobo * * * * | * | Header) * * * +---+ PS_ON# * -----+ * * * * * OFF (main * * * * | 20 pin) * * * +---------- ON The decoupling is more apparent, if you attempt to turn off the PC, after the PC is booted. They have an option to check a timer, which validates the state change on the PWR switch. You have to press the front panel switch for at least 4 seconds, to get the PC to power off. And the switch can be set in the BIOS, to either do a controlled shutdown of the PC, or do a power off instead. In the following diagram, I'm showing the "immediate" power down option in action (it's how my PC is set up right now in the BIOS). So after the four seconds is up, the power just goes OFF, without warning the OS. This gives a "dirty" shutdown, and potentially needs a CHKDSK later, to fix the file system. PWR * * *-----+ * * * * * * * +------ (Mobo * * * * | * * * * * * * | Header) * * * +---------------+ PS_ON# * * * *- 4 sec - |--------- OFF (main * * * * * * * * * * | 20 pin) *-----------------+ * * * * *ON For a sample motherboard schematic, you can take a look at this old 440BX design. http://www.intel.com/design/chipsets...x/BXDPDG10.PDF On page 32, B_SUSC drives pin 14 on the ATX power connector. That is the green wire (PS_ON# signal) in the ATX standard. B_SUSC stands for "buffered SUSC signal". On page 18, you can see the creation of the B_SUSC (PS_ON#) signal. A 74F07 open collector driver is used. That is a beefy OC driver, with lots of current sink capability to ground. Modern PCs are probably using something a bit weaker than that. To operate PS_ON#, probably requires sinking a milliamp or two (I don't know the exact figure right off hand). It shouldn't need a lot of beef, but the beauty of the 74F07, is it is more likely to survive all insults. Occasionally, on modern motherboards, the equivalent to the 74F07 function, fails to sink properly to ground (logic 0). The SUSC# signal is coming from the Southbridge. So that is where the "conditioned" control signal, comes out of the motherboard chipset in this case. Now, still on page 18, you can see in the Power Management section of the Southbridge IC, they have a "PWRBT#" (Power Button) signal, which is active low. That is the signal the Southbridge is going to be looking for a pulse on. The power button circuit is back on page 32.. And on page 32, they kinda ruined my explanation. They chose to use a momentary high pulse from the switch (switch pulls to 3VSB), plus a CMOS Schmitt trigger/inverter to clean up the edge of the signal. The 74LVC14 turns that signal upside-down again, so as the PWRBT# signal leaves page 32, it is an active low pulse. But as far as I know, modern motherboards don't have that additional step. The switch would be set up to pulse low, so the 74LVC14 would not be present. Using an ohmmeter, on the PWR/GND pair on the motherboard panel header, should have little to do with the PS_ON# signal on the main 20 or 24 pin cable, as they're separated by the logic in at least one chip. In the Intel schematic, that was the Southbridge. So ohming from PWR to PS_ON# wouldn't be telling you anything. What you want to do, is check the voltage level on PS_ON# (green wire), while you're fiddling with the front panel power button. If the motherboard open collector driver, pulls the PS_ON# signal towards ground (zero volts), then you should be seeing the power supply fan come on and the main rails pop up. On the input side, you'd monitor the voltage between PWR/GND pair, when you push the front panel button. PWR should drop to zero volts, for as long as the front power button is pushed. Alternately, you can connect the front panel PWR switch to your ohmmeter, and see if it reads zero ohms, when the button is pushed. Sometimes, the flimsy button breaks, and when you push the button, it no longer makes a proper momentary contact. HTH, * * * Paul thanks for great explanation so in modern mobos the on switch grounds a pulled up line, straight to southbridge (Possibly there is a Schmitt trigger/inverter in between the two) |
#7
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motherboard pwr_on pins resistance?
On Mon, 31 Jan 2011 20:41:31 -0800 (PST) mynick
wrote in Message id: : so in modern mobos the on switch grounds a pulled up line, straight to southbridge The term southbridge is pretty much dead now. Today it's usually referred to as the ICH or I/O controller hub. |
#8
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motherboard pwr_on pins resistance?
mynick wrote:
thanks for great explanation so in modern mobos the on switch grounds a pulled up line, straight to southbridge (Possibly there is a Schmitt trigger/inverter in between the two) +5VSB +5VSB | | Pullup Pullup Resistor Resistor | PS_ON# | PWR X----+---- Motherboard logic ---- Open -------------------+- ... Collector (to GND X----+ Driver ATX + | supply) | (Front GMD GND Panel Switch) Using an ohmmeter, between PWR and PS_ON#, doesn't tell you anything. There is a silicon chip in the way. Paul |
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