What does soft on/off mean for ATX power connector on mobo?

I'm looking at my user manual for my Gigabyte GA-K8NSNXP-939 mobo and for Pin # 14 it says PS_ON(soft on/off). What does that mean? Thanx.

I ran into a problem with this myself when converting a case from AT to ATX. Back in the prehistoric times (1995 or so) just before ATX came out, we were all using AT form factor cases. One thing about AT that ATX addressed was that the power switch was a mechanical power switch actually connected to the power supply itself. Flip the switch, it turns on, it turns off, nothing spectacular, no auto-shut down when you close Windows. This was called "hard" switching as it was a physical toggle. ATX power supplies, however, are turned on by an electric signal from the motherboard, which can also be addressed by software allowing auto shutdowns and startups. For this reason, the power switch is actually connected to the motherboard by the PS_ON you describe. There is no actual "switching", the switch is just a "momentary" switch that isn't toggled, but temporarily bridges those 2 pins when it is pressed, completing the circuit there and telling the mobo to send the signal to the power supply to turn on. After you let go, the switch goes back to its previous state of "not connecting" and not doing anything, hence the term "momentary" switch. Press it again, and it connects the circuit again and the mobo knows to tell the power supply to turn off. What does it tell you that the mobo is able to know to turn the power supply on even though it is off itself? There is only one explanation - it means there is always a small amount of current going through even when the system is off, so it is never really completely off. This is called "soft" power switching because physically, the power supply is constantly on, and you are using a "virtual" power switch that is really only sending electric signals to the motherboard and is not actually turning off power. There is often another switch on the power supply itself that physically turns it off so there is zero current running through the PC (best to do this when you got the case open fellas).

This is also why sometimes when your comp hangs, the power switch doesnt do anything and you have to unplug it from the wall or flip the mechanical power supply switch. The mobo is in an unresponsive state and your repeated attempts to send the signal to turn off by pressing the power switch don't do anything.

the way it works btw is that the mobo hold pin 14 low (gnd)

also even if your mobo is completely frozen, holding the power button for like 4~5 seconds will turn it off...

http://www.pcguide.com/ref/power/sup/func.htm

Soft Power (Power On and 5V Standby Signals)

Early PCs using the PC/XT, AT, Baby AT and LPX form factors all use a mechanical switch to turn the computer on and off. Newer form factors, starting with the ATX/NLX, and including the SFX and WTX, have changed the way the power supply is turned on and off. Instead of using a physical switch, these systems are turned on by a signal from the motherboard telling the power supply what to do. In turn, the motherboard can be told to change this signal under software control. This is what allows Windows to shut the power down to a PC, or what allows such features as turning a PC on from a button on the keyboard. This feature is called "Soft Power" and the signal that controls the power supply is called "Power On", or alternately, "PS On" or "PS_On".

This feature would seem to create a small "chicken and egg" situation however. How can the motherboard tell the power supply to turn on, electronically, when the motherboard is also off due to not having any power from the supply? :^) The answer is the other "Soft Power" signal, which is called "+5 V Standby" (or "+5VSB", or "5VSB", etc.) This signal is the same output level as the regular +5 V lines from the power supply, but is independent of the other provided voltages and is always on, even when the rest of the power supply is turned off. A small amount of current on this wire is what allows the motherboard to control the power supply when it is off. It also permits other activities that must occur while the PC is off, such as enabling wakeup from sleep mode, or allowing the PC to be turned on when activity is detected on a modem ("Wake on Ring") or network card ("Wake on LAN").

The WTX form factor also includes a similar standby signal for +3.3 V. See the next section for more

Click to expand...

which adds more to the "why" of Synful Serenity's excellent explaination

also even if your mobo is completely frozen, holding the power button for like 4~5 seconds will turn it off...

Click to expand...

Well that's true for the most part, but there are times when a serious lockup (such as due to OCing) will not allow you to turn off the comp, which is already accomplished by holding the power button down for a few seconds in most cases. You can usually change this to Instant in the BIOS so all you have to do is press it without holding it down....either way, if you find yourself unable to turn off your comp, it's because the system is in a state where it can't send any signals, so flip the switch in the back or unplug it......And thanks for the kind words Czar

Its alway nice to see new knowledgeable members
we are trying to make this a very good power supply forum, there arent many around

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