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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.
Next: Additional Power Signals