AT Power Supply (original IBM PC XT AT standard)
+12 V (typically for disk drives and cooling fans)
-5 V (typically for peripherals on the ISA bus)
-12 V (typically for RS-232 serial port negative rail)
Power Good – pin used to prevent digital circuitry operating while the power supply turns on and output is not yet properly stable.
Typical power rating 63.5 watts (most of it on the +5 V rail)
Power is turned on and off at the mains (no energy saving low-power mode).
ATX Power Supply
+3.3 V sense (should be connected to the +3.3 V at the motherboard to allow for remote sensing of the voltage drop in the power supply wiring)
-5 V – absent in current power supplies (optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3)
Power good (PWR_OK) – low when other outputs have not yet reached, or are about to leave, correct voltages (typically remains low for 100–500 ms after the PS_ON# signal is pulled low)
Power on (PS_ON#)- pulled up to +5 V by the PSU, motherboard drives low to turn on the PSU.
+5 V standby (+5VSB) – provides a small amount of standby power when the computer is “off”
Positive supply voltages must be within ±5% of their nominal values at all times. Negative supply voltages must be within ±10%.