PG = Power Good
This is a 0/1 signal that the power supply sends after it's turned on and it believes that the voltages are stable.
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Quote from UseNet:
The power good signal is *from* the PSU *to* the motherboard, Quoted from
the original IBM PC/XT hardware reference manual :-
Power on/off cycle: when the supply is turned off for a minimum of 1.0
second, and then turned back on, the power-good signal will be regenerated.
The power-good signal indicates there is adequate power to continue signal
processing. If the power goes below the specified levels, the power-good
signal triggers a system shutdown.
This signal is the logical AND of the dc output-voltage sense signal and
the ac input voltage fail signal. This signal is TTL-compatible up-level
for normal operation or down-level for fault conditions. The ac fail signal
casues power-good to go to a down-level when any output voltage falls below
the regulation limits.
The dc output-voltage sense signal holds ther power-good signal at a down
level (during power-on) until all output voltages have reached their
respective minimum sense levels. The power-good signal has a turn-on delay
of at least 100 ms but no greater than 500ms.