Hardware Reference
In-Depth Information
Main power connector. This is sometimes called a
soft-off switch
because this is the name
of the Advanced Configuration Power Interface (ACPI) state when the system is off but
still receiving standby power.
ThePS_ONsignalcanbemanipulatedphysicallybythecomputer'spowerswitchorelec-
tronically by the motherboard under software control. PS_ON is an
active low signal
,
meaning the power supply voltage outputs are disabled (the system is off) when the
PS_ONishigh(greaterthanorequalto2.0V).Thisexcludesthe+5VSB(Standby)onpin
9 of the ATX main power connector, which is active whenever the power supply is con-
nected to an AC power source. The power supply maintains the PS_ON signal at either
3.3V or +5V. This signal is then routed through the motherboard to the remote switch on
thefrontofthecase.Whentheswitchispressed,thePS_ONsignalisgrounded.Whenthe
power supply sees the PS_ON signal drop to 0.8V or less, the power supply (and system)
is turned on. Thus, the remote switch in a system using an ATX or newer power supply
carries up to only +5V of DC power, rather than the full 120V-240V AC current like that
of the older AT/LPX form factors.
The actual power switch used in ATX systems is normally a tiny momentary contact push
buttonswitch,whichisconnectedtothemotherboardfrontpanelheaderviaatinytwo-pin
connector (see
Figure 18.16
)
. When the button is pushed, the motherboard then grounds
the PS_ON signal in the main 20/24-pin power connector, causing the power supply to
turn on.
Figure 18.16
ATX power switch.
