Hardware Reference
In-Depth Information
Caution
The continuous presence of the +5VSB power on pin 9 of the ATX main connector means
the motherboard is always receiving standby power from the power supply when connected
toanACsource,evenwhenthecomputer isturnedoff.Asaresult,itisevenmoreimportant
to unplug an ATX system from the power source before working inside the case than it is on
earlier model systems.
The remote switch on ATX and newer designs can only put the system in a soft-off state,
inwhichthesystemappearsoffbutisstillreceivingstandbypower.SomeATXandnewer
power supplies include a hard override AC power switch on the back, which essentially
disconnects AC power from the system when turned off. With the AC switch off, the sys-
tem no longer receives standby power and is essentially the same as being completely un-
plugged from an AC outlet.
Tip
ThedesignoftheATXpowerswitchissuchthatthemotherboardactuallycontrolsthestatus
of the power supply. On systems with full support for ACPI, when you press the power
switch, the motherboard informs the OS to perform an orderly shutdown before the power is
actually turned off. However, if the system is locked up or corrupted, it can remain running
when you press the switch. In that situation, you can manually override the ACPI control by
pressing the switch continuously for more than 4 seconds, which overrides the software con-
trol and forcibly turns off the system.
PC/XT/AT and LPX Power Switches
Theearliestsystemshadpowerswitchesintegratedorbuiltdirectlyintothepowersupply,
which turned the main AC power to the system on and off. This was a simple design, but
becausethepowersupplywasmountedintherearrightofthesystem,itrequiredreaching
around to the right side near the back of the system to actuate the switch. Also, switch-
ing the AC power directly meant the system couldn't be remotely started without special
hardware.
Startinginthelate1980s,systemswithLPXpowersuppliesbeganusingremotefrontpan-
el switches. These were still AC switches; the only difference was that the AC switch was
now mounted remotely (usually on the front panel of the chassis), rather than integrated
in the power supply unit. The switch was connected to the power supply via a four-wire
cable,andtheendsofthecablewerefittedwithspadeconnectorlugs,whichpluggedonto
the spade connectors on the power switch. The cable from the power supply to the switch
in the case contained four color-coded wires. In addition, a fifth wire supplying a ground
connection to the case was sometimes included. The switch was usually included with the
