Chapter 14: Power Supply and Electrical Issues^321
set high (turned on) to indicate that the power supply is able to supply a good power
stream in the right voltages. If there are any problems with the power source or the power
supply’s ability to produce a certain voltage stream, the POWER_GOOD signal is not set.
The POWER_GOOD wire is attached to the microprocessor’s timing chip, and if it does not
get the POWER_GOOD signal in the right amount of time, the processor resets the startup
process, which may or may not result in a loo pthat is continually resetting the startu p pro-
cess.Whenthishappens,thePCappearstohavestoppedsomewhereinthebootprocess.Soft Switches
Beginning with the ATX form factor and including most of the form factors that followed,
the motherboard is able to power the PC on or off using what is called asoft switch. The
motherboard uses the PS_ON (power supply on) signal over one of the wires connecting
it to the power supply. How do you know if your PC has this feature? If your PC powers
off—or attempts to—when you shut down Windows 98 (or later), you have it.
On some newer PCs, the power-on switch on the front panel is directly connected to the
motherboard and not the power supply. This createsmomentary-onoralways-onsoft switches
that jump-start the boot process when pressed. When the soft switch is used, the voltage lines
(see Figure 14-4) connecting the power supply to the motherboard are activated. Each of theFigure 14-4. ATX/NLX power supply to motherboard connector and pinouts