Hardware Reference
In-Depth Information
Figure 18.15. Flex ATX power supply dimensions.
Flex ATX power supplies include either a 20-pin or 24-pin main motherboard power connector and a
4-pin +12V connector for the motherboard. They also usually include standard peripheral and floppy
power connectors, with newer units having Serial ATA power connectors as well.
Power Switches
Three main types of power switches are used on PCs. They can be described as follows:
• Front panel motherboard-controlled switch (ATX and newer)
• Front panel power supply AC switch (AT/LPX; obsolete)
• Integral power supply AC switch (PC/XT/AT; obsolete)
ATX and Newer
All ATX and newer power supplies that employ the 20- or 24-pin motherboard connector use the
PS_ON signal to power up the system. In this design, the power supply runs in standby mode when
plugged in with the system off. The PS_ON signal is routed from the power supply through the
motherboard to a low-voltage momentary contact DC switch on the front panel. As a result, the
remote power switch does not physically control the power supply's access to the 120V AC power,
as in older-style power supplies. Instead, the power supply's on or off status is toggled by the PS_ON
signal received on the ATX 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.
The PS_ON signal can be manipulated physically by the computer's power switch or electronically
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_ON is high (greater than or equal
to 2.0V). This excludes the +5VSB (Standby) on pin 9 of the ATX main power connector, which is
active whenever the power supply is connected to an AC power source. The power supply maintains
