Hardware Reference
In-Depth Information
Flex ATX power supplies include either a 20-pin or 24-pin main motherboard power con-
nector and a 4-pin +12V connector for the motherboard. They also usually include stand-
ard 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
