System information

Power Supply Form Factors
139
Disassembly
and Power
4
Tech
Tip
Tech
Tip
Tech Tip
Figure 4.19 shows a few ATX power supply connectors. Figure 4.20 shows more ATX power
supply connectors.
4-pin CPU power connector SATA power connector 6-pin PCIe power connector
Figure 4.19 Common power supply connectors
Not all 24-pin motherboard connectors accept 20-pin power supply
connectors
You can purchase a 24-pin to 20-pin power adapter. The site http://www.formfactors.org
provides information regarding power supply form factors.
CPU 8-pin
connector
Fan speed
monitor connector
PCIe power
connector: 6-pin
4-pin Molex
power connector
4-pin floppy
power connector
Main power
connector: 24-pin
P4 CPU 4-pin
connector
Figure 4.20 ATX power supply connectors
Figure 4.21 illustrates the compatibility between the ATX 20- and 24-pin motherboard
connector standards. Notice in Figure 4.21 that the power cable is only one connector, notched
so the cable inserts into the connector one way only. This is a much better design than older
power supplies, where two connectors were used and could be reversed. Also, notice that a
power good signal (labeled PWR_OK in Figure 4.21) goes to the motherboard. When the
computer is turned on, part of POST is to allow the power supply to run a test on each of
the voltage levels. The voltage levels must be correct before any other devices are tested and
allowed to initialize. If the power is okay, a power good signal is sent to the motherboard. If the
power good signal is not sent from the power supply, a timer chip on the motherboard resets
the CPU. Once a power good signal is sent, the CPU begins executing software from the BIOS.
Figure 4.21 also shows the +5vsb connection to provide standby power for features such as
Wake on LAN or Wake on Ring (covered later in this chapter).