CertiGuide to A+ (Core Hardware)  9  Chapter 0001:  Power Supplies - System Board       9  IV  Power Supply Form Factors

AT Power Supply 1 V  System Board Form Factors

The ATX power supply has a small power lead going to the case and the switch is used as a signaling device to turn the computer power supply on or off. By using signaling, other signals may be installed in the computer such as on a Network Interface Card (NIC).

This feature is marketed under the handle Wake On LAN (WOL). This makes it possible for a network administrator to wake up an entire office full of computers, download updates, for example a new version of word processor.

This saves considerable labor because the administrator did not have to visit each PC. Further, productivity is not lost because computers are upgraded when workers were at home. And users don't have the frustration of incompatible file formats between two different versions of software.

Another feature of the ATX power supply is there's only one connector for the power, making it impossible to reverse the leads as can be done with P8 and P9 on an AT power supply.

Of course differences in how the power switch operates means a different computer case must be used. With the rationale of the two different power supplies, cases are known as AT or ATX cases.

That isn't the only difference. You just learned the connectors between the two power supplies are different. That means the systemboard must be mated to accept the correct type of power connector.

Today, virtually all new computer cases, power supplies and system boards are ATX based. And that doesn't mean the Pentium computer that just blew a power supply is going to use an ATX power supply. If you are going to do any fieldwork, and you are not positive about which power supply resides in an ailing computer, take along both types of power supplies.

AT Power Supply 1 V  System Board Form Factors