ATX (Advanced Technology eXtended) is a motherboard configuration specification developed by Intel in 1995 to improve on previous de facto standards like the AT design.
It was the first major change in desktop computer enclosure, motherboard and power supply design in many years, improving standardization and interchangeability of parts. The specification defines the key mechanical dimensions, mounting point, I/O panel, power and connector interfaces between a computer case, a motherboard and a power supply.
With the improvements it offered, including lower costs, ATX overtook the AT configuration completely as the default design for new systems within a few years.
ATX addressed many of the AT’s designs annoyances which had frustrated system builders.
Other standards for smaller boards (including microATX, FlexATX and mini-ITX) usually keep the basic rear layout but reduce the size of the board and the number of expansion slots. In 2003, Intel announced the BTX standard, intended as a replacement for ATX.
As of 2009, the ATX design remains a standard for do-it-yourselfers; BTX has however made inroads into pre-made systems.
ATX motherboards were designed to solve the problems in BAT and LPX motherboards.
The official specifications were released by Intel in 1995 and have been revised numerous times since. The most recent ATX motherboard specification is version 2.2. The most recent ATX12V power supply unit specification is 2.31, released in February 2008.
A full-size ATX board is 12 × 9.6 in (305 × 244 mm). This allows many ATX chassis to accept microATX boards as well.
How to check Power Supply (ATX/AT)
Source: ElectroDroid and Wikipedia