The ATX specification is a public specification intended for widespread use in many types of system, and it has been written as a specification for the PC industry, designed to add value to the PC.
ATX was developed as an evolution of the Baby-AT form-factor and was defined to address four major areas of improvement: enhanced ease-of-use, better support for current and future I/O, better support for current and future processor technology, and reduced total system cost.
By using a power supply that is specially optimized for ATX, it is possible to reduce cooling costs and lower acoustical noise. An ATX power supply, which has a side venting, allows direct cooling of the processor and add-in cards, making a secondary fan or active heatsink unnecessary in most system applications.
Download the PDF file from Intel at: http://developer.intel.com/design/motherbd/atx.htm
Dowload the PDF file from Teleport at: http://www.teleport.com/~atx/spec/index.htm