AT extended (Motherboard) (ATX)
AT Extended (ATX) is a layout format for motherboards defined by Intel in 1995 and standardized in 1999. It is the further development of the AT form factor with good expandability, better clarity, short connection cables to the drives and the power supply and improved heat dissipation through the fan.
The ATX standard specifies all important parameters such as dimensions, mounting holes, arrangement of connectors, sockets, slots and components. The ATX mainboard has dimensions of 305 mm x 244 mm. ATX boards have 5 to 7 slots for the PCI bus. An ATX variant is the larger EATX board format (extended), which is mainly used in servers and workstations. It has dimensions of 305 mm x 330 mm.
The power supply can be switched to standby mode by software. An important change compared to the AT board is rotation of the board by 90 degrees. The position of the slots remains the same, the position of the central processing unit( CPU) improves.
On the ATX board is defined an area for the connectors and the inputs and outputs. The power supply connection is realized by a 20- pin ATX connector with reverse polarity protection. The ATX board has connectors for sound, com port, parallel ports, USB interface, keyboard and mouse.
Besides the ATX version standardized by the Formfactors organization, there is also a mini-ATX version with fewer slots for memory expansion and expansion cards. It has 4 PCI slots and dimensions of 284 mm x 244 mm. Micro-ATX is it another ATX variant, which is slightly smaller than the Mini-ATX board format with 244 mm x 244 mm. And for use in embedded systems, there is also the Flex-ATX, which has a size of 229 mm x 191 mm and is thus smaller than the Micro-ATX board format.
Motherboard manufacturers do not adhere exactly to the allowed sizes and produce smaller ATX boards, however, the hole spacing complies with the specified dimensions.