ATX
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The ATX (for Advanced Technology Extended) form factor was created by Intel in 1995. It was the first big change in computer case and motherboard design in many years. ATX overtook AT completely as the default form factor for new systems. ATX addressed many of the AT form factor's annoyances that had frustrated system builders. Other standards for smaller boards (including microATX flex-atx and mini-ITX) usually keep the basic rear layout but reduce the size of the board and the number of expansion slot positions. In 2003, Intel announced the new BTX standard, intended as a replacement for ATX, but as of February 2006 the ATX form factor remains the industry standard for do-it-yourselfers, however it has made inroads into the pre-made systems, being adopted by computer makers like Dell, Gateway, and HP.
AT-style computer cases had a power button that was directly connected to the system power supply unit (PSU). The general configuration was a double pole latching mains voltage switch with the four pins connected to wires from a four core cable. The wires were either soldered to the power button (making it difficult to replace the power supply if it failed) or blade receptacles were used. An ATX power supply does not directly connect to the system power button, allowing the computer to be powered off via software. However, many ATX power supplies have a switch on the back to ensure no power is flowing to the motherboard (a trickle of energy is normally sent to an ATX-style motherboard even if the computer appears to be "off"). Because the ATX PSU uses the motherboard's power switch, turning on the power in situations that do not utilize an ATX motherboard is possible by shorting the green wire from the ATX connector to any black wire on the connector (or ground).
The power supply's connection to the motherboard was changed. Older AT power supplies had two similar-looking connectors that could sometimes inadvertently be plugged in incorrectly, generally causing short-circuits in the motherboard and causing it to fail. ATX used one large keyed connector instead, which made installation much easier and safer. The new connector also provided a 3.3 volt connection removing the need for motherboards to derive this voltage from one of the other power rails. Some motherboards, particularly late model AT form factor ones, supported both AT and ATX PSUs. With the introduction of the Pentium 4 this connector was deemed inadequate and an extra 4-pin 12 volt connector was added and this was also later adopted by later Athlon XP and Athlon 64 systems. Some high end systems have also used other connectors to bring in more power. Some high power graphics cards also needed extra power but this was generally delivered through a 4-pin molex connector as used for hard drives and CD-ROM drives.
On the back of the system, some major changes were made. The AT standard only had a keyboard connector and the slots where the backplates of plug in cards went. Any other onboard interfaces (such as serial and parallel ports) had to be connected via flying leads to connectors which were mounted either on spaces provided by the case or brackets placed in unused expansion slot positions. ATX allowed each motherboard manufacturer to put these ports in a rectangular area on the back of the system, with an arrangement they could define themselves (though a number of general patterns depending on what ports the motherboard offers have been followed by most manufacturers). Generally the case comes with a snap out panel, also known as a I/O plate, reflecting one of the common arrangements. If necessary I/O plates can be replaced, to suit the arrangement on the motherboard that is being fitted, and are usualy included when purchasing a motherboard. Panels were also made that allowed fitting an AT motherboard in an ATX case.
ATX also made the PS/2 keyboard and mouse connectors ubiquitous. AT systems often used a 5 pin DIN connector for the keyboard, and were generally used with serial port mice (although a PS/2 mouse port was not unheard of).
The official specifications were released by Intel in 1995, and have been revised numerous times since, the most recent being version 2.2[1], released in 2004.
The ATX form factor has had five main power supply designs throughout its lifetime:
- ATX - 20 pin connector (Used through Pentium III and early Athlon XP)
- WTX - 24 pin connector (Pentium II and III, Xeon and Athlon MP)
- AMD GES - 24 pin main connector, 8 pin secondary connector (some dual-processor Athlon)
- ATX12V - 20 pin main connector, 4 pin secondary connector, 8 pin tertiary connector (Pentium 4 and mid/late Athlon XP & Athlon 64)
- EPS12V - 24 pin main connector, 8 pin secondary connector, 8 pin tertiary connector (Xeon and Opteron) defined in SSI specification
- ATX12V 2.0 - 24 pin main connector, 4 pin secondary connector (Pentium 4 and Athlon 64 with PCI Express)
A full size ATX board is 12" wide by 9.6" deep (305 mm x 244 mm). This allows many ATX form factor chassis to accept microATX boards as well.
