ATX
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, FlexATX 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 BTX standard, intended as a replacement for ATX. As of 2007 the ATX form factor remains the industry standard for do-it-yourselfers; BTX has however made inroads into pre-made systems, being adopted by computer makers like Dell, Gateway, and HP.
The official specifications were released by Intel in 1995, and have been revised numerous times since, the most recent being version 2.2, released in 2004.
A full size ATX board is 305mm wide by 244mm deep (12" x 9.6" ). This allows many ATX form factor chassis to accept microATX boards as well.
AT-style computer cases had a power button that was directly connected to the system computer power supply (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 turned off via software. However, many ATX power supplies have a manual switch on the back to ensure the computer is truly off and no power is being sent to the components. With this switch on, energy still flows to the components even when the computer appears to be "off." This is known as soft-off or standby and can be used for remote wake up through Wake-on-Ring or Wake-on-LAN, but is generally used to power on the computer through a front switch.
The power supply's connection to the motherboard was changed. Older AT power supplies had two similar connectors that could be accidentally switched, usually causing short-circuits and irreversible damage to the motherboard. ATX used one large, keyed connector instead, making a reversed connection very difficult. The new connector also provided a 3.3 volt source, removing the need for motherboards to derive this voltage from one of the other power rails. Some motherboards, particularly late model AT form factor offerings, supported both AT and ATX PSUs.
ATX was originally designed with the power supply drawing air into the case and exhausting it down onto the motherboard. The plan was to deliver cool air directly to the CPU's and power regulation circuitry's location, which was usually at the top of the motherboard in ATX designs. This was not particularly useful for a variety of reasons. Early ATX systems simply didn't have processors or components with thermal output that required special cooling considerations. Later ATX systems with significantly greater heat output would not be aided in cooling by a power supply delivering its often significantly heated exhaust into the case. As a result, the ATX specification was changed to make PSU airflow optional.
With the introduction of the Pentium 4, the standard 20-pin ATX power connector was deemed inadequate to supply increasing electrical load requirements. The standard was revised with an extra 4-pin, 12-volt connector. This was later adopted by Athlon XP and Athlon 64 systems. Various high-end systems may have other forms of supplemental power connections.
Because video card power demands have dramatically increased over the 2000s, some high-end graphics cards have power demands that exceed AGP or PCIe slot capabilities. For these cards, supplementary power was delivered through a standard power connector like those used for hard drives or floppy drives. PCI Express-based video cards manufactured after 2004 typically use a standard 6 or 8-pin PCIe power connector directly from the PSU.
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). This allows re-use of an old PC power supply for tasks other than powering a PC, but one must be careful to observe the minimum load requirements of the PSU.
The ATX form factor has had five main power supply designs throughout its lifetime:
- ATX — 20 pin connector (Used for all of pentium range)
- WTX — 24 pin connector (Xeon and Athlon MP)
- AMD GES — 24 pin main connector, 8 pin secondary connector (some dual-processor Athlon)
- EPS12V — 24 pin main connector, 8 pin secondary connector, optional 4 pin tertiary connector (Xeon and Opteron) defined in SSI specification
- ATX12V — 20 pin main connector, 4 pin secondary connector, 8 pin tertiary connector (Pentium 4 and mid/late Athlon XP & Athlon 64)
- ATX12V 1.3 — guidance for the −5 volt feed was removed. This was only used by legacy ISA add-in cards.
- ATX12V 2.0 — 20 pin main connector, 4 pin secondary connector (Pentium 4, Core 2 Duo, and Athlon 64 with PCI Express)
- ATX12V 2.1 — One 20-pin connector, one ATX12V 4 pin connector. Many power supply manufacturers include a 4 plus 4 pin, or 8 to 4 pin secondary connector instead, which can also be used as the secondary EPS12V connector.
- ATX12V 2.2 — One 24-pin connector, one ATX12V 4 pin connector. Main Power Connector changed from 20 pin to 24 pin to support PCI-Express requirements.
| Color | Signal | Pin | Pin | Signal | Color |
|---|---|---|---|---|---|
| +3.3 V | 1 | 13 | +3.3 V sense | ||
| +3.3 V | 2 | 14 | −12 V | ||
| Ground | 3 | 15 | Ground | ||
| +5 V | 4 | 16 | Power on | ||
| Ground | 5 | 17 | Ground | ||
| +5 V | 6 | 18 | Ground | ||
| Ground | 7 | 19 | Ground | ||
| Power good | 8 | 20 | −5 V (optional) | ||
| +5 V standby | 9 | 21 | +5 V | ||
| +12 V | 10 | 22 | +5 V | ||
| +12 V | 11 | 23 | +5 V | ||
| +3.3 V | 12 | 24 | Ground |
Dell power supplies
Older Dell computers, particularly those from the Pentium II and III times, are notable for using proprietary power wiring on their power supplies and motherboards. While the motherboard connectors appear to be standard ATX, and will actually fit a standard power supply, they are not compatible. Not only have wires been switched from one location to another, but the number of wires for a given voltage has been changed. Thus, the pins cannot simply be rearranged.
The change affects not only 20-pin ATX connectors, but also auxiliary 6-pin connectors. Modern Dell systems may use standard ATX connectors.Dell PC owners should be careful when attempting to mix non-Dell motherboards and power supplies, as it can cause damage to the power supply or other components. If the power supply color coding on the wiring does not match ATX standards, then it is probably proprietary. Wiring diagrams for Dell systems are usually available on Dell's support page.
Connectors
On the back of the system, some major changes were made. The AT standard had only a keyboard connector and expansion slots for add-on card backplates. 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 an 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 the I/O plates are usually 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-style mini-DIN keyboard and mouse connectors ubiquitous. AT systems used a 5 pin DIN connector for the keyboard, and were generally used with serial port mice (although PS/2 mouse ports were also found on some systems). Many modern motherboards are phasing out the PS/2-style keyboard and mouse connectors in favor of the modern standard of USB ports. Other legacy connectors that appeared on ATX motherboards but are being phased out include 25-pin parallel ports and 9-pin RS-232 serial ports. In their place, on-board Ethernet, Firewire, eSATA and audio ports are increasingly common.
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