ATX

Overview

As of 2012, ATX is the most common motherboard design. Other standards for smaller boards (including microATX, FlexATX, nano-ITX, and mini-ITX) keep the basic rear layout but reduce the size of the board and the number of expansion slots. Dimensions of a full-size ATX board are 12 × 9.6 in (305 × 244 mm) which means microATX boards fit in many ATX chassis. The ATX specifications were patented by David Dent at Intel and co-developed with Astec America, a major manufacturer of power supplies at the time. The inaugural version 1.0 of the ATX specification was unveiled by Intel in July 1995. Since then, ATX has been revised numerous times. The most recent ATX motherboard specification is version 2.2. The most recent ATX12V power supply unit specification is ATX 3.0 released in February 2022.

EATX, or E-ATX (Extended ATX), is a term that refers to motherboards larger than the ATX standard. Motherboards labeled E-ATX commonly share dimensions with the SSI-EEB (Server System Infrastructure - Enterprise Electronics Bay) standard (12 x 13 in), however, it is not a guarantee. While some dual-CPU-socket motherboards have been implemented in ATX, the extra size of EATX makes it the typical form factor for dual-socket systems and for single-socket systems with a large number of memory slots (or more than two memory channels).

BTX

In 2004, Intel announced the BTX (Balanced Technology eXtended) standard, intended as a replacement for ATX. While some manufacturers adopted the new standard, Intel discontinued any future development of BTX in 2006. As of 2026, the ATX design still remains the standard for personal computers.

Connectors

On the back of the computer case, some major changes were made to the AT standard. Originally AT style cases 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.

The ATX standard specified a rectangular area on the back of the system where motherboard manufacturers can arrange ports any way they want. A number of general patterns (depending on which ports the motherboard offers) have been used by most manufacturers. Cases are usually fitted with a snap-out panel, also known as an I/O plate or I/O shield, in one of the common arrangements. I/O plates are usually included with retail motherboards for installation in any suitable case. The computer will operate correctly without a plate fitted, but there will be open gaps in the case which may compromise the EMI/RFI screening and allow the ingress of dirt and random foreign bodies. Panels were made that fit an AT motherboard in an ATX case. Some ATX motherboards come with an integrated I/O plate.

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 more modern Universal Serial Bus. Other legacy connectors that are slowly being phased out of modern ATX motherboards include 25-pin parallel ports and 9-pin RS-232 serial ports. In their place are onboard peripheral ports such as Ethernet, FireWire, eSATA, audio ports (both analog and S/PDIF), video (analog D-sub, DVI, HDMI, or DisplayPort), extra USB ports, and Wi-Fi.

A notable issue with the ATX specification was that it was last revised when power supplies were normally placed at the top of many old computer cases rather than at the bottom, as with many modern computer cases. This has led to some problematic standard locations for ports, in particular the 4/8 pin CPU power, which is normally located along the top edge of the board, convenient for top-mounted power supplies. This makes it very difficult for cables from bottom-mounted power supplies to reach, and commonly requires a special cutout in the back plane for the cable to come in from behind and bend around the board, making insertion and wire management very difficult. Many power supply cables barely or fail to reach, or are too stiff to make the bend, and extensions are sometimes required due to this placement. Modern power supplies often have longer cables to alleviate this issue.

Variants

Several ATX-derived designs have been specified that use the same power supply, mountings and basic back panel arrangement, but set different standards for the size of the board and number of expansion slots. Standard ATX provides seven slots at 0.8 in (20 mm) spacing; the popular microATX size removes 2.4 inches (61 mm) and three slots, leaving four. Here width refers to the distance along the external connector edge, while depth is from front to rear. Note each larger size inherits all previous (smaller) colors area.

AOpen has conflated the term Mini ATX with a more recent 15 × 15 cm (5.9 × 5.9 in) design. Since references to Mini ATX have been removed from ATX specifications since the adoption of microATX, the AOpen definition is the more contemporary term and the one listed above is apparently only of historical significance. This sounds contradictory to the now common Mini-ITX standard (17 × 17 cm (6.7 × 6.7 in)), which is a potential source of confusion. A number of manufacturers have added one to three additional expansion slots (at the standard 0.8 inch spacing) to the standard 12-inch ATX motherboard width.

Form factors considered obsolete in 1999 included Baby-AT, full size AT, and the semi-proprietary LPX for low-profile cases. Proprietary motherboard designs such as those by Compaq, Packard-Bell, Hewlett Packard and others existed, and were not interchangeable with multi-manufacturer boards and cases. Portable and notebook computers and some 19-inch rackmount servers have custom motherboards unique to their particular products.

Although true E-ATX is 12 × 13 in (305 × 330 mm) most motherboard manufacturers also refer to motherboards with measurements 12 × 10.1 in (305 × 257 mm), 12 × 10.4 in (305 × 264 mm), 12 × 10.5 in (305 × 267 mm) and 12 × 10.7 in (305 × 272 mm) as E-ATX. While E-ATX and SSI EEB (Server System Infrastructure (SSI) Forum's Enterprise Electronics Bay (EEB)) share the same dimensions, the screw holes of the two standards do not all align; rendering them incompatible.

In 2008, Foxconn unveiled a Foxconn F1 motherboard prototype, which has the same width as a standard ATX motherboard, but an extended 14.4" length to accommodate 10 slots. The firm called the new 14.4 × 9.6 in (366 × 244 mm) design of this motherboard "Ultra ATX" in its CES 2008 showing. Also unveiled during the January 2008 CES was the Lian Li Armorsuit PC-P80 case with 10 slots designed for the motherboard.

Power supply

The ATX specification requires the power supply to produce three main outputs, +3.3 V, +5 V and +12 V. Low-power −12 V and +5 VSB (standby) supplies are also required. The −12 V supply can be used used to provide the negative supply voltage for RS-232 ports, by one pin on conventional PCI slots primarily to provide a reference voltage for some models of sound cards, or as power for motherboard audio. The 5 VSB supply is used to produce trickle power to provide the soft-power feature of ATX when a PC is turned off, as well as powering the real-time clock to conserve the charge of the CMOS battery. A −5 V output was originally required because it was supplied on the ISA bus. However, it was removed in later versions of the ATX standard once ISA expansion slots became obsolete.

Originally, the motherboard was powered by one 20-pin connector. An ATX power supply provides a number of peripheral power connectors and (in modern systems) two connectors for the motherboard: an 8-pin (or 4+4-pin) auxiliary connector providing additional power to the CPU and a main 24-pin power supply connector, an extension of the original 20-pin version. 20-pin Molex 39-29-9202 at the motherboard. 20-pin Molex 39-01-2200 at the cable. The connector pin pitch is 4.20 mm (about one sixth of an inch).

Four wires have special functions:

PS_ON# (power on) is a signal from the motherboard to the power supply. When the line is connected to ground (by the motherboard), the power supply turns on. It is internally pulled up to +5 V inside the power supply.

PWR_OK ("power good") is an output from the power supply that indicates that its output has stabilized and is ready for use. It remains low for a brief time (100–500 ms) after the PS_ON# signal is pulled low.

+5 VSB (+5 V standby) supplies power even when the rest of the supply wire lines are off. This can be used to power the circuitry that controls the power-on signal.

+3.3 V sense should be connected to the +3.3 V on the motherboard or its power connector. This connection allows remote sensing of the voltage drop in the power-supply wiring. Some manufacturers also provided a +5 V sense wire (typically colored pink) connected to one of the red +5 V wires on some models of power supply; however, the inclusion of such wire was a non-standard practice and was never part of any official ATX standard.

Physical characteristics

ATX power supplies generally have the dimensions of 150 × 86 × 140 mm (5.9 × 3.4 × 5.5 in), with the width and height being the same as the preceding LPX (Low Profile eXtension) form factor (which are often incorrectly referred to as "AT" power supplies due to their ubiquitous use in later AT and Baby AT systems, even though the actual AT and Baby AT power supply form factors were physically larger) and share a common mounting layout of four screws arranged on the back side of the unit. That last dimension, the 140 mm depth, is frequently varied, with depths of 160, 180, 200 and 230 mm used to accommodate higher power, larger fan and/or modular connectors.

Main changes from AT and LPX designs

Original AT cases (flat case style) have an integrated power switch that protruded from the power supply and

sits flush with a hole in the AT chassis. It utilizes a paddle-style DPST switch and is similar to the PC and PC-XT style power supplies.

Later AT (so-called "Baby AT") and LPX style computer cases have a power button that is directly connected to the system computer power supply (PSU). The general configuration is a double-pole latching mains voltage switch with the four pins connected to wires from a four-core cable. The wires are 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 is typically controlled by an electronic switch connected to the power button on the computer case and allows the computer to be turned off by the operating system. In addition, many ATX power supplies have a manual switch on the back that also ensures no power is being sent to the components. When the switch on the power supply is turned off, the computer cannot be turned on with the front power button.

The power supply's connection to the motherboard was changed from the older AT and LPX standards; AT and LPX had two similar connectors that could be accidentally interchanged by forcing the different keyed connectors into place, usually causing short-circuits and irreversible damage to the motherboard (the rule of thumb for safe operation was to connect the side-by-side connectors with the black wires together). ATX uses one large, keyed connector which can not be connected incorrectly. The new connector also provides a 3.3 volt source, removing the need for motherboards to derive this voltage from the 5 V rail. Some motherboards, particularly those manufactured after the introduction of ATX but whilst LPX equipment was still in use, support both LPX and ATX PSUs.

If using an ATX PSU for purposes other than powering an ATX motherboard, power can be fully turned on (it is always partly on to operate "wake-up" devices) by shorting the "power-on" pin on the ATX connector (pin 16, green wire) to a black wire (ground), which is what the power button on an ATX system does. A minimum load on one or more voltages may be required (varies by model and vendor); the standard does not specify operation without a minimum load and a conforming PSU may shut down, output incorrect voltages, or otherwise malfunction, but will not be hazardous or damaged. An ATX power supply is not a replacement for a current-limited bench laboratory DC power supply; instead it is better described as a bulk DC power supply.

ATX power supply revisions

ATX, introduced in late 1995, defined three types of power connectors:

4-pin "Molex connector" – transferred directly from AT standard: +5 V and +12 V for P-ATA hard disks, CD-ROMs, 5.25 inch floppy drives and other peripherals.

4-pin Berg floppy connector – transferred directly from AT standard: +5 V and +12 V for 3.5 inch floppy drives and other peripherals.

20-pin Molex Mini-fit Jr. ATX motherboard connector – new to the ATX standard.

A supplemental 6-pin AUX connector providing additional 3.3 V and 5 V supplies to the motherboard, if needed. This was used to power the CPU in motherboards with CPU voltage regulator modules which required 3.3 volt and/or 5 volt rails and could not get enough power through the regular 20-pin header.

The power distribution specification defined that most of the PSU's power should be provided on 5 V and 3.3 V rails, because most of the electronic components (CPU, RAM, chipset, PCI, AGP and ISA cards) used 5 V or 3.3 V for power supply. The 12 V rail was only used by computer fans and motors of peripheral devices (HDD, FDD, CD-ROM, etc.)

While designing the Pentium 4 platform in 1999/2000, the standard 20-pin ATX power connector was found insufficient to meet increasing power-line requirements; the standard was significantly revised into ATX12V 1.0 (ATX12V 1.x is sometimes inaccurately called ATX-P4). ATX12V 1.x was also adopted by AMD Athlon XP and Athlon 64 systems. However, some early model Athlon XP and MP boards (including some server boards) and later model lower-end motherboards do not have the 4-pin connector as described below.

Numbering of the ATX revisions may be a little confusing: ATX refers to the design, and goes up to version 2.2 in 2004 (with the 24 pins of ATX12V 2.0), while ATX12V describes only the PSU.

For instance, ATX 2.03 is quite commonly seen on PSUs from 2000 and 2001 and often includes the P4 12V connector, even if the norm itself did not define it yet.

ATX power supply derivatives

Standing for ATX 12-volt-only, this is a new specification published by Intel in 2019, aimed at pre-built systems in the first run, and stricter power efficiency requirements by the California Energy Commission going into effect in 2021. Several OEMs were already using a similar design with proprietary connectors and this effectively standardizes those.

Under this standard, power supplies provide only a 12 V output. ATX12VO introduces a new 10-pin connector to supply the motherboard, replacing the 24-pin ATX12V connector. This greatly simplifies power supplies, but moves DC-to-DC conversion and some connectors to the motherboard instead. Notably, SATA power connectors, which include 3.3 V and 5 V pins, need to move to the motherboard instead of being connected directly to the power supply.

a. Connector pinout per Intel ATX12VO (12V Only) Desktop Power Supply Design Guide.

SFX (Small Form-Factor eXtended) is merely a design for a small form factor (SFF) power supply casing (such as those using microATX, FlexATX, nano-ITX, mini-ITX, and NLX), with the power specifications almost identical to ATX. Thus, an SFX power supply is mostly pin-compatible with the ATX power supply as the main difference is its reduced dimensions; the only electrical difference is that the SFX specifications do not require the −5 V rail. Since −5 V is required only by some ISA-bus expansion cards, this is not an issue with modern hardware and decreases productions costs. As a result, ATX pin 20, which carried −5 V, is absent in current power supplies; it was optional in ATX and ATX12V version 1.2 and deleted as of ATX version 1.3.

SFX has dimensions of 125 × 63.5 × 100 mm (width × height × depth), with a 60 mm fan, compared with the standard ATX dimensions of 150 × 86 × 140 mm. Optional 80 or 40 mm fan replacement increases or decreases the height of an SFX unit.

Some manufacturers and retailers incorrectly market SFX power supplies as μATX or MicroATX power supplies.

Some manufacturers make SFX-L dimensions of 125 × 63.5 × 130 mm to accommodate a 120 mm fan.

Specification changes and additions

High-performance video card power demands dramatically increased during the 2000s and 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 4-pin peripheral or floppy power connector. Midrange and high-end PCIe graphics cards manufactured after 2004 typically use a standard 6- or 8-pin PCIe power connector directly from the PSU.

Efficiency

Efficiency in power supplies means the extent to which power is not wasted in converting electricity from a household supply to regulated DC. Computer power supplies vary from around 70% to over 90% efficiency.

Various initiatives exist to improve the efficiency of computer power supplies. Climate Savers Computing Initiative promotes energy saving and reduction of greenhouse gas emissions by encouraging development and use of more efficient power supplies. 80 PLUS certifies a variety of efficiency levels for power supplies and encourages their use via financial incentives. Efficient power supplies also save money by wasting less power; as a result they use less electricity to power the same computer, and they emit less waste heat which results in significant energy savings on central air conditioning in the summer. The gains of using an efficient power supply are more substantial in computers that use a lot of power.

Although a power supply with a larger than needed power rating will have an extra margin of safety against overloading, such a unit is often less efficient and wastes more electricity at lower loads than a more appropriately sized unit. For example, a 900-watt power supply with the 80 Plus Silver efficiency rating (which means that such a power supply is designed to be at least 85 percent efficient for loads above 180 W) may only be 73% efficient when the load is lower than 100 W, which is a typical idle power for a desktop computer. Thus, for a 100 W load, losses for this supply would be 27 W; if the same power supply was put under a 450 W load, for which the supply's efficiency peaks at 89%, the loss would be only 56 W despite supplying 4.5 times the useful power. For a comparison, a 500-watt power supply carrying the 80 Plus Bronze efficiency rating (which means that such a power supply is designed to be at least 82-percent efficient for loads above 100 W) may provide an 84 percent efficiency for a 100 W load, wasting only 19 W.