ATX

An ATX motherboard

ATX (Advanced Technology eXtended) is a motherboard form factor specification developed by Intel in 1995 to improve on previous de facto standards like the AT form factor. It was the first major change in desktop computer enclosure, motherboard, and power supply design in many years, improving standardization and interchangeability of parts. The specification defines the key mechanical dimensions, mounting point, I/O panel, power and connector interfaces between a computer case, a motherboard, and a power supply. With the improvements it offered, including lower costs, ATX overtook AT completely as the default form factor for new systems within a few years. ATX addressed many of the AT form factors 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 slots. In 2003, Intel announced the BTX standard, intended as a replacement for ATX. As of 2009^[update], the ATX form factor remains a standard for do-it-yourselfers; BTX has however made inroads into pre-made systems. This^{[clarification needed]} was designed to solve the problems in BAT and LPX motherboards.^{[citation needed]}

The official specifications were released by Intel in 1995, and have been revised numerous times since, the most recent being version 2.3,^[1] released in 2007.

A full-size ATX board is 12 in × 9.6 in (305 mm × 244 mm). This allows many ATX form factor chassis to accept microATX boards as well.

Connectors

The original AT standard only had a single keyboard port opening on the back of the case. Any additional connectors such as video, audio, printer, serial, network, and so forth would require either mounting in a slot, or using custom-cut holes.

ATX I/O plates for motherboard rear 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. 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. 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. If necessary, I/O plates can be replaced to suit a motherboard that is being fitted; the I/O plates are usually included with motherboards not designed for a particular computer. The computer will operate correctly without a plate fitted, although there will be open gaps in the case and the EMI/RFI screening will be compromised. Panels were 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 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), and extra USB ports.

Variants

ATX motherboard size comparison; rear is on left.

  FlexATX (229×191 mm)

  MicroATX/Embedded ATX (244×244 mm)

  Mini ATX (284x208mm)

  Standard ATX (305×244 mm)

  Extended ATX (EATX) (305×330 mm)

  enhanced Extended ATX (EEATX) (347×330 mm)

  Workstation ATX (WATX) (356×425 mm)

Several ATX-derived form factors 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 7 slots at 0.8 in (20 mm) spacing; the popular Micro-ATX size removes 2.4 inches and 3 slots, leaving 4. 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.

Larger variants

A number of manufacturers have added one, two or three additional expansion slots (at the standard 0.8 inch spacing) to the standard 12-inch ATX motherboard width.

Ultra ATX

In 2008, Foxconn unveiled a Foxconn F1 motherboard prototype, which has the same depth as a standard ATX motherboard, but an extended 14.4" width to accommodate 10 slots.^[2] The firm called the new form factor for this motherboard "Ultra ATX"^[3] 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.^[4]

XL-ATX

The name "XL-ATX" has been used by at least two companies in different ways. In September 2009, EVGA Corporation had already released a 13.5" wide by 10.3" deep "XL-ATX" motherboard as its EVGA X58 Classified 4-Way SLI.^[5] In Q2/2010, Gigabyte launched another XL-ATX Mainboard with model number GA-X58A-UD9.

In April 2010, Gigabyte Technology announced its 12.8" wide by 9.6" deep GA-890FXA-UD7 motherboard that allowed all seven slots to be moved downward by one slot position. The added length could have allowed placement of up to eight expansion slots, but the top slot position is vacant on this particular model.

Although these boards have room for additional expansion slots (9 and 8 total, respectively), all three provide only 7 expansion connectors; the topmost positions are left vacant to provide more room for the CPU, chipset, and associated cooling.

HPTX

In 2010, EVGA Corporation released a new motherboard, the "Super Record 2", or SR-2, whose size surpasses that of the "EVGA X58 Classified 4-Way SLI". The new board is designed to accommodate two Dual QPI LGA1366 socket CPUs (e.g. Intel Xeon), similar to that of the Intel Skulltrail motherboard that could accommodate two Intel Core 2 Quad processors, and has a total of seven PCI-E slots and 12 DDR3 RAM slots. The new form factor is dubbed "HPTX", and is 13.6 by 15 inches (34.5 cm by 38.1 cm).^[6]

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 V_SB (standby) supplies are also required. A −5 V output was originally required because it was supplied on the ISA bus, but it became obsolete with the removal of the ISA bus in modern PCs and has been removed in later versions of the ATX standard.

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: a 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.

ATX 2.0 motherboard power connectors (bottom view of plug). There is an error in the 4-pin +12V power connector; the square and chamfered pins are reversed. The pin numbers are correct, but pins 1 and 4 should be square, while pins 2 and 3 should be chamfered.^[7]:20

Four wires have special functions:

• PS_ON# or 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.^[7][9]
• PWR_OK or 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.^[10]
• +5 V_SB or +5 V standby supplies power even when the rest of the supply 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 for remote sensing of the voltage drop in the power supply wiring.

Generally, supply voltages must be within ±5% of their nominal values at all times. The little-used negative supply voltages, however, have a ±10% tolerance. There is a specification for ripple in a 10 Hz–20 MHz bandwidth:^[7]

The Molex Mini-Fit Jr. has a power rating of 600 volts, 13 amps maximum per pin.^[11] As large server motherboards and 3D graphics cards have required progressively more and more power to operate, it has been necessary to revise and extend the standard beyond the original 24-pin connector, to allow more current using multiple additional pins in parallel. The low circuit voltage is the restriction on power flow through each connector pin; at the maximum rated voltage, a single Mini-Fit Jr pin would be capable of 7800 watts.

Physical characteristics

ATX power supplies generally have the dimensions of 150 × 86 × 140 mm (5.9 × 3.4 × 5.5 in)^[12]:23–24 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 or modular connectors.

Main changes from AT design

Power switch

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.

Typical ATX 1.3 power supply. From left to right, the connectors are 20-pin motherboard, 4-pin "P4 connector", fan RPM monitor (note the lack of a power wire), SATA power connector (black), "Molex connector", and floppy connector.

Interior view in an ATX power supply.

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 an equivalent-function 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, however, the computer cannot be turned on with the front power button.

Power connection to the motherboard

The power supply's connection to the motherboard was changed from the older AT standard; ATs 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 used one large, keyed connector which could 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 5V rail. Some motherboards, particularly those manufactured after the introduction of ATX but while AT equipment was still in use, supported both AT and ATX PSUs.

If using an ATX PSU for other purposes 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. At least the specified minimum load required by the PSU should be present; the standard does not specify operation without load, and a conforming PSU may shut down, output incorrect voltages, or otherwise malfunction, but will not be hazardous or damaged.

Airflow

The original ATX specification called for a power supply to be located near to the CPU with the power supply fan drawing in cooling air from outside the chassis and directing it onto the processor. It was thought that in this configuration, cooling of the processor would be achievable without the need of an active heatsink.[1] This recommendation was removed from later specifications; modern ATX power supplies usually exhaust air from the case.

ATX power supply revisions

Original ATX

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.^[13]
• 4-pin Berg floppy connector — transferred directly from AT standard: +5 V and +12 V for 3.5 inch floppy drives and other peripherals.^[14]
• 20-pin Molex Mini-fit Jr. main 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 fans and motors of peripheral devices (HDD, FDD, CD-ROM, etc.).

The original ATX power supply specification was little revised until 2000.

ATX12V 1.x

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.

ATX12V 1.0

The main changes and additions in ATX12V 1.0 (released in February 2000) were:

• Increased the power on the 12 V rail (power on 5 V and 3.3 V rails remained mostly the same).

• An extra 4-pin mini fit JR (Molex 39-01-2040), 12-volt connector to power the CPU.^[12] Formally called the +12 V Power Connector, this is commonly referred to as the P4 connector because this was first needed to support the Pentium 4 processor.

Before the Pentium 4, processors were generally powered from the 5V rail. Later processors operate at much lower voltages, typically around 1 V, and some draw over 100 A. It is infeasible to provide power at such low voltages and high currents from a standard system power supply, so the Pentium 4 established the practice of generating it with a DC-to-DC converter on the motherboard next to the processor, powered by the 4-pin 12V connector.

ATX12V 1.1

This is a minor revision from August 2000. The power on the 3.3 V rail was slightly increased, and other smaller changes were made.

ATX12V 1.2

A relatively minor revision from January 2002. The only significant change was that the −5 V rail was no longer required (it became optional). This voltage was used only on some old systems with certain ISA add-on cards.

ATX12V 1.3

Introduced in April 2003 (a month after 2.0). This standard introduced some changes, mostly minor. Some of them are:

• Slightly increased the power on 12 V rail.
• Defined minimal required PSU efficiencies for light and normal load.
• Defined acoustic levels.
• Introduction of Serial ATA power connector (but defined as optional).
• Guidance for the −5 V rail was removed (but it was not prohibited).^[15]

ATX12V 2.x

ATX12V 2.x brought a very significant design change regarding power distribution. On analyzing the then-current PC architecture's power demands, it was determined that it would be much cheaper and more practical to power most PC components from 12 V rails, instead of from 3.3 V and 5 V rails.

ATX12V 2.0

ATX-450PNF by FSP Group

The above conclusion was incorporated in ATX12V 2.0 (introduced in February 2003), which defined quite different power distribution from ATX12V 1.x:

• Most power is now provided on 12 V rails. The standard specifies that two independent 12 V rails (12 V₂ for the 4 pin connector and 12 V₁ for everything else) with independent overcurrent protection are needed to meet the power requirements safely (some very high power PSUs have more than two rails, recommendations for such large PSUs are not given by the standard).
• The power on 3.3 V and 5 V rails was significantly reduced.
• The main ATX power connector was extended to 24 pins. The extra four pins provide one additional 3.3 V, 5 V and 12 V circuit.
• The 6-pin AUX connector from ATX12V 1.x was removed because the extra 3.3 V and 5 V circuits which it provided are now incorporated in the 24-pin main connector.
• The power supply is required to include a Serial ATA power cable.
• Many other specification changes and additions

ATX12V v2.01

This is a minor revision from June 2004. An errant reference for the −5V rail was removed. Other minor changes were introduced.^[16]

ATX12V v2.1

This is a minor revision from March 2005. The power was slightly increased on all rails. Efficiency requirements changed.

ATX12V v2.2

Another minor revision.

• Specified High Current Series wire terminals for 24-pin main and 4-pin +12V power connectors.
• Corrections.

ATX12V v2.3

Effective March 2007 and current as of 2012^[update]. Recommended efficiency was increased to 80% (with at least 70% required), and the 12 V minimum load requirement was lowered. Higher efficiency generally results in less power consumption (and less waste heat), and the 80% recommendation brings supplies in line with new Energy Star 4.0 mandates.^[17] The reduced load requirement allows compatibility with processors that draw very little power during startup.^[18] The absolute over-current limit of 240VA per rail was removed, allowing 12V lines to provide more than 20A per rail.

ATX power supply derivatives

SFX

SFX is merely a form factor for a power supply casing and the power specifications are almost identical. Thus, an SFX power supply is mostly interchangeable with the ATX power supply. The only difference is that the SFX specifications do not require the -5V rail. Since -5V 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 -5V, is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3.

SFX has dimensions of 100 x 125 x 63.5 (width x depth x height in mm) with 60 mm fan. Optional 80 or 40 mm fan replacement increases or decreases the height of the unit.^[19]

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

TFX

Another small form factor power supply with standard ATX specification connectors. Generally 5.75 in × 3.25 in × 2.5 in (D) × (W) × (H) (146 mm x 83 mm x 64 mm).

WTX

Provides a WTX style motherboard connector which is incompatible with the standard ATX motherboard connector.

AMD GES

This is an ATX12V power supply derivative made by AMD to power its Athlon MP (dual processor) platform. It was used only on high-end Athlon MP motherboards. It has a special 8-pin supplemental connector for motherboard, so an AMD GES PSU is required for such motherboards (those motherboards will not work with ATX(12 V) PSUs).

EPS12V

EPS12V is defined in SSI, and used primarily by SMP/multi-core systems such as Core 2, Core i7, Opteron and Xeon. It has a 24-pin main connector (same as ATX12V v2.x), an 8-pin secondary connector, and an optional 4-pin tertiary connector. Rather than include the extra cable, many power supply makers implement the 8-pin connector as two combinable 4-pin connectors to ensure backwards compatibility with ATX12V motherboards.

Recent 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.

Interchanging PSUs

Although the ATX power supply specifications are mostly vertically compatible in both ways (both electrically and physically), there are potential issues with mixing old motherboards/systems with new PSUs, and vice versa. The main issues to consider are the following:

• The power distribution biases across 3.3 V, 5 V, and 12 V rails are very different between older and newer ATX PSU designs, as well as between older and newer PC system designs.
• Older PSUs may not have connectors which are required for newer PC systems to properly operate.
• Newer systems generally have higher power requirements than older systems.

This is a practical guidance what to mix and what not to mix:

• Older systems (before Pentium 4 and Athlon XP platforms) were designed to draw most power from 5 V and 3.3 V rails.
• Because of the DC-DC converters on the motherboard that convert 12 V to the low voltages required by the Intel Pentium 4 and AMD Athlon XP (and subsequent) processors, such systems draw most of their power from the 12 V rail.
• Original ATX PSUs have power distribution designed for pre-P4/XP PCs. They lack the supplemental 4-pin 12-volt CPU power connector, so they most likely cannot be used with P4/XP or newer motherboards. Adapters do exist but power drain on the 12 V rail must be checked very carefully. There is a chance it can work without connecting the 4-pin 12 V connector, but caution is advised.^{[20][dead link]}
• ATX12V 1.x PSUs have power distribution designed for P4/XP PCs, but they are also greatly suitable for older PCs, since they give plenty of power (relative to old PCs' needs) both on 12 V and on 5 V/3.3 V. It is not recommended to use ATX12V 1.x PSUs on ATX12V 2.x motherboards because those systems require much more power on 12 V, and much less on 3.3 V/5 V than ATX12V 1.x PSUs provide.
• ATX12V 2.x PSUs have power distribution designed for late P4/XP PCs and for Athlon 64 and Core Duo PCs. They can be used with earlier P4/XP PCs, but the power distribution will be significantly suboptimal, so a more powerful ATX12V 2.0 PSU should be used to compensate for that discrepancy. ATX12V 2.x PSUs can also be used with pre-P4/XP systems, but the power distribution will be greatly suboptimal (12 V rails will be mostly unused, while the 3.3 V/5 V rails will be overloaded), so this is not recommended.
• Systems that use an ISA bus should have a PSU that provides the −5 V rail, which became optional in ATX12V 1.2 and was subsequently phased out by manufacturers.

Not all computers use standard, interchangeable ATX power supplies. In particular, some proprietary brand-name machines and high-end workstation and server designs do not, and require an exactly matching power supply unit.

See also

• AT (form factor)
• BTX (form factor)
• Computer form factor
• Mini-ITX
• Power supply rail
• Power supply unit (computer)
• SSI CEB

References

External links

• ATX Motherboard Specification, v2.2
• ATX Riser Card Specification Revision 1.0
• Power supply information
  • Power Supply Form Factors
  • ATX12V Power Supply Design Guide, v2.01
  • ATX12V Power Supply Design Guide, v2.2
  • Power Supply Design Guide for Desktop Platform Form Factors (ATX12V specification v2.3)
  • Power Supply Design Guide for Desktop Platform Form Factors Revision 1.2
  • EPS12V Power Supply Design Guide Version 2.92
  • EPS12V Power Supply Design Guide, v2.91
  • EPS12V Power Supply Design Guide, v2.0
  • Various power supply cables and connectors
  • A short history of power supply voltage rails
  • ATX power supply connectors with pinouts
  • More ATX power supply connectors with pinouts
  • ATX Power Supply Terminology