| Name | PCB size (mm) |
|---|---|
| WTX | 356 × 425 |
| AT | 350 × 305 |
| Baby-AT | 330 × 216 |
| BTX | 325 × 266 |
| ATX | 305 × 244 |
| EATX (Extended) | 305 × 330 |
| LPX | 330 × 229 |
| microBTX | 264 × 267 |
| NLX | 254 × 228 |
| Ultra ATX | 244 × 367 |
| microATX | 244 × 244 |
| DTX | 244 × 203 |
| FlexATX | 229 × 191 |
| Mini-DTX | 203 × 170 |
| EBX | 203 × 146 |
| microATX (min.) | 171 × 171 |
| Mini-ITX | 170 × 170 |
| EPIC (Express) | 165 × 115 |
| ESM | 149 × 71 |
| Nano-ITX | 120 × 120 |
| COM Express | 125 × 95 |
| ESMexpress | 125 × 95 |
| ETX/XTX | 114 × 95 |
| Pico-ITX | 100 × 72 |
| PC/104 (-Plus) | 96 × 90 |
| ESMini | 95 × 55 |
| Qseven | 70 × 70 |
| mobile-ITX | 60 × 60 |
| CoreExpress | 58 × 65 |
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 big change in computer case, 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 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 2009[update], the ATX form factor remains a standard for do-it-yourselfers; BTX has however made inroads into pre-made systems. This was designed to solve the problems in BAT and LPX Motherboards
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 × 9.6 in (305 × 244 mm). This allows many ATX form factor chassis to accept microATX boards as well.
Contents
|
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. 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, or HDMI), and extra USB ports.
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. The two most popular are the Standard (6 slots) and Micro ATX (4 slots) sizes. Here length refers to the distance along the external connector edge
| width | length | color in image | |
|---|---|---|---|
| FlexATX | 9 in (229 mm) | 7.5 in (191 mm) | |
| microATX and EmbATX | 9.6 in (244 mm) | 9.6 in (244 mm) | |
| Mini ATX | 11.2 in (284 mm) | 8.2 in (208 mm) | |
| Standard ATX | 12 in (305 mm) | 9.6 in (244 mm) | |
| EATX (extended ATX) | 12 in (305 mm) | 13 in (330 mm) | |
| EEATX (enhanced extended ATX) | 13.68 in (347 mm) | 13 in (330 mm) | |
| WTX (workstation ATX) | 14 in (356 mm) | 16.75 in (425 mm) |
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.[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]
Unlike Ultra-ATX which was fully defined by a single company, XL-ATX does not yet have established dimensional standards. In April 2010, Gigabyte Technology announced its 12.8" long by 9.6" wide 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. Meanwhile, EVGA Corporation had already released a 13.5" long by 10.3" wide "XL-ATX" motherboard as its EVGA X58 Classified 4-Way SLI. EVGA's version of XL-ATX has room for up to nine expansion slots, but the top two positions are vacant.[5] Note that even though both of these boards have room for extra expansion slots, neither makes use of that extra room for card placement. In Q2/2010 Gigabyte launched another XL-ATX Mainboard with model number GA-X58A-UD9, but it also only implements 7 PCI-Express x16 Slots (the extra space from XL-ATX form factor seems to be needed for chipset cooling).
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 slot 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]
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. 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.
| Color | Signal | Pin | Pin | Signal | Color |
|---|---|---|---|---|---|
| Orange | +3.3 V | 1 | 13 | +3.3 V | Orange |
| +3.3 V sense | Brown | ||||
| Orange | +3.3 V | 2 | 14 | −12 V | Blue |
| Black | Ground | 3 | 15 | Ground | Black |
| Red | +5 V | 4 | 16 | Power on | Green |
| Black | Ground | 5 | 17 | Ground | Black |
| Red | +5 V | 6 | 18 | Ground | Black |
| Black | Ground | 7 | 19 | Ground | Black |
| Grey | Power good | 8 | 20 | Reserved | N/C |
| Purple | +5 V standby | 9 | 21 | +5 V | Red |
| Yellow | +12 V | 10 | 22 | +5 V | Red |
| Yellow | +12 V | 11 | 23 | +5 V | Red |
| Orange | +3.3 V | 12 | 24 | Ground | Black |
|
|||||
Four wires have special functions:
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:[8]
| Supply [V] | Tolerance | Range (min. to max.) | Ripple (p. to p. max.) |
|---|---|---|---|
| +5 VDC | ±5% (±0.25 V) | +4.75 V to +5.25 V | 50 mV |
| −5 VDC | ±10% (±0.50 V) | –4.50 V to –5.50 V | 50 mV |
| +12 VDC | ±5% (±0.60 V) | +11.40 V to +12.60 V | 120 mV |
| −12 VDC | ±10% (±1.2 V) | –10.8 V to –13.2 V | 120 mV |
| +3.3 VDC | ±5% (±0.165 V) | +3.135 V to +3.465 V | 50 mV |
| +5 VSB | ±5% (±0.25 V) | +4.75 V to +5.25 V | 50 mV |
ATX power supplies generally have the dimensions of 6 × 3.4 × 5.5 (inches) and in metric 150 mm × 86 mm × 140 mm and share a common mounting layout of four screws arranged on the back side of the unit.
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 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 wrongly. 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.
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, introduced in late 1995, defined three types of power connectors:
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.
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.
The main changes and additions in ATX12V 1.0 (released in February 2000) were:
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.
This is a minor revision from August 2000. The power on the 3.3 V rail was slightly increased, and other lesser changes made.
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.
Introduced in April 2003 (a month after 2.0). This standard introduced some changes, mostly minor. Some of them are:
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.
The above conclusion was incorporated in ATX12V 2.0 (introduced in February 2003), which defined quite different power distribution from ATX12V 1.x:
This is a minor revision from June 2004. An errant reference for the -5V rail was removed. Other minor changes were introduced.
This is a minor revision from March 2005. The power was slightly increased on all rails. Efficiency requirements changed. Added 6-pin connector for PCIe graphics cards, that aids the PCIe slot in the motherboard, delivering 75 watts.
Another minor revision. Added 8-pin connector for PCIe graphics cards, that delivers another 150 watts.
Effective March 2007 and current as of 2011[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.[14] The reduced load requirement allows compatibility with processors that draw very little power during startup.[15] The absolute over-current limit of 240VA per rail was removed, allowing 12V lines to provide more than 20A per rail.
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.[16]
Some manufacturers and retailers incorrectly market SFX power supplies as µATX or MicroATX power supplies.
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)
Provides a WTX style motherboard connector which is incompatible with the standard ATX motherboard connector.
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 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.
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.
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:
This is a practical guidance what to mix and what not to mix:
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.
Older Dell computers, particularly those from the Pentium II and III lines, 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 have been changed. Thus, the pins cannot simply be rearranged.[2]
The change affects not only 20-pin ATX connectors, but also auxiliary 6-pin connectors. Modern Dell systems might use standard ATX connectors.[3] Dell PC owners should be careful when attempting to change Dell motherboards and power supplies from the original setup, 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.
To determine if a Dell PC has this proprietary ATX (non industrial standard), view the power pin layout in the on-line Dell 'SERVICE' manual (not user manual) and compare it with the ATX pin diagram above.[18] A more reliable method is to measure the voltages on the connector.
| Color | Signal | Pin | Pin | Signal | Color |
|---|---|---|---|---|---|
| Red | +5 V | 1 | 11 | PS_On | Grey |
| Black | Ground | 2 | 12 | Ground | Black |
| Red | +5 V | 3 | 13 | Ground | Black |
| Black | Ground | 4 | 14 | Ground | Black |
| Orange | Power good | 5 | 15 | −5 V | White |
| Purple | +5 VSB (standby) | 6 | 16 | +5 V | Red |
| Yellow | +12 V | 7 | 17 | +5 V | Red |
| Blue | −12 V | 8 | 18 | +5 V | Red |
| Black | Ground | 9 | 19 | - | KEY (blank) |
| Black | Ground | 10 | 20 | +5 V | Red |
|
|||||