ATX

ATX motherboard.

ATX (Advanced Technology Extended) is a computer 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.

[edit] Connectors

ATX I/O plates

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 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.

[edit] Variants

ATX motherboard size comparison.     FlexATX (229×191 mm)     MicroATX/Embedded ATX (244×244 mm)     Mini ATX (284x208mm)     Standard ATX (305×244 mm)     Extended ATX (EATX) (305×330 mm)     Workstation ATX (WATX) (356×425 mm)

There exist several ATx-derived form factors that use the same power supply, mountings and basic back panel arrangement, but set different standards for the size of the board. The two most popular are the Standard and Micro ATX sizes.

[edit] Ultra ATX, XL-ATX

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 defined by one company, XL-ATX is not yet an established standard. 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 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).

[edit] HPTX

In 2010, EVGA Corporation has revealed plans for a new motherboard, the "Super Record 2", or SR-2, that is claimed to have a size surpassing 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 appears to have 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 (34.5cm by 38.1cm). ^[6]

[edit] 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.

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 GND (by the motherboard), the power supply turns on. It is internally pulled up to +5 V inside the power supply.^[8][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:^[8]

[edit] Main changes from AT design

[edit] 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 of an ATX power supply.

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.

[edit] Power connection to the motherboard

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.

If not working with an ATX motherboard, one can fully turn on the power (it is always partly on) by shorting from pin 16 (the green wire) on the ATX connector to a black wire (ground), since it is the motherboard's power switch which the ATX PSU uses. In order to use an old PC power supply for tasks other than powering a PC, one must also be careful to observe the minimum load requirements of the PSU; if some load is not provided, the supply may shut down, output incorrect voltages, or otherwise malfunction.

[edit] Airflow

The original ATX specification called for a power supply to be located near to the CPU with the power supply fan directed to draw in cool air from outside the chassis and exhaust it directly 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 and modern ATX power supplies prevailingly exhaust air from the case.

[edit] ATX power supply revisions

[edit] 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.^[11]
• 4-pin Berg floppy connector — transferred directly from AT standard: +5 V and +12 V for 3.5 inch floppy drives and other peripherals.^[12]
• 20-pin Molex Mini-fit Jr. main motherboard connector — new to the ATX standard.

The power distribution specification defined that most of 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 remained mostly unrevised until 2000.

[edit] ATX12V 1.x

While designing the Pentium 4 platform in 1999/2000, the standard 20-pin ATX power connector was deemed inadequate to supply increasing electrical load requirements. So, ATX was significantly revised into ATX12V 1.0 standard (that is why ATX12V 1.x is sometimes inaccurately called ATX-P4). ATX12V 1.x was also adopted by Athlon XP and Athlon 64 systems.

[edit] ATX12V 1.0

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

• An extra 4-pin mini fit JR, 12-volt connector to power the CPU. 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. (Older processors were powered from the 5V rail.)
• A supplemental 6-pin AUX connector providing additional 3.3 V and 5 V supplies to the motherboard, if it needed it. Rarely used by motherboards.
• Increased the power on the 12 V rail (power on 5 V and 3.3 V rails remained mostly the same).

[edit] ATX12V 1.1

This is a minor revision from August 2000. The power on 3.3 V rail was slightly increased, among other much lesser changes.

[edit] 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 very rarely used, only on some old systems with some ISA add-on cards.

[edit] ATX12V 1.3

Introduced in April 2003 (a month after 2.0). This standard introduced some changes, with most of them being 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).
• The -5V rail is prohibited

[edit] ATX12V 2.x

ATX12V 2.x brought a very significant design change regarding power distribution. When analyzing the then-current PC architectures' power demands, it was determined that it would be much easier (both from economical and engineering perspectives) to power most PC components from 12 V rails, instead of from 3.3 V and 5 V rails.

[edit] ATX12V 2.0

ATX 450 PHF.

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

• The main ATX power connector was extended to 24 pins (it is backwards compatible). 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.
• 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 power supply is required to include a Serial ATA power cable.
• Many other specification changes and additions.

[edit] 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.

[edit] ATX12V v2.1

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.

[edit] ATX12V v2.2

Another minor revision. Added 8-pin connector for PCIe graphics cards, that delivers another 150 watts.

[edit] ATX12V v2.3

The most recent revision, effective March 2007. Efficiency recommendations were increased to 80% (with at least 70% efficiency required), and the 12 V 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.^[13] The reduced load requirement allows compatibility with processors that draw very little power during startup.^[14] The absolute over current limit (240VA per rail) is no longer present, enabling 12V line to provide more than 20A per rail.

[edit] ATX power supply derivatives

[edit] 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).

[edit] 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. To ensure backwards compatibility with ATX12V, many power supply makers implement the 8-pin connector as two combinable 4-pin connectors.

[edit] Recent specification changes and additions

Because the 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 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.

[edit] Interchanging old/new systems with old/new 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 require larger power supplies than older systems.

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

• Older systems (until Pentium 4 and Athlon XP platforms) were designed to draw most power from 5 V and 3.3 V rails.
• Pentium 4 and Athlon XP systems draw much more power from the 12 V rail, than the 5 V and 3.3 V rails.
• Newer systems (Athlon 64, Core 2 Duo, etc.) draw most power from 12 V rails.
• Original ATX PSUs have power distribution designed for pre-P4/XP PCs. They lack the supplemental 4-pin 12-volt connector for the CPU, so they most likely cannot be used with P4/XP era or newer motherboards (adapters do exist but power drain on the 12 V rail must be checked very carefully if using them). There is a chance it can work without connecting the 4-pin 12 V connector, but caution is to be advised.^[15]
• 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 require ATX/ATX12V 1.2 or earlier because the ISA bus requires a −5 V power rail unless the board provides a DC to DC converter that supplies −5 V. ATX/ATX12V 1.3 and later prohibit the −5 V rail so power supplies built to these versions are usually unsuitable for ISA systems.

Special note: Proprietary brand-name or high-end workstation/server designs do not fit into these guidelines. They usually require an exactly matching power supply unit.^{[citation needed]}

[edit] Issues with 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 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 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.

[edit] References

[edit] See also

• AT power connector
• Mini-ITX
• BTX
• SSI CEB
• Power supply rail
• Entry-Level Power Supply Specification

[edit] 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)
  • 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