ATX format

from Wikipedia, the free encyclopedia

The ATX format ( A dvanced T echnology E x tended ) is a standardization for housings, power supplies , motherboards and plug-in cards of microcomputers . The ATX form factor was introduced by Intel in 1996 as the successor to the AT form factor that had prevailed up to that point in time . Despite attempts to establish the BTX format as the standard, the ATX format is the dominant format for PCs and desktop computers . Big towers usually have the E-ATX format.


Mainboard formats:
top left: ATX board with 12 ″ × 7.5 ″
top right: ATX board with 12 ″ × 8.25 ″
bottom right: Micro ATX board in standard size (9.6 ″ × 9.6 ″ )

The board dimensions:

  • E-ATX format: 305 mm × 330 mm (12 ″ × 13 ″)
  • ATX: 305 mm × 244 mm (12 ″ × 9.6 ″)
  • XL-ATX: 345 mm × 262 mm (13.5 ″ × 10.3 ″)
  • ATX-EXtended: 308 mm × 340 mm (server board format)
  • Mini-ATX: 284 mm × 208 mm (11.2 ″ × 8.2 ″)
  • Micro-ATX (µATX): 244 mm × 244 mm (9.6 ″ × 9.6 ″)
  • Flex-ATX: 229 mm × 191 mm (9 ″ × 7.5 ″),
  • Mini-ITX : 170 mm × 170 mm (6.7 ″ × 6.7 ″)
  • Nano-ITX: 120 mm × 120 mm (4.7 ″ × 4.7 ″)
  • Pico-ITX: 100 mm × 72 mm (4 ″ × 2.8 ″)

Except for the ATX standard itself, these are specifications based on ATX. Server mainboards for two processors often use the larger E-ATX format. The format is particularly decisive for the maximum number of externally accessible expansion card slots that can be available. For the most important formats these are 7 (ATX), 4 (µATX), 3 (Flex-ATX) and 1 (Mini-ITX). Of the boards just mentioned, smaller boards fit into larger housings, but in some cases the corresponding brackets have to be removed from the housing for individual fastening holes, usually by unscrewing them. Since the end of the 2010s there have been an increasing number of motherboards that use any dimensions between those of µATX and Mini-ITX. The full depth of 9.6 ″ was previously not exhausted by many ATX and µATX models (see ATX boards in the adjacent picture).

The advantages of this standard

  • Integration of the I / O connections on the motherboard itself
  • "Rotation by 90 °" compared to previous formats, i. H. The processor and memory banks are now next to the bus slots instead of behind them. Long cards therefore do not collide with coolers or fans.
  • Reverse polarity protected power supply connection for the main board.
  • The power supply unit is switched on and off internally via a control voltage, so no mains voltage cable has to be pulled through the computer housing.
  • Originally it was planned that the fan of the power supply should also cool the processor, which was only equipped with a passive heat sink.


  • Standby power consumption if the computer is not switched off with the main switch directly on the power supply unit. However, this is not always available.
  • The cooling of the power supply is impaired when other components in the computer case (especially the CPU and graphics card) release a lot of heat into the air. This is because the power supply unit is cooled with the air inside the case. Many ATX cases therefore place the power supply unit at the bottom - contrary to the ATX standard. It gets the fresh air from outside and has its own air flow, separate from that in the rest of the computer case.

With even faster and thus initially warmer processors and graphics cards, the ATX format reaches its thermal limits, so that Intel defined the BTX format as its successor in 2003/2004 , which, however, did not establish itself on the market until 2006. In October 2006, Intel stopped the production of BTX mainboards because, according to Intel, the Core 2 Duo is so good in terms of waste heat that it no longer needs BTX - although high-end graphics cards, for example, would still have theoretical advantages. However, these were never widely used as typical "hobbyist goods" in the BTX systems, which were mainly sold completely, and are accordingly not adapted to its cooling concept.


ATX IO panels with different layout

The AT standard practically only had a keyboard connection on the back of the system. Further interfaces had to be accommodated individually in the housing. ATX allows motherboard manufacturers to arrange these ports in a rectangular area on the rear of the case. How they use the space is up to the manufacturers (although most of them followed a basic pattern). So that no holes remain in the rear wall of the case, a suitable cover ( I / O shield , also "ATX cover") must be enclosed with each mainboard . Their size is 160 × 45 mm.

The colors of the interfaces that are common today are shown in revision 2.2 of the ATX specification.

Sound ( AC'97 )
colour function
  pink Input for microphone ( mono ).
  green Line-out / output for headphones or (front) speakers (stereo)
  blue Line-in / input for AUX / external sources ( stereo ).
  Gray Output for side speakers side speakers ] (stereo)
  orange Output for center and sub-bass speakers center speaker, subwoofer ]
  black Output for rear speakers rear speakers ] (stereo)
Standard connections of an integrated sound chip

Power supply

There is basically no binding, but version-dependent compatibility between ATX mainboards and ATX power supplies . It may be that an older generation ATX power supply is not suitable for supplying a newer standard ATX mainboard with suitable connectors. Matching total power of a power supply does not say anything about the output power of the required busbars.

While in the old AT format the power button was located directly in the primary circuit of the power supply , in the ATX it is connected to the mainboard and only switches one control input of the electronics to ground (so-called soft switch ). This makes it z. B. possible that the computer switches itself off by software. However, it is only a "soft off" state, which means that the mainboard continues to be supplied with standby voltage, which can also be passed on to expansion cards. This enables functions such as Wake-on-LAN or Wake-on-Modem , in which the "switched off" computer switches itself on again when a special "wake-up packet" ( Magic Packet ) comes in via the LAN or the modem receives a call . Time-controlled switching on of the computer is also possible. Disadvantages are the power consumption in standby and the risk of damage from voltage peaks from the power supply, even when the computer is not running.

To start an ATX power supply without a connected mainboard, the (mostly) green PowerSupplyOn line must be connected to ground (mostly black). It should be noted, however, that the power supply unit should never be switched on without a connected load (e.g. a hard drive), otherwise it can be damaged. Many power supplies do not start at all without a load on the 5-volt and 12-volt branches.

The development towards ever more power-hungry processors and graphics cards soon overwhelmed the current capacity of the original 20-pin ATX connector. First of all, Intel introduced the additional four-pole P4 connector, which supplies the switching regulator for the processor via two contacts each for +12 V and ground, thus relieving the main power connector. A similar gain is achieved by expanding the previously 20-pin ATX connector to 24 pins, which provides an additional line for +12 V, +5 V, +3.3 V and ground. Since the graphics cards also used more and more power at the same time (nowadays a high-end graphics card easily consumes a multiple of the processor), the 6-pin PCI Express power connector was introduced in ATX 2.2, which supplies the graphics card with additional power, in addition to the 75 W through the mainboard.

With some ATX 2.x power supplies, the 24-pin plug can also be split into a 20-pin plug and a 4-pin additional plug - this additional plug must not be confused with the ATX12V-P4 connector. It is therefore coded differently in the connector shape and has a different fastening lock.


ATX power supplies typically have dimensions of approximately 6 ″ × 3.5 ″ × 5.5 ″ or 15 cm × 8.6 cm × 14 cm (width × height × depth). Power supplies with high performance ( EPS ) are deeper.

Pin assignment

The picture shows the ATX 2.2 socket with 24 contacts on the computer board. The 4 contacts on the right are separated from the remaining 20 contacts and can be left out for older mainboards. The geometry of the plug contacts ensures that the plug can only be inserted the right way round. A plastic hook on the plug engages in the rail on the socket and mechanically secures the plug. Seen from the rail, pin 1 is on the outside left, rotated on the connector depending on the direction of view. Pin 16 (with ATX 1.0 pin 14) controls the power supply. If it is grounded, the power pack switches on the main voltages.

ATX 2.2 power supply connector (male)
ATX 2.2 power supply connector (plug) side view
Assignment of the connector of an ATX 1.3 power supply unit (the 4-pin "P4" connector is assigned differently than the 4 additional pins of an ATX 24-pin connector and cannot be used for this purpose [also mechanically])
Pin code signal Cable color a Function b
ATX 1.0 to 2.1 ATX 2.2
01 01 3.3V orange
02 02 3.3V orange
03 03 Dimensions black
04th 04th 5 V red
05 05 Dimensions black
06th 06th 5 V red
07th 07th Dimensions black
08th 08th PWR_OK Gray Power Ok f
09 09 5 CDB violet +5 V standby voltage
10 10 12 V yellow
10 e 11 12 V yellow
02 e 12 3.3V orange
11 13 3.3V orange
12 14th −12 V blue
13 15th Dimensions black
14th 16 PS ON green Power supply on c
15th 17th Dimensions black
16 18th Dimensions black
17th 19th Dimensions black
18th 20th −5 V White only with ATX-1.x d
19th 21st 5 V red
20th 22nd 5 V red
20 e 23 5 V red
17 e 24 Dimensions black
aThe cable colors show the most common assignment. There may be deviations from individual manufacturers or models.
bTwo wires are usually connected to pin 11 (pin 13 on the 24-pin connector), and occasionally also to pin 1 or pin 2 . One of the two, often with a smaller cross-section, serves as a sensor line for better regulation of the 3.3 volt rail.
cControl input. When connected to ground, the power pack switches on.
d With power supplies according to the ATX-2.x standard "reserved" and mostly unused.
e Assignment of an adapter from a 20-pin connector (up to ATX-2.1) to a 24-pin connector (from ATX-2.2)
fPower-OK-Signal (also Power-Good-Signal). This is a status signal which shows whether the output voltages of the power supply unit are stable. HIGH (+ 5VDC) means: voltages stable. LOW (0VDC) means: Error (If the signal from the mainboard is evaluated, the mainboard performs a reset in the event of an error or does not start the boot process.)

Voltage tolerances

The following tolerances must be observed both when idling and under load. In the event of a mains voltage disturbance, they must be observed for at least 17 ms even under full load in order to be able to bridge short disturbances.

output tolerance Minimum
in V

in V
in V
+12 V1DC ± 5% +11.40 +12.00 +12.60
+12 V2DC ± 5% +11.40 +12.00 +12.60
+5 VDC ± 5% +4.75 +5.00 +5.25
+3.3 VDC ± 5% +3.14 +3.30 +3.47
−12 VDC ± 10% −13.20 −12.00 −10.80
−5 VDC ± 10% −5.50 −5.00 −4.50
+5 VSB ± 5% +4.75 +5.00 +5.25

Power supply standards

Adapter to provide ATX12V with older power supplies
Mainboard power supply
connector (2 × 2)
ATX 1.0 20-pin (2 × 10) Yes No
ATX 1.3 20-pin (2 × 10) Yes Yes 6-pin (1 × 6) AUX power
ATX EPS 24-pin (2 × 12) Yes No 8-pin (2 × 4) supply connector
ATX 2.0 20-pin (2 × 10) Yes Yes
ATX 2.2 24-pin (2 × 12) Yes Yes 6-pin (2 × 3) PCI-E connector; Power factor correction filter
ATX 2.4 24-pin (2 × 12) Yes Yes Minimum load on 12V V2DC less than 0.05 amps possible
  • The 24-pin plug of ATX 2.2 power supplies also fits into the 20-pin socket of an ATX 1.x to 2.1 mainboard (2 × 2 protruding pins), provided that the mainboard is equipped in the connection area . With some ATX 2.x power supplies, the 24-pin connector can also be split into a 20-pin connector and a 4-pin additional connector. (This additional plug must not be confused with the ATX12V-P4 connection).
  • The 20-pin plug of ATX 1.x to 2.1 power supplies also fits into the 24-pin socket of an ATX 2.x mainboard (2 × 2-pin sockets remain empty). Some ATX 2.2 mainboards also require a 20 to 24 pin adapter in order to be able to operate successfully with an ATX 1.x to 2.1 power supply unit.
  • The ATX 2.3 specification differs only slightly from the previous version. The required efficiency has been increased from 75 to 80%. The required switch-on current consumption of the 12-volt processor line has been reduced so that the system now starts even when the processor current consumption is very low. The upper limit of 240 VA per line has been removed, which means that a current of more than 20 A is possible on the 12 V line.
  • The ATX 2.4 specification supports Intel CPUs from the Haswell generation. The minimum load was set to 0.05 A, the recommended is 0.00 A. Power supplies that do not meet this specification cannot start or switch off during operation. Almost all ATX power supplies lower than the ATX 2.4 specification require at least 0.5 A load at 12V V2DC.

Web links

Individual evidence

  1. BTX. The end? PC Games Hardware, September 11, 2006.
  2. ATX12V Power Supply Design Guide v2.01 from
  3. ^ Power Supply - Design Guide for Desktop Platform Form Factors. (PDF) Intel, April 2013, accessed on September 10, 2017 .