ATX - AbsoluteAstronomy.com

ATX is a motherboard form factor specification developed by Intel in 1995 to improve on previous de facto standard

De facto standard

A de facto standard is a custom, convention, product, or system that has achieved a dominant position by public acceptance or market forces...

s like the AT form factor

AT form factor

In the area of IBM compatible personal computers, the AT form factor referred to the dimensions and layout of the motherboard for the IBM AT. Like the IBM PC and IBM XT models before it, many third-party manufacturers produced motherboards compatible with the IBM AT form factor, allowing end users...

. It was the first big change in computer case

Computer case

A computer case is the enclosure that contains most of the components of a computer...

, motherboard

Motherboard

In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...

, and power supply

Power supply unit (computer)

A power supply unit converts mains AC to low-voltage regulated DC power for the internal components of the computer. Modern personal computers universally use a switched-mode 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

Computer case

A computer case is the enclosure that contains most of the components of a computer...

, a motherboard

Motherboard

, and a power supply

Power supply unit (computer)

A power supply unit converts mains AC to low-voltage regulated DC power for the internal components of the computer. Modern personal computers universally use a switched-mode power supply...

. With the improvements it offered, including lower costs, ATX overtook AT

AT form factor

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

MicroATX

microATX, also known as µATX is a standard for motherboards that was introduced in December 1997. The maximum size of a microATX motherboard is 244 mm × 244 mm , but some microATX boards can be as small as 171.45 mm × 171.45 mm...

, FlexATX

FlexATX

FlexATX is a motherboard form factor derived from ATX. The specification was released in 1999 by Intel as an addendum to the microATX specification...

and mini-ITX

Mini-ITX

Mini-ITX is a 17 x 17 cm low-power motherboard form factor developed by VIA Technologies in 2001. Mini-ITX is slightly smaller than microATX. Mini-ITX boards can often be passively cooled due to their low power consumption architecture, which makes them useful for home theater PC systems,...

) 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

BTX (form factor)

BTX is a form factor for motherboards, originally intended to be the replacement for the aging ATX motherboard form factor in late 2004 and early 2005...

standard, intended as a replacement for ATX. , the ATX form factor remains a standard for do-it-yourselfers; BTX

BTX (form factor)

BTX is a form factor for motherboards, originally intended to be the replacement for the aging ATX motherboard form factor in late 2004 and early 2005...

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, released in 2007.

A full-size ATX board is 12 ×. This allows many ATX form factor chassis to accept microATX

MicroATX

boards as well.

Connectors

On the back of the system, some major changes were made. The AT standard had only a keyboard

Computer keyboard

In computing, a keyboard is a typewriter-style keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches...

connector and expansion slots for add-on card backplates. Any other onboard interfaces (such as serial

Serial port

In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...

and parallel port

Parallel port

A parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...

s) 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

PS/2 connector

The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987...

mini-DIN keyboard and mouse connectors ubiquitous. AT systems used a 5-pin DIN connector

DIN connector

A DIN connector is a connector that was originally standardized by the , the German national standards organization. There are DIN standards for a large number of different connectors, therefore the term "DIN connector" alone does not unambiguously identify any particular type of connector unless...

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

Universal Serial Bus

USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....

. Other legacy connectors that are slowly being phased out of modern ATX motherboards include 25-pin parallel port

Parallel port

s and 9-pin RS-232

RS-232

In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...

serial port

Serial port

In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...

s. In their place are onboard peripheral ports such as Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

, FireWire, eSATA, audio ports

Sound card

A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...

(both analog and S/PDIF

S/PDIF

S/PDIF is a digital audio interconnect used in consumer audio equipment over relatively short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high...

), video (analog D-sub, DVI

Digital Visual Interface

The Digital Visual Interface is a video interface standard covering the transmission of video between a source device and a display device. The DVI standard has achieved widespread acceptance in the PC industry, both in desktop PCs and monitors...

, or HDMI

High-Definition Multimedia Interface

HDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...

), and extra USB

Universal Serial Bus

ports.

Variants

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 FlexATX FlexATX is a motherboard form factor derived from ATX. The specification was released in 1999 by Intel as an addendum to the microATX specification...	9 in (229 mm)	7.5 in (191 mm)
microATX MicroATX microATX, also known as µATX is a standard for motherboards that was introduced in December 1997. The maximum size of a microATX motherboard is 244 mm × 244 mm , but some microATX boards can be as small as 171.45 mm × 171.45 mm... and EmbATX	9.6 in (244 mm)	9.6 in (244 mm)
Mini ATX Mini ATX Mini-ATX is a 15 x 15 cm motherboard form factor developed by AOpen Inc. Mini-ATX is slightly smaller than Mini-ITX...	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)

Ultra ATX, XL-ATX

In 2008, Foxconn

Foxconn

The Foxconn Technology Group is a multinational business group anchored by the Hon Hai Precision Industry Co., Ltd. , a Taiwan-registered corporation headquartered in Tucheng, Taiwan...

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 "form factor" for this motherboard "Ultra ATX" in its CES 2008 showing. Also unveiled during the January 2008 CES was the Lian Li

Lian Li

Lian Li Industrial Co., Ltd. is a Taiwanese computer case and accessories manufacturer...

Armorsuit PC-P80 case with 10 slots designed for the motherboard.

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

Gigabyte Technology

Gigabyte Technology Co., Ltd. is a Taiwan-based manufacturer of computer hardware products best known for its motherboards. The company is publicly held and traded on the Taiwan Stock Exchange .-Company:...

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

EVGA Corporation

EVGA Corporation is a company that produces NVIDIA based consumer computer hardware, as well as Intel based motherboards. Founded in July 1999, its current headquarters is in Brea, California.- Products :...

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. 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).

HPTX

In 2010, EVGA Corporation

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).

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 ground (by the motherboard), the power supply turns on. It is internally pulled up to +5 V inside the power supply.
PWR_OK or Power good
Power Good Signal
In addition to the voltages and currents that a computer needs to operate, power supplies also provide a signal called the Power Good signal, sometimes written as Power_OK or Power_Good or it can be distinguished by its gray color. Its purpose is to tell the computer all is well with the power...

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
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....

) after the PS_ON# signal is pulled low.
+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
Four-terminal sensing
Four-terminal sensing , 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal 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:

Supply [V]	Tolerance	Range (min. to max.)	Ripple (p. to p. max.)
+5 V_DC	±5% (±0.25 V)	+4.75 V to +5.25 V	50 mV
−5 V_DC	±10% (±0.50 V)	–4.50 V to –5.50 V	50 mV
+12 V_DC	±5% (±0.60 V)	+11.40 V to +12.60 V	120 mV
−12 V_DC	±10% (±1.2 V)	–10.8 V to –13.2 V	120 mV
+3.3 V_DC	±5% (±0.165 V)	+3.135 V to +3.465 V	50 mV
+5 V_SB	±5% (±0.25 V)	+4.75 V to +5.25 V	50 mV

Physical characteristics

ATX power supplies generally have the dimensions of 6 × 3.4 × 5.5 (inches) and in metric and share a common mounting layout of four screws arranged on the back side of the unit.

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 solder

Solder

Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...

ed 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

Wake-on-ring

Wake-on-Ring , sometimes referred to as Wake-on-Modem , is a specification that allows supported computers and devices to "wake up" or turn on from a sleeping, hibernating or "soft off" state Wake-on-Ring (WOR), sometimes referred to as Wake-on-Modem (WOM), is a specification that allows supported...

or Wake-on-LAN

Wake-on-LAN

Wake-on-LAN is an Ethernet computer networking standard that allows a computer to be turned on or woken up by a network message....

, but is generally used to power on the computer through a front switch.

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 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.

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.http://www.intel.com/support/processors/pentium/sb/cs-011025.htm This recommendation was removed from later specifications; modern ATX power supplies usually exhaust air from the case.

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.
4-pin Berg floppy connector
Berg connector
A Berg connector is a brand of electrical connector used in computer hardware. Berg connectors are manufactured by Berg Electronics Corporation of St. Louis, Missouri, a division of Framatome Connectors International....

— 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. 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 module
Voltage regulator module
A voltage regulator module or VRM, sometimes called PPM , is a buck converter that provides a microprocessor the appropriate supply voltage, converting +5 V or +12 V to a much lower voltage required by the CPU. Some are soldered to the motherboard while others are installed in an open slot...

s 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.

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. 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
Pentium 4
Pentium 4 was a line of single-core desktop and laptop central processing units , introduced by Intel on November 20, 2000 and shipped through August 8, 2008. They had a 7th-generation x86 microarchitecture, called NetBurst, which was the company's first all-new design since the introduction of the...

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 lesser changes 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).
The −5 V rail is prohibited.

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

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. 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.

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.

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

PCI Express

PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...

graphics cards, that aids the PCIe slot in the motherboard, delivering 75 watts.

ATX12V v2.2

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

ATX12V v2.3

Effective March 2007 and current . 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. The reduced load requirement allows compatibility with processors that draw very little power during startup. The absolute over-current limit of 240VA per rail was removed, allowing 12V lines to provide more than 20A per rail.

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.

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

Symmetric multiprocessing

In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...

/multi-core systems such as Core 2, Core i7, Opteron

Opteron

Opteron is AMD's x86 server and workstation processor line, and was the first processor which supported the AMD64 instruction set architecture . It was released on April 22, 2003 with the SledgeHammer core and was intended to compete in the server and workstation markets, particularly in the same...

and Xeon

Xeon

The Xeon is a brand of multiprocessing- or multi-socket-capable x86 microprocessors from Intel Corporation targeted at the non-consumer server, workstation and embedded system markets.-Overview:...

. 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

Accelerated Graphics Port

The Accelerated Graphics Port is a high-speed point-to-point channel for attaching a video card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Since 2004 AGP has been progressively phased out in favor of PCI Express...

or PCIe

PCI Express

PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...

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 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.
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.
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.

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.

Issues with Dell power supplies

Older Dell computers, particularly those from the Pentium II

Pentium II

The Pentium II brand refers to Intel's sixth-generation microarchitecture and x86-compatible microprocessors introduced on May 7, 1997. Containing 7.5 million transistors, the Pentium II featured an improved version of the first P6-generation core of the Pentium Pro, which contained 5.5 million...

and III

Pentium III

The Pentium III brand refers to Intel's 32-bit x86 desktop and mobile microprocessors based on the sixth-generation P6 microarchitecture introduced on February 26, 1999. The brand's initial processors were very similar to the earlier Pentium II-branded microprocessors...

lines, are notable for using proprietary

Proprietary hardware

Proprietary hardware is computer hardware which is owned by the proprietor.Historically, most early computer hardware was designed as proprietary until the 1980s, when IBM PC changed this paradigm...

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.http://www.informit.com/articles/article.aspx?p=339053

The change affects not only 20-pin ATX connectors, but also auxiliary 6-pin connectors. Modern Dell systems might use standard ATX connectors.http://pinouts.ru/Power/dell_atxpower_pinout.shtml 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 propriety 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. A more reliable method is to measure the voltages on the connector.

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