The ATX Modern Form Factor For Computer Motherboards
This document provides an overview of the ATX motherboards for personal computers.
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Saturday, October 19, 2013 6:14 PM
The ATX Motherboard
The ATX form factor was the first of a dramatic evolution in motherboard form factors. ATX is a combination of the best features of the Baby-AT and LPX motherboard designs, with many new enhancements and features thrown in. The ATX form factor is essentially a Baby-AT motherboard turned sideways in the chassis, along with a modified power supply location and connector. The most important thing to know initially about the ATX form factor is that it is physically incompatible with either the previous Baby-AT or the LPX design. In other words, a different case and power supply are required to match the ATX motherboard. These case and power supply designs have become common and are found in most new systems.
Intel initially released the official ATX specification in July 1995. It was written as an open specification for the industry. ATX boards didn’t hit the market in force until mid-1996, when they rapidly began replacing Baby-AT boards in new systems. The ATX specification was updated to version 2.01 in February 1997, 2.03 in May 2000, 2.1 in June 2002, and 2.2 in February 2004. Intel publishes these detailed specifications so other manufacturers can use the interchangeable ATX design in their systems. The current specifications for ATX and other current motherboard types are available online from the Desktop Form Factors site: www.formfactors.org. ATX is the most popular motherboard form factor for new systems and will continue to be popular in the future. An ATX system will be upgradeable for many years to come, exactly like Baby-AT was in the past.
Figure 4.13 shows the typical ATX system layout and chassis features, as you would see them looking in with the lid off on a desktop, or sideways in a tower with the side panel removed. Notice how virtually the entire motherboard is clear of the drive bays and how the devices such as CPU, memory, and internal drive connectors are easy to access and do not interfere with the bus slots.
ATX improved on the Baby-AT and LPX motherboard designs in several major areas:
Built-in double high external I/O connector panel— The rear portion of the motherboard includes a stacked I/O connector area that is 6 1/4 inches wide by 1 3/4 inches tall. This enables external I/O connectors to be located directly on the board and minimizes the need for cables running from internal connectors to the back of the case as with Baby-AT designs.
Single main keyed internal power supply connector— The ATX specification includes a keyed and shrouded main power connector that is easy to plug in and install. This connector also features pins for supplying 3.3V to the motherboard, helping to minimize the use of built-in voltage regulators that are susceptible to failure.
Relocated CPU and memory— The CPU and memory modules are relocated so they can’t interfere with bus expansion cards and can easily be accessed for upgrade without removing any of the installed bus adapters.
Relocated internal I/O connectors— The internal I/O connectors for the floppy and hard disk drives are relocated to be near the drive bays and out from under the expansion board slot and drive bay areas.
Improved cooling— The CPU and main memory are designed and positioned to improve overall system cooling compared to Baby-AT and older designs.
Lower cost to manufacture— The ATX specification eliminates the need for the rat’s nest of cables to external I/O port connectors found on Baby-AT motherboards.
Figure 4.13. Typical ATX system layout.
Although most ATX systems mount the power supply near the processor (on top in most tower arrangements), this is not a requirement of the standard. Some systems mount the power supply in other locations (such as on the bottom).
The ATX motherboard shape is basically a Baby-AT design rotated sideways 90°. The expansion slots are now parallel to the shorter side dimension and do not interfere with the CPU, memory, or I/O connector sockets (see Figure 4.14). In addition to a full-size ATX layout, Intel originally specified a Mini-ATX design, which is a fully compatible subset of ATX that fits into the same case:
A full-size ATX board is 12 inches wide×9.6 inches deep (305mm×244mm).
The Mini-ATX board is 11.2 inches×8.2 inches (284mm×208mm).
Figure 4.14. ATX specification 2.2 motherboard dimensions. Most recent ATX motherboards no longer use ISA expansion slots.
Mini-ATX is not an official standard; instead, it is simply referenced as a slightly smaller version of ATX. In fact, all references to Mini-ATX were removed from the ATX 2.1 and later specifications. Two smaller official versions of ATX exist, called microATX and FlexATX. They are discussed in the following sections.
Although the case holes are similar to the Baby-AT case, cases for Baby-AT and ATX are generally incompatible. The ATX power supply design is identical in physical size to the standard Slimline power supply used with Baby-AT systems; however, they also use different connectors and supply different voltages.
The best way to tell whether your system has an ATX-family motherboard design without removing the lid is to look at the back of the system. Two distinguishing features identify ATX. One is that the expansion boards plug directly into the motherboard. There is usually no riser card as with LPX and NLX (except for certain Slimline systems, such as rack-mounted servers), so the slots are usually perpendicular to the plane of the motherboard. Also, ATX boards have a unique double-high connector area for all the built-in connectors on the motherboard (see Figure 4.15 and Table 4.3). This is found just to the side of the bus slot area and can be used to easily identify an ATX board.
Figure 4.15. ATX motherboard and rear panel connections from systems with onboard sound and video (all), networking and IEEE 1394/FireWire (middle and bottom), and a “legacy-free” system with integrated DVI-I, HDMI, eSATA, and USB 3.0 ports (bottom).
Table 4.3. Built-In Ports Usually Found on ATX Motherboards
PS/2 mouse port
PS/2 keyboard port
USB 2.0 ports
Dual Stack USB
USB 3.0 ports
Dual Stack USB
VGA analog video port
15-pin HD D-Submini
Audio ports: L/R in, front L/R out, rear L/R out, center/LFE out, Microphone L/R in
1/8 in. (3.5mm) Mini-Phone
Light blue, lime green, black, black, pink
S-Video TV out (not shown)
IEEE 1394/FireWire port
6-pin IEEE 1394
10/100/1000 Ethernet LAN
Optical S/PDIF audio out
DVI digital video out
Digital S/PDIF audio out
SCSI (not shown)
SCSI Modem (not shown)
Composite video out (not shown)
DIN = Deutsches Institut für Normung e.V.
USB = Universal serial bus
VGA = Video graphics array
HD = High density
MIDI = Musical Instrument Digital Interface
L/R = Left and right channel
LFE = Low frequency effects (subwoofer)
S-Video = Super Video
IEEE = Institute of Electrical and Electronics Engineers
LAN = Local area network
RJ = Registered jack
S/PDIF = Sony/Philips Digital Interface
TOSLINK = Toshiba optical link
DVI = Digital visual interface
DDWG = Digital Display Working Group
RCA = Radio Corporation of America
SCSI = Small computer systems interface
HDMI = High definition multimedia interface
A/V = audio/video
eSATA = external SATA
Most ATX motherboards feature connectors with industry-standardized color codes (shown in the previous table). This makes plugging in devices much easier and more foolproof: You merely match up the colors. For example, most USB keyboards have a cable with a purple plug, whereas most mice have a cable with a green plug. Even though the keyboard and mouse connectors on the motherboard appear the same (both are 6-pin Mini-DIN types), their color-coding matches the plugs on the respective devices. Therefore, to plug them in properly, you merely insert the purple plug into the purple connector and the green plug into the green connector. This saves you from having to bend down to try to decipher small labels on the connectors to ensure you get them right. Some systems have a single PS/2 port (sometimes marked in both green and purple) that can be used for either device.
The specifications and related information covering the ATX and related form factors are available from the Form Factors website at www.formfactors.org. The Form Factors site provides form factor specifications and design guides, as well as design considerations for new technologies, information on initiative supporters, vendor products, and a form factor discussion forum.
Some motherboards, especially those used in server systems, come in nonstandard ATX variations collectively called extended ATX. This is a term applied to boards that are compatible with ATX but that are deeper. Standard ATX is 12 inches×9.6 inches (305mm×244mm), whereas extended ATX boards are up to 12 inches×13 inches (305mm×330mm). Because technically no official “extended ATX” standard exists, compatibility problems can exist with boards and chassis claiming to support extended ATX. When purchasing an extended ATX board, be sure it will fit in the chassis you intend to use. Dual Xeon processors fit in a standard ATX-size board, so choose a standard ATX-size board for maximum compatibility with the existing ATX chassis.