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Modern motherboards can be any size or shape that suits the design of the computer. Only the need for standard-size expansion slot connectors limits the freedom of design. Many computers, however, are built around motherboards of a few standard sizes. This standardization is a matter of convenience. It allows a computer manufacturer flexibility in the choice of suppliers—standardized dimensions make motherboards interchangeable.

For you as the purchaser of a new computer, motherboard standardization has its downside. You face the problem of the computer manufacturer selling systems equipped with a motherboard du jour, whatever OEM motherboard was available cheapest on the day the computer was put together. On the other hand, a computer built around a standard-sized motherboard gives you upgrade freedom. Should you become dissatisfied in the performance of your computer, you can replace a standard-size motherboard with a more powerful one.

The earliest motherboard standards followed the leads set by IBM. They duplicated the physical dimensions of the motherboards used by the most popular IBM machines. Even when they lopped off vast areas of board to trim costs, most manufacturers retained compatibility with IBM's designs, keeping mounting holes in the same locations so that one board could be substituted for another. This heritage continues even in some of the latest designs.

After IBM ceded its influence as the setter of the standard dimensions of motherboards, the industry was essentially adrift. Major manufacturers developed their own designs without regard to older products, while smaller manufacturers clung to the old board layouts. The situation is changing, though, with new motherboards standards now promulgated by Intel. The most recent of these, the ATX motherboard design, goes further than ever before and specifies not only dimensions and mounting holes but also connector placement and even connector designs. To standardize the motherboards of more powerful computer workstations, the computer industry recently developed and then dropped the WTX specification.

Motherboard design divergence first arose among makers of small-footprint computers, machines designed with smaller dimensions to cover less of your desktop. These machines compromised expansion by reducing the number of expansion slots and drive bays to gain their more modest measurements. Many manufacturers developed low-profile computers that reduced system height by turning expansion boards on their sides. These designs necessitated changes from the more standardized motherboard layout. The computer industry has rallied around the low-profile design and produced two standards that support the concept. First came the LPX motherboard, and then came a smaller derivative, the Mini-LPX. To accommodate the needs of new microprocessor and memory technologies (and to add a wealth of other features), the industry adopted the NLX motherboard.

In general, smaller manufacturers are more likely to use standard-size motherboards. Larger manufacturers are better able to afford the price of custom-designing cases and motherboards to match. Even large computer-makers have moved to standard-size boards at least for their offerings that take advantage of the latest microprocessors. In truth, systems that use the latest microprocessors often all have exactly the same motherboard design inside, using a motherboard designed and manufactured by Intel. Even large manufacturers may rely on Intel motherboards until their engineers develop a familiarity with new chips. For example, all but a handful of the first Pentium Pro computer models uniformly used standard-size Intel-manufactured motherboards.


To bring a degree of uniformity to motherboard design, the computer industry created a new motherboard standard that roughly conforms to the Mini-AT board size but with a few design twists that result in lower-cost engineering. Called ATX, the standard is promulgated by Intel but is openly published. Intel released the most recent version, 2.1, in February 1996, fine-tuning the design based on industry feedback.

The ATX standard defines the number and position of the motherboard mounting holes and offers recommendations as to component, expansion board, and port connector placement. Although the standard does not demand any particular slot type or configuration, it's aimed primarily at ISA, PCI, and ISA/PCI combination designs. It also allows for both 5.0- and 3.3-volt system operation (or both simultaneously).

The odd orientation of the board facilitates port placement. It provides the maximum space for expansion boards and port connectors at the rear of the host computer chassis. The design also envisions that the microprocessor will be located near the right edge of the board, where it will be in close proximity to both the power supply and the cooling fan. In the recommended configuration, memory sockets can be readily accessed between the microprocessor and expansion slots.

The ATX board itself measures a maximum of 12 by 9.6 inches (305 by 244 millimeters). This size is not a random choice but, according to Intel, was selected to allow manufacturers to cut two boards from a standard-size 18-by-24-inch raw printed circuit panel. It provides sufficient space for about seven expansion slots, which are spaced at the conventional 0.8 inches apart. It incorporates nine mandatory and one optional mounting holes, most of which are in the same positions as the holes in a Mini-AT motherboard. Figure 29.1 shows the dimensions and mounting hole placement for an ATX motherboard.

Figure 29.1. Dimensions and hole placement of an ATX motherboard.


The ATX specification goes further than simply indicating mechanical board dimensions. The standard also embraces the PS/2 size of power supply and specifies a new motherboard power connector (see Chapter 31, "Power").

Besides uniformity, the ATX design aims at trimming costs for computer-makers. By putting port connectors on the motherboard, even in multiple layers, the cost of connecting cables as well as the labor required for assembly is eliminated. Eliminating cables also helps minimize potential radio frequency interference. In its recommended configuration, the ATX layout also allows the use of shorter floppy and hard disk connecting cables, with similar benefits. The power supply choice and location also trims cost for the computer manufacturer as well as helps the computer run cooler and even quieter.


The designers of the ATX board realized that the one certainty in computer circuit design is that functions get combined and made more compact. As more and more of the functions of a computer squeezed into one or two chips, the ATX designers imagined that soon much of the ATX real estate would be superfluous. In that one of the primary goals in the design of ATX was trimming costs, they figured that trimming motherboards to a size smaller than ATX as the technology permitted would reap savings in materials cost. Consequently, they included a standard size for Mini-ATX motherboards in the original ATX specification.

The Mini-ATX design chops the ATX motherboard down to 11.2 by 8.2 inches (284 by 208 millimeters). When installed in a computer, the Mini-ATX motherboard still sits at the rear edge of the chassis so that port connectors can be mounted directly to it without cables. In most chassis, the left edge still aligns with the left side of the case to allow space for a full complement of expansion boards. Because of this placement, the smaller size of the Mini-ATX board cuts off one row of ATX mounting holes. As a result, the lower row of mounting holes is displaced on the Mini-ATX design, as shown in Figure 29.2.

Figure 29.2. Mini-ATX motherboard dimensions and layout.



The Mini-ATX design has one chief benefit. It reduces the materials costs for a motherboard by about 30 percent when compared to a full-size ATX board. But that wasn't enough for the creators of the ATX specifications. They created another small-size motherboard called microATX, which took over the role of Mini-ATX. The current incarnation of the standard, version 1.0, was published in 1997.

The microATX specification represents more rethinking than a simple extra shrinking. The new design is actually a bit larger than Mini-ATX. A microATX motherboard measures 9.6 inches (244 millimeters) square.

The chief difference between Mini-ATX and microATX is how the two mate with the full-size ATX standard. Mini-ATX motherboards pay little heed to the larger standard; microATX boards fit neatly inside it. Although microATX motherboards are narrower than full-size ATX designs, they use nearly the same mounting scheme, so a microATX board will fit into a full-size chassis. Expansion slots appear in the same position, except microATX allows for fewer slots because of its reduced size—a maximum of four slots, typically three PCI and one AGP. (ISA slots are also allowed.) In addition, most of the screw holes used for mounting a microATX motherboard match the positions of those on a full-size ATX motherboard.

To mate with the conventional ATX chassis, the microATX motherboard reserves the same space for port connectors as does the full-size board, and the two boards use exactly the same power connectors. Figure 29.3 illustrates the microATX motherboard and compares it to its full-size sibling.

Figure 29.3. The microATX motherboard layout.



If small is good, even smaller is better. In 1999, Intel's engineers unveiled yet another motherboard format, FlexATX, which shaves inches off the microATX design. The name reflects the design goal: FlexATX is meant to be flexible and allows for the development of devices beyond the traditional personal computer (but based on the traditional personal computer motherboard). In the classification system of its promoters, FlexATX is not a standalone standard but an addendum to the microATX specification.

A FlexATX motherboard measures no more than 9 inches wide and 7.5 inches deep. It retains the same area (in both size and location) for ports and connectors as that used by ATX and microATX motherboards.

Seeing a wide application of compact motherboards, however, the FlexATX design fits inside the conventional ATX chassis. The placement of its mounting holes matches those of an ATX motherboard, although (as with microATX) two added holes provide mounting security on the edge adjacent to where excess glass-epoxy was pared off.

The FlexATX design does not specify the number and location of expansion slots because the creators of the standard envisioned its use in systems that provide no board-style expansion (although the standard does permit engineers to use their imagination and add slots to the design).

Table 29.1 lists the dimensions for ATX motherboards and those derived from that standard. The complete ATX specifications and those related to it are available on the Web at www.formfactor.org.

Table 29.1. Dimensions of Modern Motherboards
Motherboard Width (in) Length (mm) Area (in) Area (mm) Area (Square in) Area (Square cm)
ATX 12.0 305 9.6 244 115.2 744.2
Mini-ATX 11.2 284 8.2 208 91.8 590.7
microATX 9.6 244 9.6 244 92.2 595.4
FlexATX 9.0 229 7.5 191 67.5 437.4


Announced in September 1996, by Intel, the NLX motherboard design was a cooperative effort among several manufacturers, including IBM. At the time of its introduction, 12 computer-makers and one motherboard-only manufacturer (ASUStek) had announced support of the specification. The computer-makers included AST, Digital Equipment Corporation (now part of Compaq), Fujitsu, Gateway 2000, Hewlett-Packard, IBM, ICL Personal Computers, Micron Electronics, NEC Computer Systems, Sony, Toshiba, and Tulip Computers.

NLX is an improved low-profile layout. It was created specifically to overcome some shortcomings of earlier, no longer supported designs (LPX and its reduced-size version, Mini-LPX), in that those designs interfered with adapting to the latest technologies. NLX is meant to support all current Intel microprocessor designs and memory technologies as well as the advanced graphics port (AGP) for high-speed interconnections with video boards. In addition, the NLX design enhances the physical packaging of systems to allow greater mechanical integrity, better cooling, and more space for peripheral ports.

Key to the NLX design is the riser board. Although it's similar to the riser boards of LPX systems, because you plug expansion boards into it, under the NLX design the riser takes on a greater role. In effect, it operates as a backplane and the NLX motherboard itself is a glorified processor board. The riser board attaches permanently to the chassis of its host computer while the NLX motherboard readily slides in and out of the case like a steroid-enhanced expansion board. In purest form of the NLX implementation, all cables in the system—including the power supply—attach to the riser, and none attach to the motherboard. Typically the cables for the floppy disk and hard disk drives in a computer will plug into the riser.

Although the design of the NLX motherboard is fixed by the specification, the riser board is not. Computer-makers can customize its design to accommodate different system designs. Although the typical riser board has four expansion connectors, the NLX specification allows a great deal of freedom to accommodate not only low-profile computers but also tower-style systems. The specification describes signals for up to five PCI expansion slots and an unlimited number of ISA slots on a single riser. All boards slide into the riser parallel to the motherboard.

The one aberration in the NLX motherboard/riser design is its accommodation for AGP video boards. The specification reserves a special area on the left side of the motherboard, opposite the riser board, for a single AGP slot. The AGP board slides into the motherboard slot parallel to the riser board. Microprocessors (the NLX design accommodates up to two) and memory reside on the right side of the motherboard so that they do not interfere with expansion boards. Figure 29.4 shows the basic layout of this two-board system.

Figure 29.4. NLX motherboard layout.


An L-shaped integral rear panel of the board provides space for peripheral connectors. The higher part of the panel allows designers to stack multiple connectors, one over the other, on the right side of the chassis, away from expansion boards. The NLX design envisions the motherboard to be readily removable without removing an expansion board (except the AGP board) from inside the computer case. In a typical computer, the motherboard slides out the side of the chassis, guided by rails at the bottom of the chassis. Four screws inside the computer secure the board to the chassis and electrically ground the two together. A latch that's part of the chassis and under the motherboard holds the board horizontally in place and also serves as a board ejector to aid in removing the motherboard. Spring-like contact fingers around the periphery of the rear panel shield electrically integrate the board with the rest of the chassis.

The mechanical specification of NLX motherboards allows for a small degree of freedom in their size. The specs allow any width between eight and nine inches (inclusive), except for systems integrating AGP video boards. These must be nine inches wide because the space reserved for the AGP connector is on the last inch of the board. NLX motherboards may be between 10.0 and 13.6 inches long. The smallest NLX motherboard is approximately the size of a mini-LPX motherboard; the largest about the size of an LPX board.

The NLX specifications provide for three different mounting screw patterns, depending on the length of the board. An NLX motherboard may use any of the three patterns, but each NLX chassis design must accommodate all three.

NLX uses a 340-pin connector between the motherboard and riser. This single connector carries signals for the ISA and PCI buses, IDE drives, and miscellaneous system functions. In addition, the specification reserves space for a larger connector to accommodate future, wider expansion buses and an optional connector for other system features.


Workstations require greater power and design flexibility from ordinary general-purpose computers, so in September, 1998, Intel introduced a new specification aimed at standardizing many aspects of workstation design. Under the specification, motherboards could be up to 14 by 16.75 inches or any size smaller. This design proved superfluous, however, and although the WTX specification is still published on the Web at www.wtx.org, its promoters no longer advocate its use.

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