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Motherboard Form Factors

There are several compatible form factors used for motherboards. The form factor refers to the physical dimensions and size of the board, and dictates what type of case the board will fit into. The types of motherboard form factors generally available are the following:

Backplane Systems

Not all systems have a motherboard in the true sense of the word. In some systems, the components normally found on a motherboard are located instead on an expansion adapter card plugged into a slot. In these systems, the board with the slots is called a backplane, rather than a motherboard. Systems using this type of construction are called backplane systems.

Backplane systems come in two main types: passive andactive. A passive backplane means the main backplane board does not contain any circuitry at all except for the bus connectors and maybe some buffer and driver circuits. All the circuitry found on a conventional motherboard is contained on one or more expansion cards installed in slots on the backplane. Some backplane systems use a passive design that incorporates the entire system circuitry into a single mothercard. The mothercard is essentially a complete motherboard that is designed to plug into a slot in the passive backplane. The passive backplane/mothercard concept allows the entire system to be easily upgraded by changing one or more cards. Because of the expense of the high function mothercard, this type of system design is rarely found in PC systems. The passive backplane design does enjoy popularity in industrial systems, which are often rack-mounted. Some high-end file servers also feature this design.

An active backplane means the main backplane board contains bus control and usually other circuitry as well. Most active backplane systems contain all the circuitry found on a typical motherboard except for the processor complex. Theprocessor complex is the name of the circuit board that contains the main system processor and any other circuitry directly related to it, such as clock control, cache, and so forth. The processor complex design allows the user to easily upgrade the system later to a new processor type by changing one card. In effect, it amounts to a modular motherboard with a replaceable processor section. Most modern PC systems that use a backplane design use an active backplane/processor complex. Both IBM and Compaq have used this type of design in some of their high-end (server class) systems, for example. This allows an easier and generally more affordable upgrade than the passive backplane/mothercard design since the processor complex board is usually much cheaper than a mothercard. Unfortunately, because there are no standards for the processor complex interface to the system, these boards are proprietary and can only be purchased from the system manufacturer. This limited market and availability causes the prices of these boards to be higher than most complete motherboards from other manufacturers.

The motherboard system design and the backplane system design have both advantages and disadvantages. Most original personal computers were designed as backplanes in the late 1970s. Apple and IBM shifted the market to the now traditional motherboard with a slot-type design because this type of system generally is cheaper to mass-produce than one with the backplane design. The theoretical advantage of a backplane system, however, is that you can upgrade it easily to a new processor and new level of performance by changing a single card. For example, you can upgrade a system's processor just by changing the card. In a motherboard-design system, you often must change the motherboard itself, a seemingly more formidable task. Unfortunately, the reality of the situation is that a backplane design is often much more expensive to upgrade, and because the bus remains fixed on the backplane, the backplane design precludes more comprehensive upgrades that involve adding local bus slots, for example.

Another nail in the coffin of backplane designs is the upgradable processor. Intel has designed all 486, Pentium, Pentium MMX, and Pentium Pro processors to be upgradable to faster (sometimes called OverDrive) processors in the future by simply swapping (or adding) the new processor chip. Changing only the processor chip for a faster one is the easiest and generally most cost-effective way to upgrade without changing the entire motherboard.

Because of the limited availability of the processor complex boards or mothercards, they usually end up being more expensive than a complete new motherboard that uses an industry standard form factor. Intel recently announced the new NLX form factor for the Pentium II, and it shares some traits with traditional backplane systems. The NLX has been promised considerable industry support, so we may well see affordable backplane systems in the near future.

Full-Size AT

The full-size AT motherboard is so named because it matches the original IBM AT motherboard design. This allows for a very large board of up to 12 inches wide by 13.8 inches deep. The keyboard connector and slot connectors must conform to specific placement requirements to fit the holes in the case. This type of board will fit into full-size AT or Tower cases only. Because these motherboards will not fit into the popular Baby-AT or Mini-Tower cases, and because of advances in component miniaturization, they are no longer being produced by most motherboard manufacturers.


The Baby-AT form factor is essentially the same as the original IBM XT motherboard, with modifications in screw hole positions to fit into an AT-style case (see Figure 4.1). These motherboards also have specific placement of the keyboard connector and slot connectors to match the holes in the case. Note that virtually all full-size AT and Baby-AT motherboards use the standard 5-pin DIN type connector for the keyboard. Baby-AT motherboards will fit into every type of case except the Low Profile or Slimline cases. Because of their flexibility, this is now the most popular motherboard form factor. Figure 4.1 shows the dimensions and layout of a Baby-AT motherboard.


FIG.  Baby-AT motherboard form factor.


Another popular form factor used in motherboards today is the LPX and Mini-LPX form factors. This form factor was first developed by Western Digital for some of their motherboards. Although they no longer produce PC motherboards, the form factor lives on and has been duplicated by many other motherboard manufacturers. These are used in the Low Profile or Slimline case systems sold widely today. These are often lower-cost systems like those sold at retail electronics superstores. It should be noted that systems using LPX boards may have other differences which can cause compatibility problems similar to those of proprietary systems.

The LPX boards are characterized by several distinctive features. The most noticeable is that the expansion slots are mounted on a bus riser card that plugs into the mother- board. Expansion cards must plug sideways into the riser card. This sideways placement allows for the low profile case design. Slots are located on one or both sides of the riser card depending on the system and case design.

Another distinguishing feature of the LPX design is the standard placement of connectors on the back of the board. An LPX board has a row of connectors for video (VGA 15-pin), parallel (25-pin), two serial ports (9-pin each), and mini-DIN PS/2 style Mouse and Keyboard connectors. All of these connectors are mounted across the rear of the motherboard and protrude through a slot in the case. Some LPX motherboards may have additional connectors for other internal ports such as Network or SCSI adapters. Figure 4.2 shows the standard form factors for the LPX and Mini-LPX motherboards used in many systems today.


FIG.  LPX and Mini-LPX motherboard form factors.


The ATX form factor is a recent 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 LPX designs. In other words, a different case and power supply are required to match the ATX motherboard. These new case and power supply designs have become common, and can be found in many new systems.

The official ATX specification was released by Intel in July 1995, and has been written as an open specification for the industry. The latest revision of the specification is Version 2.01, published in February 1997. Intel has published detailed specifications so other manufacturers can use the ATX design in their systems.

ATX improves on the Baby-AT and LPX motherboard designs in several major areas:

Figure 4.3 shows the new ATX system layout and chassis features. Notice how the entire motherboard is virtually clear of the drive bays, and how the devices like CPU, memory, and internal drive connectors are easy to access and do not interfere with the bus slots. Also notice the power supply orientation and the single power supply fan that blows into the case directly over the high heat, generating items like the CPU and memory.


FIG.  ATX system chassis layout and features.

The ATX motherboard is basically a Baby-AT design rotated sideways. The expansion slots are now parallel to the shorter side dimension and do not interfere with the CPU, memory, or I/O connector sockets. In addition to a full-sized ATX layout, Intel also has specified a mini-ATX design as well, which will fit into the same case. Although the case holes are similar to the Baby-AT case, cases for the two formats are generally not compatible. The power supplies would require a connector adapter to be interchangeable, but the basic ATX power supply design is similar to the standard Slimline power supply. The ATX and mini-ATX motherboard dimensions are shown in Figure 4.4.


FIG.  ATX and Mini-ATX motherboard form factors.

Clearly, the advantages of the ATX form factor make it a good choice for high-end systems. For backwards compatibility, Baby-AT is still hard to beat, and there are still more Baby-AT motherboards, cases, and power supplies on the market than the ATX versions. With the coming of NLX motherboards and the support that form factor is receiving from the industry, it seems unlikely that ATX will be the all encompassing wave of the future.

For complete specifications, check out the ATX Motherboard Specification page at


NLX is the latest development in desktop motherboard technology, and may prove to be the form factor of choice in the near future. It is a low-profile form factor similar in appearance to LPX, but with a number of improvements designed to allow full integration of the latest technologies. Whereas the primary limitation of LPX boards includes an inability to handle the physical size of newer processors, as well as their higher thermal characteristics, the NLX form factor has been designed specifically to address these problems.

Specific advantages of the NLX form factor include:

With the basic NLX system layout notice that, like ATX, the system is clear of the drive bays and other chassis-mounted components. Also, the motherboard and I/O cards (which, like the LPX form factor, are mounted parallel to the motherboard) can easily be slid in and out of the side of the chassis, leaving the riser card and other cards in place. The processor itself can be easily accessed and enjoys greater cooling than in a more closed in layout.

As you can see, the NLX form factor has been designed for maximum flexibility and space efficiency. Even extremely long I/O cards will fit easily, without fouling on other system components as has been such a problem with Baby-AT form factor systems.

Complete design specifications and information on NLX boards can be found online at the official NLX Motherboard Specification page, located at:

ATX and NLX form factors will probably be used in most future systems. I usually do not recommend LPX style systems if upgradability is a factor because it is not only difficult to locate a new motherboard that will fit, but LPX systems are also limited in expansion slots and drive bays as well. Baby-AT systems still offer a great deal of flexibility at present, but for future systems, ATX and NLX configurations are the way to go.