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  • Motherboardsand Buses


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  • 196 Chapter 4 Motherboards and Buses

    Motherboard Form FactorsWithout a doubt, the most important component in a PC system is the main board or motherboard.Some companies refer to the motherboard as a system board or planar. The terms motherboard, mainboard, system board, and planar are interchangeable, although I prefer the motherboard designation.This chapter examines the various types of motherboards available and those components typicallycontained on the motherboard and motherboard interface connectors.

    Several common form factors are used for PC motherboards. The form factor refers to the physicaldimensions (size and shape) as well as certain connector, screw hole, and other positions that dictateinto which type of case the board will fit. Some are true standards (meaning that all boards with thatform factor are interchangeable), whereas others are not standardized enough to allow for inter-changeability. Unfortunately, these nonstandard form factors preclude any easy upgrade or inexpen-sive replacement, which generally means they should be avoided. The more commonly known PCmotherboard form factors include the following:

    Obsolete Form Factors


    Full-size AT

    LPX (semiproprietary)

    WTX (no longer in production)

    ITX (flex-ATX variation, neverproduced)

    Modern Form Factors




    Mini-ITX (flex-ATXvariation)


    All Others

    Fully proprietary designs(certain Compaq, PackardBell, Hewlett-Packard,notebook/portable sys-tems, and so on)

    Motherboards have evolved over the years from the original Baby-AT form factor boards used in theoriginal IBM PC and XT to the current ATX and NLX boards used in most full-size desktop and towersystems. ATX has a number of variants, including micro-ATX (which is a smaller version of the ATXform factor used in the smaller systems) and flex-ATX (an even smaller version for the lowest-costhome PCs). A new form factor called mini-ITX is also available; its really just a minimum-size versionof flex-ATX designed for very small systems. NLX is designed for corporate desktoptype systems;WTX was designed for workstations and medium-duty servers, but never became popular. Table 4.1shows the modern industry-standard form factors and their recommended uses.

    Table 4.1 Current Industry-Standard Motherboard Form Factors

    Form Factor Use

    ATX Standard desktop, mini-tower, and full-tower systems; most common form factor today; mostflexible design for power users, enthusiasts, low-end servers/workstations, and higher-endhome systems; ATX boards support up to seven expansion slots.

    Mini-ATX A slightly smaller version of ATX that fits into the same case as ATX. Many so-called ATX motherboards are actually mini-ATX motherboards; mini-ATX boards support up to six expansion slots.

    micro-ATX A smaller version of ATX, used in Mid-range desktop or mini-tower systems. Fits micro-ATX orATX chassis.

    Flex-ATX Smallest version of ATX, used in expensive or low-end small desktop or mini-tower systems; entertainment or appliance systems. Fits in flex-ATX, micro-ATX, or ATX chassis.

    Mini-ITX Minimum-size flex-ATX version, used in set-top boxes and compact/small form factor comput-ers; highly integrated with one PCI expansion slot. Fits in mini-ITX, flex-ATX, micro-ATX, or ATXchassis.

    NLX Corporate desktop or mini-tower systems; fast and easy serviceability.

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  • Motherboard Form Factors 197

    Although the Baby-AT, Full-size AT, and LPX boards were once popular, they have all but beenreplaced by more modern and interchangeable form factors. The modern form factors are true stan-dards, which guarantees improved interchangeability within each type. This means that ATX boardscan interchange with other ATX boards, NLX with other NLX, and so on. The additional featuresfound on these boards as compared to the obsolete form factors, combined with true interchangeabil-ity, has made the migration to these newer form factors quick and easy. Today I recommend purchas-ing only systems with one of the modern industry-standard form factors. Each of these form factors,however, is discussed in more detail in the following sections.

    Anything that does not fit into one of the industry-standard form factors is considered proprietary.Unless there are special circumstances, I do not recommend purchasing systems with proprietaryboard designs. They will be virtually impossible to upgrade and very expensive to repair later becausethe motherboard, case, and often power supply will not be interchangeable with other models. I callproprietary form factor systems disposable PCs because thats what you must normally do withthem when they are too slow or need repair out of warranty.

    CautionDisposable PCs might be more common than ever. Some estimate that as much as 60% of all PCs sold today are dis-posable models, not so much because of the motherboards used, but because of the tiny power supplies and crampedmicro-tower cases that are favored on most retail-market PCs today. Although low-cost PCs using small chassis and powersupplies are theoretically more upgradeable than past disposable type systems, youll still hit the wall over time if you needmore than three expansion slots or want to use more than two or three internal drives. Because mini-tower systems are socramped and limited, I consider them to be almost as disposable as the LPX systems they have largely replaced.

    You also need to watch out for systems that only appear to meet industry standards, such as certain Dell computer modelsbuilt from 1996 to the presentespecially the XPS line of systems. These computers often use rewired versions of the ATXpower supply (or even some that are completely nonstandard in size and shape) and modified motherboard power con-nectors, which makes both components completely incompatible with standard motherboards and power supplies. In someof the systems, the power supply has a completely proprietary shape as well and the motherboards are not fully standardATX either. If you want to upgrade the power supply, you must use a special Dell-compatible power supply. And if youwant to upgrade the motherboard (assuming you can find one that fits), you must buy a standard power supply to match.The best alternative is to replace the motherboard, power supply, and possibly the case with industry-standard componentssimultaneously. For more details about how to determine whether your Dell computer uses nonstandard power connectors,see Chapter 21, Power Supply and Chassis/Case.

    If you want to have a truly upgradeable system, insist on systems that use ATX motherboards in a mid-tower or larger casewith at least five drive bays.

    PC and XTThe first popular PC motherboard was, of course, the original IBM PC released in August 1981. Figure4.1 shows how this board looked. IBM followed the PC with the XT motherboard in March 1983,which had the same size and shape as the PC board but had eight slots instead of five. Both the IBMPC and XT motherboards were 9''13'' in size. Also, the slots were spaced 0.8'' apart in the XT insteadof 1'' apart as in the PC (see Figure 4.2). The XT also eliminated the little used cassette port in theback, which was supposed to be used to save BASIC programs on cassette tape instead of the muchmore expensive (at the time) floppy drive.

    NoteThe Technical Reference section of the DVD accompanying this book contains detailed information on the PC (5150) andXT (5160). All the information there is printable.

    Chapter 4

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  • 198 Chapter 4 Motherboards and Buses

    The minor differences in the slot positions and the deleted cassette connector on the back required aminor redesign of the case. In essence, the XT was a mildly enhanced PC, with a motherboard thatwas the same overall size and shape, used the same processor, and came in a case that was identicalexcept for slot bracketry and the lack of a hole for the cassette port. Eventually, the XT motherboarddesign became very popular, and many other PC motherboard manufacturers of the day copied IBMsXT design and produced similar boards.

    Full-Size ATThe full-size AT motherboard form factor matches the original IBM AT motherboard design. This allowsfor a very large board of up to 12'' wide by 13.8'' deep. The full-size AT board first debuted in August1984, when IBM introduced the Personal Computer AT (advanced technology). To accommodate the 16-bit 286 processor and all the necessary support components at the time, IBM needed more room thanthe original PC/XT-sized boards could provide. So for the AT, IBM increased the size of the motherboardbut retained the same screw hole and connector positions of the XT design. To accomplish this, IBMessentially started with a PC/XT-sized board and extended it in two directions (see Figure 4.3).

    NoteThe Technical Reference section of the DVD enclosed with this book contains detailed coverage of the AT and the XT Model 286.

    A little more than a year after being introduced, the appearance of chipsets and other circuit consolida-tion allowed the same motherboard functionality to be built using fewer chips, so the board wasredesigned to make it slightly smaller. Then, it was redesigned again as IBM shrank the board down toXT-size in a system it called the XT-286 (introduced in September 1986). The XT-286 board was virtuallyidentical in size and shape to the original XT, a form factor which would later be known as Baby-AT.

    {Pin 1 Speaker output

    System Configuration DIPswitches



    Intel 8088processor

    Intel 8087 mathcoprocessor

    System-board p