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Design Approaches

Nothing about computers requires a motherboard. You could build a computer without one—at least if you had sufficient knowledge of digital circuits and electronic fabrication, not to mention patience that would make Job seem a member of the television generation. Building a computer around a single centralized circuit board seems obvious, even natural, only because of its nearly universal use. Engineers designed the very first mass-market computers around a big green motherboard layout, and this design persists to this day.

Motherboards exist from more than force of habit, however. For the computer manufacturer, the motherboard design approach has immediate allure. Building a computer with a single large motherboard is often the most economical way to go, at least if your aim is soldering together systems and pushing them out the loading dock. There are alternatives, however, that can be more versatile and are suited to some applications. The more modular approach used in some of these alternatives allows you more freedom in putting together or upgrading a system to try to keep up with the race of technology.

The motherboard-centered design of most computers is actually a compromise approach between two diametrically opposed design philosophies. One approach aims at diversity, adaptability, and expandability by putting the individual functional elements (microprocessor, memory, and input/output circuitry) on separate boards that plug into connectors that link them together through a circuit bus. You can change the power and personality of such a computer as easily as swapping boards. Such machines are known as bus-oriented computers because everything connects through a bus, akin to an expansion bus. The alternative concentrates on economy and simplicity by uniting all the essential components of the computer on a single large board, thus making a single-board computer. Each of these designs has its strengths and weaknesses.

Bus-Oriented Computers

At the time the computer was developed, the bus-oriented design was the conservative approach. A true bus-oriented design seems the exact opposite of the motherboard. Instead of centralizing all circuitry, the bus-oriented design spreads it among multiple circuit boards. It's sort of the Los Angeles approach to computer design—it sprawls out all over without a distinct downtown. Only a freeway system links everything together to make a working community. In the bus-oriented computer, that freeway system is the bus.

The bus approach enabled each computer to be custom-configured for its particular purpose and business. You attached whatever components the computer application required to the bus. When you needed them, you could plug larger, more powerful processors, even multiple processors, into the bus. This modular design enabled the system to expand as business needs expanded. It also allowed for easier service. Any individual board that failed could be quickly removed and replaced without circuit-level surgery.

Actually, among smaller computers that preceded the introduction of the first IBM PC in 1981, the bus-oriented design originated as a matter of necessity simply because all the components required to make a computer would not fit on a circuit board of practical size. The overflowing circuitry had to be spread among multiple boards, and the bus was the easiest way to link them all. Although miniaturization has nearly eliminated such needs for board space, the bus-oriented design still occasionally resurfaces. You'll sometimes find special-purpose computers, such as numerical control systems and network servers, that use the bus-oriented approach for the sake of its modularity.

Single-Board Computers

The advent of integrated circuits, microprocessors, and miniaturized assemblies that put multiple electronic circuit components into a single package, often as small as a fingernail, greatly reduced the amount of circuit board required for building a computer. By the end of the 1970s, putting an entire digital computer on a single circuit board became practical.

Reducing a computer to a single circuit board was also desirable for a number of reasons. Primary among them was cost. Fewer boards means less fabrication expense and lower materials cost. Not only can the board be made smaller, but the circuitry that's necessary to match each board to the bus can be eliminated. Moreover, single-board computers have an advantage in reliability. Connectors are the most failure-prone part of any computer system. The single-board design eliminates the bus connectors as a potential source for system failure.

On the downside, however, the single-board computer design is decidedly less flexible than the bus-oriented approach. The single board has its capabilities forever fixed the moment it is soldered together at the factory. It can never become more powerful or transcend its original design. It cannot adapt to new technologic developments.

Modern technology has made most of us accept the inevitable obsolescence of computers, so the sting of the single-board approach now is much milder than with the first machines. Computers have almost reached the point of being throwaway devices. Locking a machine to a given level of technology is no hardship.

Notebook and sub-notebook computers all follow the single-board design for the sake of compactness. Squeezing everything onto a single board nearly eliminates the need for expansion boards sprouting up everywhere. By minimizing (or eliminating) connections between boards, the single-board design also improves the reliability of notebook machines—there's less to jostle and work itself loose.

But the single-board approach is also becoming more prevalent on the desktop. The motivation is mostly price. Putting everything on a single motherboard allows computer-makers to save the cost of extra boards and connectors, which helps pare down the retail price of machines. Many of today's computers are true single-board designs but still offer expansion slots for later adding options.

Some manufacturers have tried using today's high-speed ports in lieu of an expansion bus to make sealed-box computers, designed to make maintenance costs for businesses essentially zero. With nothing to service or upgrade inside, these machines have no service costs during their operating lifetimes. So far, however, such an approach has not been a marketplace success.

Compromise Designs

Until computers became true mass-market products, most followed a design compromise. Rather than strictly following either the single-board or bus-oriented approach, computer-makers brought the two philosophies together, mixing the best features of the single-board computer and the bus-oriented design in one box. This was the design that IBM chose for its first Personal Computer, which set the design for almost two decades of desktop computers. In it, one large board hosts the essential circuitry that defines the computer, but it relies on additional circuits on secondary circuit boards and provides further space for expansion and adaptability.

Throughout the history of the computer, functions have migrated from auxiliary circuit boards to the motherboard to bring desktop systems closer to the ideal single-board computer design. Early computers required extra circuit boards for their serial and parallel ports. Modern computers pack those ports and USB (and even FireWire) ports on the motherboard. At one time, most system memory, mass-storage interfaces, high-quality sound circuitry, network adapters, and video circuitry all required additional boards. Now many systems incorporate all these functions on their motherboards.

At least three motivations underlie this migration: expectations, cost, and capability. As the power and potential of personal computers have increased, people expect more from their computers. The basic requirements for a personal computer have risen so that features that were once options and afterthoughts are now required. To broaden the market for personal computers, manufacturers have striven to push prices down. Putting the basics required in a computer on the main circuit board lowers the overall cost of the system for exactly the same reasons that a single-board computer is cheaper to make than the equivalent bus-oriented machine. Moreover, using the most modern technologies, manufacturers simply can fit more features on a single circuit board. The original computer had hardly a spare square inch for additional functions. Today, all the features of a computer hundreds of times more powerful than the original IBM PC will fit into a couple of chips.

As with any trend, however, aberrant counter-trends in computer design appear and disappear occasionally. Some system designers have chosen to complicate their systems to make them explicitly upgradable by pulling essential features, such as the microprocessor, off the main board. The rationale underlying this more modular design is that it gives the manufacturer (and your dealer) more flexibility. The computer-makers can introduce new models as fast as they can slide a new expansion board into a box—motherboard support circuitry need not be reengineered. Dealers can minimize their inventories. Instead of stocking several models, the dealer (and manufacturer) need only keep a single box on the shelf, shuffling the appropriate microprocessor module into it as the demand arises. For you, as the computer purchaser, these modular systems also promise upgradability, which is a concept that's desirable in the abstract (your computer need never become obsolete) but often impractical (upgrading is rarely a cost-effective strategy).

Today, the compromise design survives in mainstream machines, although most motherboards delegate only their video circuitry to an expansion board—video being the one place where manufacturers are constantly adding improvements that benefit (mostly) game players. Most machines retain the capability for adding additional circuit boards, even though most people are not apt to bother.

Blade Servers

In the computer rooms of today's businesses, a further variation on motherboard design is making itself prominent. Called the blade server, this new design puts an entire computer—actually a powerful server computer—on a single board that's often the size of an expansion board (see "Expansion Boards," later). These motherboards earn their name because the boards are wide and flat like a knife blade. The blades slide into a large board in a rack-mounted chassis like component boards would slide into a bus-oriented computer. The bus, however, only provides power and, sometimes, a network connection between the blades.

Blade servers have become popular because they allow several computers to fit where only one used to. They lower costs because each server requires no cabinet or power supply of its own. Also, individual servers can be replaced almost instantly by sliding the bad board out and a replacement in should there be a failure.

Despite the new technology, each blade is simply a single-board computer. The only change is that microelectronics have made its circuits more compact.

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