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Mechanical Construction

The obvious function of the case is mechanical—you can see it and touch it as a distinct object. And it steals part of your desktop, floor, or lap when you put it to work. It has a definite size—always too small when you want to add one more thing but too large when you need to find a place to put it (and particularly when that place happens to be inside your carry-on luggage). The case also has a shape, which may be functional to allow you the best access to all those computer accoutrements, such as the slot into which you shove your backup tapes. But shape and color also are part of your computer's style, which can set one system apart from the boring sameness of its computer kin.

Modern computers have wings, waves, and flares—all only aesthetics calculated to make a given manufacturer's machines stand out among the masses, style to make you think they are more modern and capable. The features, such as some of the more interesting paint shades with which some manufacturers have started experimenting, are design gimmicks, the tail-fins of the new century. There's nothing wrong with a computer that looks like you've stolen it from the deck of an aircraft carrier, but there's nothing inherently better about it, either. Beneath the plastic you'll find that same basic mechanical design and construction of systems built a decade ago.

When a computer is being built, two issues are paramount: how the case is put together and how a computer is put together inside it. In the first case, how the case gets constructed—whether with screws, rivets, or welds—is not so important as the size of the finished product. The size of the case determines what it can hold, which in turn limits how much you can expand the system to add all the features you want. How components install inside the case affect how much you want to expand your system. If sliding in an expansion board or just getting at a drive bay to install a new disk makes you think the system was designed by someone in league with the devil (or another employee of Microsoft), you're not going to look on expansion with much favor.

Some computer companies actually sympathize with your plight, knowing the happy customer brings goodwill, and a satisfied customer is less likely to make calls to the support staff that's so expensive to pay. Over the years, they have developed several schemes to make working on and expanding your computer easier—everything from cases you can open without tools to cases you cannot open at all, relying instead on external expansion.

In computers, form dictates function as much (if not more) than it does for any other type of office equipment. Computers have the shape they have so that they can hold what you want to put inside them—primarily all those expansion options that give your machine power and capabilities. It has to be large enough to accommodate the expansion boards you want to plug in as well as provide adequate space for all the disk drives your computer and life would not be complete without—hard, floppy, DVD, and the like.

For most manufacturers, the sizes of both boards and drives are preordained. Both are covered by industry standards that were set years ago and remain invariant. The case must be designed around these standards if it is to accommodate the generally available motherboards, expansion boards, and disk drives.


All current desktop computer cases follow the motherboard mounting dimensions laid down by the ATX motherboard standard, the microATX standard, or the NLX standard. These standards describe not only the minimum dimensions for a computer case but also the locations of mounting standards for the screws that hold the motherboard in place.

The guidance given by these standards is, in fact, minimal. The standards define only the footprint of the motherboard and a space on the rear panel of the case for port connectors. Even so, you can be sure that an ATX motherboard will fit into an ATX case.

With notebook computers, all bets are off. Despite attempts at defining a notebook motherboard standard, every manufacturer goes its own way—both in motherboard and case design.


A computer can be any size to carry out its job. Thanks to miniaturization, even the smallest computers can rival the power of the largest—with one exception. You can stuff more stuff into a big box than you can a small one.

That's the essence of size differences among computers. Bigger means more expansion, more places to put power-enhancing accessories. As long as you don't need an extraordinary amount of expansion, even the physically smallest computer will suffice.

A number of terms are used to describe the size of computers, although none approaches being an official designation.


The basic computer—that is, the smallest with the least future potential—uses a compact device that's sometimes called a small footprint computer. In the jargon of the computer industry, the footprint is the space a computer occupies on your desk, so the small footprint steals less desk space, making it an ideal office companion. Compact desktop computers are sometimes called low-profile computers because they are short, usually as short as possible, and still hold a whole computer.

To achieve compact size, computer-makers must do some serious trimming from the designs of larger computers. The principal loss is in expansion. Compact systems have fewer slots and bays than other computers. They have a minimum of extra space for expansion—once filled with standard factory equipment, perhaps as little as a single open slot and a single unfilled bay. The underlying philosophy is that the compact computer comes with everything you need already installed, so there's no need to add anything more.

To shave down the height of low-profile cases while conserving the capability of handling tall expansion boards, compact machines move their options on edge. Instead of boards sliding into slots vertically, these tiny computers align them horizontally. The motherboard boards remain horizontal in the case and have a single master expansion slot. A special expansion board rises vertically with additional slot connectors on one side to allow you to slide in several ordinary expansion boards (typically three) parallel to the system board.

Although the exact assortment of drive possibilities varies with the design of compact systems, most small-footprint machines offer similar drive options. They are constrained by what you expect and need in any computer. A floppy disk drive and a CD or DVD drive leer out from the front panel, and a hard disk hides safely inside. If any drive expansion is available inside these systems, it comprises a single bay suitable for an inch-tall hard disk.


At one time, desktop meant a particular case design—a low, flat cabinet meant to sit on a desk and hold a monitor on top. The basic configuration and even the standard dimensions carried over from the original IBM Personal Computer. The result was a case measuring 21 inches wide by 17 inches deep, designed to accommodate two large disk drives measuring 5.25 inches across, side by side. Its height, to allow for expansion boards and three-eighths inch feet underneath for airflow (just enough so you could lose a pencil—maybe the designers did have some kind of inspiration!), measured 5.5 inches.

Because few people need more than one large (5.25-inch) drive bay, most new desktop computers opt to shrink the other bay area to the 3.5-inch size, shaving about two inches of the computer in the process. This intermediate package, measuring about 5.5 by 19 by 17 inches is today's most popular in desktop systems.


Stand a conventional computer on edge and you get a tower. At least that's how the tower format evolved. The design has proved so popular that these days, one form of tower or another probably is the standard computer.

The upright design actually makes the ultimate small-footprint computer. It steals no space from your desk if you set it next to the furniture. A smaller footprint is hard to imagine.

Freed from restraints required when sharing a desktop, tower computers could expand to suit even the most fanciful dreams. The largest have nearly as many bays as the Eastern seaboard—as many as 11 in a single system. On the other hand, they usually do not extend the number of expansion slots over eight. The reason is not a spatial limitation but an economic one. Most towers use standard, off-the-shelf motherboards. Raising the slot number would involve expensive redesign (as well as more motherboard, circuitry, and assembly costs).

With modern components, vertically mounting computer components causes no problems. Electronic circuits don't know which way is up. Although mechanical devices such as disk drives may care, the only practical effects with current hardware are really your problem. Putting a CD drive on edge can turn loading a disc into a dash across your office floor and down the hallway as you race after the errant disc that dropped and rolled away. Most towers keep their bays horizontal to avoid the office chase scene. Compaq even developed a computer transformer—by remounting the drive bay and changing the front panel bezel, you can switch the Deskpro EP series from desktop to tower and back.

The tower design has been so successful, designers have fine-tuned the vertical package to meet a variety of expansion needs. Mini-tower cases are the most compact and usually accommodate only Mini-AT and smaller motherboards and a few drive options. Full-size tower cases hold full-size motherboards and more drive bays than most people know what to do with. Recently, midi-tower cases, with accommodation falling in-between, have become a popular option. There is no standardization of these terms. One manufacturer's mini is another's midi.

Choose a computer with a tower-style case for its greater physical capacity for internal peripherals and its flexibility of installation wherever there are a few vacant feet of floor space. You also need to be critical about the provisions for physically mounting mass-storage devices. Some towers provide only flimsy mounting means or require you to work through a Chinese puzzle of interlocking parts to install a drive. You need a system that provides sufficient drive-mounting options.


Back in the days before micro-miniaturization, anything instantly became portable the moment you attached a handle. The first generation of portable televisions, for example, was eminently portable—at least for anyone accustomed to carrying a carboy of mineral water under each arm. The first generation of computers had similar pretenses of portability, challenging your wherewithal with a weighty bottle of gas and photons and a small but hardly lightweight picture tube. The typical weight of a first-generation portable computer was about 40 pounds—about the limit of what the market (or any reasonable human being) would bear. These portables were essentially nothing more than a repackaging that combined a conventional computer with an integral monitor.

Replacing the bottle with a flat-panel display gave designers a quick way to cut half the weight and repackage systems into lunchbox computers. The name referred to the slab-sided design with a handle on top, reminiscent of what every kid not party to the school lunch program toted to class. But with some weighing in at 20 to 25 pounds, these packages were enough to provide Paul Bunyan with his midday meal. The largest of these did allow the use of conventional motherboards with space for several conventional expansion slots. Overall, however, the design was one that only a mother could love, at least if she advocated an aggressive weight-training program.

The ultimate in computer compression is the notebook computer, machines shrunk as small as possible while allowing your hands a grip on their keyboards (and eyes a good look at the screen) and as thin as componentry allows. Manufacturers have managed to squeeze the typical notebook computer down to a little more than six pounds. Size is limited by the choice of display screen. With bleary-eyed users demanding the biggest possible screens—one manufacturer now offers a notebook computer with a 16-inch screen—cases have been tailored to match. Although easy on your eyes when you're working or playing games, the big screens make today's notebook a bit more troublesome to carry and almost impossible to use in an economy-class airline seat.

The notebook design spawned the subnotebook for people on the move who want to carry the minimal computer. Typically, subnotebooks forgo several features, such as port connectors and optical drives, to achieve their smaller size. In general, a subnotebook is a portable computer weighing under four pounds, has a 12-inch or smaller LCD screen, and is a little larger than an inch-thick stack of writing tablets.

Fitting into the small notebook and subnotebook packages forces manufacturers to sacrifice some degree of convenience. For example, a number of subnotebook machines have been developed with keyboards reduced to 80 percent of the standard size. Most people adapt to slightly cramped keyboards and continue to touch-type without difficulty. Smaller than that, however, and touch-typing becomes challenging. In other words, handheld computers are not for extensive data entry.

Besides length and width, notebook computer–makers also have trimmed the depth of their keyboards, reducing the height of keytops—not a noticeable change—as well as reducing key travel. The latter can have a dramatic effect on typing feel and usability. Although the feel and travel of a keyboard are mostly user preference, odds favor greater dissatisfaction with the shrunken, truncated keyboards in miniaturized computers compared to full-size machines.

Notebook computers have forced manufacturers to miniaturize a number of components that don't naturally lend themselves to miniaturization. Both CD and DVD-ROM drives now fit into a space one-third their desktop size, but they suffer the penalty of lower top speeds.

Hard disk drives suffer the same fate. Drive-makers have shrunk their products to fit a 2.5-inch form factor with little capacity penalty. Instead, they have sacrificed speed. Notebook drives spin more slowly than desktop models (which helps them gain some of the capacity they need).

Internal Access

Two philosophies surround the issue of access into your computer's case. One school of thought holds that it should be as easy as possible so that you can make upgrades quickly and easily. The other school believes that you can only get yourself into trouble inside your computer. Students of this school believe that the best case is one that's sealed against access. If they had their way, your computer would be embedded in a block of Lucite so you could see its pretty lights but not touch a thing inside.

The curriculum divides roughly on application lines. Computers used at home or in a small business—that is, one small enough that you're responsible for your own computer—are best with open access. Large businesses that have a unified Information Systems department headed by an all-powerful fuehrer want to keep in absolute control—which means keeping you out. The needs of these folks led the move to the Network Computer and other abortive attempts at electronic mind control.

Tool-free entry allows you to open and upgrade your system without special tools or any tools at all. You won't have to steal a knife from the silverware drawer to get into your computer or buy a special screwdriver to pull out an expansion board. In the ideal case, the lid unlatches and slides off your computer with minimum effort. Disk drives snap into place, and knurled posts that you can tighten with your fingers lock expansion boards into place. Some current systems approach this ideal, but few go all the way.


Bays come in two basic varieties: those with front panel access and those without. Front-panel-access bays are for devices using removable media. You have to have access to the front of the drive to slide in a disk or tape cartridge. Floppy disk drives, CD and DVD drives, and tape drives all must be mounted in bays with front panel access.

Internal bays lack front panel access. They may be tucked away inside the computer or just below the bays with front panel access. Internal bays suit devices that require no direct user interaction, those you don't have to see or touch. The chief occupant of an internal bay is the hard disk drive.

Form Factors

Disk drives come in a variety of heights and widths. The basic unit of measurement of the size of a drive is the form factor, which is simply the volume of a standard drive that handles a particular medium. Several form factors regularly find their way into discussions of personal computers, ranging in size from 8 inches to 1.3 inches, most of which allow for one or more device heights.

The full-size drive, the unit that defines the form factor and occupies all of its volume, is usually a first-generation machine. Its exact dimensions, chosen for whatever particular reason, seemed fitting, perhaps allowing for the mood of the mechanical engineer on the day he was drafting the blueprints. If the drive is a reasonable size and proves particularly successful—successful enough that other manufacturers eagerly want to cash in, too—others follow suit and copy the dimension of the product, making it a standard.

Device Heights

The second generation of any variety of hardware inevitably results in some sort of size reduction. Cost cutting, greater precision, experience in manufacturing, and the inevitable need to put more in less space gang up to shrink things down.

The succession of heights first appeared among 5.25-inch drives. At 5.25 inches, devices are measured in increments of the original full-height package. Devices that are two-thirds height, half-height, one-third, or quarter-height have all been manufactured at one time or another.

At the 3.5-inch form factor, sizes are more pragmatic, measured as the actual height in inches. The original 3.5-inch drives may be considered full height and typically measure about 1.6 inches high. The next most widely used size was an even inch in height (defining five-eighths height, for the fractious folk who prefer fractions). Sub-inch heights have been used for some devices, some as small as 0.6 inches.

At 2.5 inches, the size of most notebook computer drives, height is described as actually measured. Typical high-capacity drives measure 12.5 millimeters high (roughly half an inch) with 9.5 millimeters preferred for the drives in subnotebook computers.

Smaller drives have also been manufactured, although the one standard for tiny drives fits the CompactFlash package. These have superceded the 1.8-inch size that won particular favor for fitting into Type 3 PC Cards that follow the most recent standards promulgated by PCMCIA.


Most drive-mounting schemes are meant for permanent installations. Your hard disk drive is supposed to last for the life of your computer. In certain situations, however, you may want something less permanent.

If you deal in secrets or have valuable information that you store on your computer, you might not want to leave it out on your desk untended all night long. Although packing up and putting your computer away for the evening is more trouble than most people want to bother with, pulling out a disk drive and sliding it into a safe is not.

Although hard disks are quite reliable, every one you add to your system increases the chances that one will fail. Multiple disk drives and the need for reliable operation go hand-in-hand in RAID systems. To speed repairs, most RAID systems have moved to mounting drives for hot-swapping.

In either case, the choice is mounting your drive in a removable rack. The typical hot-swap rack allows you to install a 3.5-inch hard disk in a 5.25-inch bay. More than an adapter, it puts the drive on a submodule that slides into the rack, connects, and locks in place. You can remove a drive for the evening or swap a new drive for an old one at any time. Each rack manufacturer uses its own designs, so there's no intercompatibility among different rack systems.


All expansion slots are not the same. Put another way, a slot is not a slot is not a slot. Expansion slots differ in two primary ways—the standard followed and length.

To push up slot totals, many manufacturers also add that their systems (or motherboards) include an accelerated graphics port (AGP) slot. Technically they are correct. The AGP slot works exactly like an expansion slot. But it is irrelevant when analyzing the availability of expansion. Any computer with an AGP slot likely fills it with an AGP graphics adapter (why else would the slot be there?), so the AGP slot is never available for expansion (although it does allow you to upgrade your video system). Ordinary expansion boards—both PCI and ISA—will not fit into the AGP slot in a computer.

Slots also vary in length. A full-size slot allows you to install any length of expansion board. A short slot cannot hold a full-length expansion board. Although PCI describes the dimensions of a short card, computer-makers make short slots of any length that suits their designs. In general, these short slots are more than half the length of a full-size slot and will accommodate most short cards. Most modern expansion boards are short, so you ordinarily won't have problems matching slot length.

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