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Besides holding the essential circuitry of a computer, the motherboard of a computer must accommodate some form of expansion. In desktop computers, the motherboard is home to special electrical jacks called expansion connectors that allow you to plug in additional printed circuit boards. The space potentially occupied by an expansion board is an expansion slot, usually referred to simply as a slot.
Notebook computers and their ilk also incorporate expansion boards but in a different form. A protective shell, usually sheathed with aluminum, encases their circuit boards to make a near monolithic assembly that's termed a PCMCIA card or PC Card. PCMCIA stands for the organization that sets the standard for these cards, the Personal Computer Memory Card International Association. PC Card is actually a term of art that more particularly describes one of the interface standards used by these cards. Most people call the space potentially occupied by the cards a PCMCIA slot or simply a card slot.
Manufacturers cannot designate expansion areas on their motherboards willy-nilly. There is order and reason behind their designs—all backed by standards.
Beyond standardization, other concerns include the number, size, and arrangement of the slots. These considerations determine how expandable a given computer really is. In addition, the spacing of the bus connectors in the slots is a concern. Put them too close together, and you'll limit the designer's choice of circuit components to those that are short or force the designer to put the components flat against the board, thus wasting its expensive space. Even the number of expansion connectors may be set or limited by the bus standard.
The basic characterization of motherboards is by the standard they follow for the physical and electrical characteristics of their expansion slots. The choice dictates which boards plug into the motherboard—in other words, which products you can use to upgrade your computer. Although the choice is no longer wide, variations remain.
Expansion standards for desktop computer motherboards have gone through a lengthy evolution, with a number of interesting but now essentially irrelevant side trips. Today, however, one standard (PCI) dominates, and the only option is whether a given motherboard or computer condescends to accommodate legacy (old) expansion boards that follow the ISA standard. The situation is simpler among notebook computers because their evolution has been shorter and they missed the chaotic early days of computer development. You'll find a complete discussion about the history of expansion standards in Chapter 9, "Expansion Buses."
The number of expansion slots in a given computer is a compromise with multiple considerations. More is better, but a computer with an infinite number of slots would be infinitely wide. The practical dimensions of the motherboard and case limit the space available for slots.
Electrical considerations also keep the slot count modest. The high-speed signals used by today's expansion buses are hard to control. The higher the speed, the fewer slots that are practical. Even at 33MHz, today's most popular expansion bus standard allows for only three slots per controller.
Adding more slots requires additional control circuitry, which adds to the price of the computer (as does the cost of the expansion connectors themselves). With manufacturers slugging it out over every penny of their prices, the tendency is to constrain costs and slot count.
How many expansion slots you need on a motherboard depends on how many functions the motherboard-maker has integrated into the board. Most practical computers fill one or more of their slots with standard equipment (such as a video board and modem). Over the three-to-five-year life of the typical computer, you're likely to add more than one new accessory to it, possibly something you might not have conceived of when you bought the machine. Therefore, you need to plan in advance for the need for expansion.
Notebook computers get away with two or fewer slots because they encapsulate all the normal functions of a computer on their motherboards. Nevertheless, two is still better than one, and you're likely to use what's available. Note, too, that super-thin computers, which are most likely to have single slots, are the ones that will more likely need two. For example, these machines may require a slot to run a CD drive that would otherwise be built in to a thicker system.
Expansion boards are three-dimensional objects (as is everything else in the real world). In addition to length and width, they also have thickness. PCMCIA explicitly gives all three dimensions of conforming cards. The thickness of desktop computer expansion boards is more implicit, determined by the spacing of the expansion slots. If slot spacing is too narrow, boards simply won't fit. Worse, one board might touch another and short-circuit a computer signal, leading to erratic operation or complete non-operation of the computer. If slot spacing is too wide, fewer will fit in a computer of reasonable size, thus limiting your expansion options.
A printed circuit board itself is quite thin, about one-eighth inch, and can be made even thinner when board dimensions become small. The thickness of most expansion boards and the requirement for adequate slot spacing arise from the components installed on the board—which may include another printed circuit board clinging to the first like a remora.
Surface-mounted components make thinner boards practical, but taller components remain prevalent enough that the spacing of slots has remained the same since 1982. On all motherboards, expansion connectors are located on 0.8-inch centers.
Not only does this standard set the maximum thickness of any expansion board, it also dictates the number of slots that may be available for a given size of motherboard. The board must be as least wide enough to accommodate all the slots it is to hold. Some motherboards are hardly wider than that.
In any case, the spacing of expansion slots was originally set arbitrarily. It represents what the developers of the first computers thought was the optimum compromise between compact layout and adequate allowance for the height of circuit components. The choice was wise enough that it still reigns today.
In desktop computers, the layout of the slots is mostly a concern of the system designer. As long as your boards fit, you shouldn't have to worry. The designer, however, must fret about the electrical characteristics of signals that are close to invading the territory of microwave ovens. Some are purely electronic issues that cause high-speed buses to operate erratically and interact detrimentally with other electronic circuits by generating interference.
Expansion buses pose more engineering problems than other circuits because they require the balancing of several conflicting needs. The expansion bus must operate as fast as possible to achieve the highest possible transfer rate. The expansion bus also must provide a number of connectors for attaching peripherals, and these connectors must be spread apart to allow a reasonable thickness for each peripheral. In other words, the bus itself must stretch over several inches. The common solution is to use several short buses instead of one long one. Although the connectors on the motherboard may appear to link all expansion boards into a single bus, if your system has more than three high-speed expansion slots, it likely has multiple buses, each with its own controller, powering the slots.
There's another design consideration in desktop and tower computers—whether expansion boards slide into their slots vertically or horizontally. In full-size systems, expansion boards plug into the motherboard perpendicularly. The result is that the main plane of each expansion board is vertical in desktop computers and horizontal in tower-style machines. A few compact desktop computers, termed low-profile designs, align expansion boards parallel to the motherboard, horizontal in desktop systems.
Although alignment of the expansion slots appears to be only an aesthetic consideration, it also has a practical importance. Vertical boards cool naturally through convection. Air currents can rise across each board and cool its innermost components. Horizontal boards defeat convective cooling because the boards themselves block vertical air currents. In general, computers with horizontal expansion boards should have some kind of active cooling system—translated from technical language, that means they need a fan to blow a cooling breeze across them.
Slot layout is more complicated in notebook computers in which the layout of PCMCIA slots may determine what cards you can plug into your computer. Most notebook computers have two slots, and they are arranged like the barrels of a shotgun—either in a side-by-side or over-and-under fashion. These options are not functionally equivalent. PC Cards come in any of three thicknesses (discussed later), and most slots accommodate the two slimmer cards. An over-and-under arrangement of two slots will hold either two slim cards or one of the thickest sort. Although the over-and-under configuration gives you the largest number of expansion options, many of today's ultra-thin notebooks can only accommodate side-by-side slot mountings.
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