A riser board looks like an expansion board and works like an expansion board. Using the quack-standard—if it looks like a duck and quacks like a duck—a riser board is an expansion board. But really it’s not, at least according to the people who have created the standards that make it possible to consider riser boards as being expansion boards. In its various riser board standards, Intel claims that its riser boards are not an expansion board standard.

Rest assured you’re not the only one confused by riser boards. The concept underlying them is the same as an expansion board—put extra circuitry on a special card so that it can be easily removed and changed. The boards look like expansion boards, pegged at the same height and using similar expansion connectors. Risers slide into a motherboard slot placed adjacent to ordinary expansion boards so that, at least from the outside, they look like simply another expansion board.

Intel doesn’t want you to think of riser boards as expansion boards because it doesn’t want to confuse you with more than one expansion board standard. It doesn’t want third-party manufacturers putting riser boards in boxes and selling them in retail stores. Likely the company fears someone will buy a riser and discover they can’t slide it into a PCI slot, and the lonely worker handling support will get a barrage of telephone calls asking for help.

Functionally, a riser board is simply a bend in a motherboard. It rises perpendicularly from the motherboard (much like an expansion board) to hold circuitry that otherwise would not fit on the motherboard (much like an expansion board). Riser boards can even use a slot position, much like an expansion board, but they follow their own standards, which have absolutely nothing to do with PCI. This independence, in fact, is one of the graces of the riser board. It allows you to change or customize your system using a low-cost product that avoids the complications (and cost) of a full PCI interface. Moreover, the Audio Modem Riser (AMR) slot doesn’t count against the tiny maximum number of PCI boards that can be connected to your computer’s PCI bridge circuitry.

Put another way, riser boards offer a system designer three big benefits: space, timeliness, and isolation. The physical size of the riser board amounts to additional real estate on which to locate electronic circuitry, as well as bracket space at the back of the computer for extra connectors. Using a riser board, a motherboard can be more compact. In fact, Intel created the original standard riser, the Audio Modem Riser, as an adjunct to its small-footprint microATX motherboard.

Today, communication and networking standards change faster than most computer standards. Keeping up with these changes may require motherboard-makers to revise their designs more often than they would otherwise like (they would otherwise like never to change their designs, because each change racks up design expenses). By restricting the changes to a small riser board, the manufacturer trims the cost of keeping its motherboards timely.

There’s another reason for putting audio circuits on a separate board. Audio circuits are the most prone to picking up interference from the rest of your computer. Analog audio uses signals that vary in a wide range, over more than three orders of magnitude. Interference typically produces tiny signals that are ignored by logic circuits but are amplified into bothersome noise by analog circuits. A signal one-thousandth the strength of a normal logic signal would have zero effect on the operation of your computer but would be audible in high-quality speakers attached to your computer. By putting the interference-prone circuits on a separate board, designers can physically isolate them from the other signals on the motherboard, which adds a degree of electrical isolation as well.

With these ideas in mind, Intel and other companies have created three standards for riser boards meant to work in modern computers. These include the original Audio Modem Riser standard, the Communications and Networking Riser standard, and the Advanced Communications Riser standard.

Audio Modem Riser

The first of the riser-board standards independent of a specific motherboard design was Intel’s original Audio Modem Riser (AMR). The AMR specification was originally published on June 30, 1998, and updated to its final form (version 1.01) on September 10, 1998. It envisions a board capable of holding up to four audio codecs on a single board that’s at most 6.875 inches long and 4.2 inches high. It uses a single 42-contact edge connector located between the AGP port and other port connectors on the motherboard. It takes the space of an ordinary expansion board but not the connector or the circuitry, so it doesn’t count against the maximum number of PCI boards that can be inside a single computer.

The interface was designed to allow a great deal of flexibility. Not only does it allow for four codecs where two would typically be used—one to implement a modem, one for the equivalent of a soundboard—a special set of split I/O signals allows, for example, a sound system implementation on the motherboard to communicate with a modem on the riser board. These signals take the form of four separate serial data channels from the chipset (typically) to the audio circuitry on the riser board. The interface also provides the power and clock signals needed to make the audio circuits work, as well as signals for future applications (including the USB bus).

Although you’ll still find some references to the AMR design, Intel has replaced it with the Communications Network Riser standard (discussed next) and has removed the AMR specification from its Web site.

Communications Network Riser

On February 7, 2000, Intel released its replacement for the AMR design, which it called the Communications and Networking Riser (CNR). The “networking” part of the name marks the major difference. CNR incorporates provisions for networking and USB interfaces as well as the audio and modem technologies of the earlier design. The full specification for the CNR board and its interfaces is available at the Intel Web site at www.intel.com.

The CNR design uses a new 60-pin connector with a preferred location at the edge of the motherboard: outboard of its normal PCI expansion slots, a position formerly used by a legacy ISA slot on many motherboards. Significantly, the new connector is incompatible with AMR boards.

Despite the physical incompatibility, the CNR standard starts with some of the basic functional compatibilities as with boards made for the AMR specification. At heart, CNR incorporates the Audio Codec (AC) ‘97 interface. Unlike AMR, which permits four AC ‘97 channels, the CNR specification originally allowed only two, although it was revised (to version 1.2) on November 8, 2001, to permit three.

The networking side of the CNR design defines two different board implementations, each using the same connector but with different pinouts. The “A” variation uses eight contacts for a platform LAN connection interface, meaning that the hard work of generating the network signals is on the motherboard. The riser only packages them, for example, putting them on the same RJ-11 jack as the modem signals. The “B” variation uses 17 contacts for a Media Independent Interface (MII, as defined under the IEEE 802.3u specification) bus, allowing the riser board circuitry to set up any kind of physical connection (which makes the riser board circuitry more complex). In other words, the “A” variation allows for less complex riser boards, whereas the “B” variation allows for greater versatility (for example, permitting the manufacturer to install different network interfaces by changing the riser board).

The CNR design also allows for the integration of a USB interface on the riser board. With the CNR version 1.1 revision of October 18, 2000, the specification added support for USB version 2.0 at 480Mbps.

So that the CNR board will be properly identified by the host computer, each board has five signal lines for a System Management Bus (SMBus) interface. The host computer can use the bus to interrogate ROM memory on the board to identify its function and its resource needs each time the host boots up. The CNR design also includes several lines to provide power to the circuitry on the riser board at 12, 5, and 3.3 volts (DC) as well as sleep-mode voltage so the circuitry of the CNR board may remain active while the host is in sleep mode. (This voltage allows the CNR circuitry to wake the host computer upon modem ring or network request.)

The physical specifications for the CNR board are similar to those of the AMR board. Both boards are limited to the same maximum height, 4.2 inches (106.68 millimeters), dictated by host height restrictions within the PCI standard. CNR boards have a slightly shorter maximum length than AMR boards, at 6.579 inches (167.11 millimeters). The connector is, of course, different, unique to the CNR board, although the pin spacing on its edge connector matches that of the standard PCI connector.

Advanced Communications Riser

This need for yet another new connector and other shortcomings of the CNR design led an industry group to design its own alternative to CNR on February 11, 2000. That organization, which called itself the Advanced Communications Riser Special Interest Group (ACR SIG), has grown to 55 members at the time of this writing, including, significantly, Advanced Micro Devices, but not Intel.

Working together, they created the Advanced Communications Riser (ACR) board to add network and Digital Subscriber Line (DSL) capabilities to simple add-on boards without losing compatibility with AMR boards or adding the need for a new connector.

Physically, the ACR slot uses a 120-pin PCI connector (for its ready availability), but the connector is reversed and offset in its position in its slot. This change in orientation and position makes it compatible with AMR boards. The first 42 contact positions on the ACR board have the same functions and signals as the AMR board. The ACR design envisions its slot position on the outside edge of the motherboard, replacing the legacy ISA slot in many motherboard designs.

An AMR board can slide directly into an ACR slot. An ACR board will, too, although it won’t slide into an AMR slot because of the longer connector on the ACR board.

The electrical functions of the ACR board start with the four Audio Codec ‘97 channels of the AMR board and add two more AC ‘97 channels, for a total of six.

In addition, the ACR design provides two new pins as a serial data channel for board identification. Each ACR board includes an onboard ROM chip to identify the board and its capabilities to the host computer as part of the Plug-and-Play setup process.

One of the new sets of 14 signals is the Integrated Packet Bus, developed in conjunction with the ACR design to directly link the host microprocessor with the communications subsystem and control high-speed Internet connections. Two sets of 18 signals each provide a pair of channels that separately link your chipset to phone-line networking and ordinary twisted-pair networks following the 10Base-T or 100Base-T standards. The ACR specification reserves an additional eight contacts for future applications, such as wireless networking adapters.

The ACR design specifically recognizes home phone-line networking (HomePNA). It envisions modem-makers offering a single multifunction board that includes a V.92 modem, a HomePNA network adapter, and a DSL adapter. A single ACR card will link to your home phone-line network, your telephone line as a regular modem (for data as well as dialing and Internet phone service), and your DSL line, without the need for external adapters. All phone-line communications functions can then use a single RJ-11 jack on the ACR card.

Although the ACR SIG claims that its standard is open, the specification is available only to members of the group. Its Web site is located at www.acrsig.org.

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Modern motherboards can be any size or shape that suits the design of the computer. Only the need for standard-size expansion slot connectors limits the freedom of design. Many computers, however, are built around motherboards of a few standard sizes. This standardization is a matter of convenience. It allows a computer manufacturer flexibility in the choice of suppliers—standardized dimensions make motherboards interchangeable.

For you as the purchaser of a new computer, motherboard standardization has its downside. You face the problem of the computer manufacturer selling systems equipped with a motherboard du jour, whatever OEM motherboard was available cheapest on the day the computer was put together. On the other hand, a computer built around a standard-sized motherboard gives you upgrade freedom. Should you become dissatisfied in the performance of your computer, you can replace a standard-size motherboard with a more powerful one.

The earliest motherboard standards followed the leads set by IBM. They duplicated the physical dimensions of the motherboards used by the most popular IBM machines. Even when they lopped off vast areas of board to trim costs, most manufacturers retained compatibility with IBM’s designs, keeping mounting holes in the same locations so that one board could be substituted for another. This heritage continues even in some of the latest designs.

After IBM ceded its influence as the setter of the standard dimensions of motherboards, the industry was essentially adrift. Major manufacturers developed their own designs without regard to older products, while smaller manufacturers clung to the old board layouts. The situation is changing, though, with new motherboards standards now promulgated by Intel. The most recent of these, the ATX motherboard design, goes further than ever before and specifies not only dimensions and mounting holes but also connector placement and even connector designs. To standardize the motherboards of more powerful computer workstations, the computer industry recently developed and then dropped the WTX specification.

Motherboard design divergence first arose among makers of small-footprint computers, machines designed with smaller dimensions to cover less of your desktop. These machines compromised expansion by reducing the number of expansion slots and drive bays to gain their more modest measurements. Many manufacturers developed low-profile computers that reduced system height by turning expansion boards on their sides. These designs necessitated changes from the more standardized motherboard layout. The computer industry has rallied around the low-profile design and produced two standards that support the concept. First came the LPX motherboard, and then came a smaller derivative, the Mini-LPX. To accommodate the needs of new microprocessor and memory technologies (and to add a wealth of other features), the industry adopted the NLX motherboard.

In general, smaller manufacturers are more likely to use standard-size motherboards. Larger manufacturers are better able to afford the price of custom-designing cases and motherboards to match. Even large computer-makers have moved to standard-size boards at least for their offerings that take advantage of the latest microprocessors. In truth, systems that use the latest microprocessors often all have exactly the same motherboard design inside, using a motherboard designed and manufactured by Intel. Even large manufacturers may rely on Intel motherboards until their engineers develop a familiarity with new chips. For example, all but a handful of the first Pentium Pro computer models uniformly used standard-size Intel-manufactured motherboards.

ATX

To bring a degree of uniformity to motherboard design, the computer industry created a new motherboard standard that roughly conforms to the Mini-AT board size but with a few design twists that result in lower-cost engineering. Called ATX, the standard is promulgated by Intel but is openly published. Intel released the most recent version, 2.1, in February 1996, fine-tuning the design based on industry feedback.

The ATX standard defines the number and position of the motherboard mounting holes and offers recommendations as to component, expansion board, and port connector placement. Although the standard does not demand any particular slot type or configuration, it’s aimed primarily at ISA, PCI, and ISA/PCI combination designs. It also allows for both 5.0- and 3.3-volt system operation (or both simultaneously).

The odd orientation of the board facilitates port placement. It provides the maximum space for expansion boards and port connectors at the rear of the host computer chassis. The design also envisions that the microprocessor will be located near the right edge of the board, where it will be in close proximity to both the power supply and the cooling fan. In the recommended configuration, memory sockets can be readily accessed between the microprocessor and expansion slots.

The ATX board itself measures a maximum of 12 by 9.6 inches (305 by 244 millimeters). This size is not a random choice but, according to Intel, was selected to allow manufacturers to cut two boards from a standard-size 18-by-24-inch raw printed circuit panel. It provides sufficient space for about seven expansion slots, which are spaced at the conventional 0.8 inches apart. It incorporates nine mandatory and one optional mounting holes, most of which are in the same positions as the holes in a Mini-AT motherboard. Figure 29.1 shows the dimensions and mounting hole placement for an ATX motherboard.

The ATX specification goes further than simply indicating mechanical board dimensions. The standard also embraces the PS/2 size of power supply and specifies a new motherboard power connector (see Chapter 31, “Power”).

Besides uniformity, the ATX design aims at trimming costs for computer-makers. By putting port connectors on the motherboard, even in multiple layers, the cost of connecting cables as well as the labor required for assembly is eliminated. Eliminating cables also helps minimize potential radio frequency interference. In its recommended configuration, the ATX layout also allows the use of shorter floppy and hard disk connecting cables, with similar benefits. The power supply choice and location also trims cost for the computer manufacturer as well as helps the computer run cooler and even quieter.

Mini-ATX

The designers of the ATX board realized that the one certainty in computer circuit design is that functions get combined and made more compact. As more and more of the functions of a computer squeezed into one or two chips, the ATX designers imagined that soon much of the ATX real estate would be superfluous. In that one of the primary goals in the design of ATX was trimming costs, they figured that trimming motherboards to a size smaller than ATX as the technology permitted would reap savings in materials cost. Consequently, they included a standard size for Mini-ATX motherboards in the original ATX specification.

The Mini-ATX design chops the ATX motherboard down to 11.2 by 8.2 inches (284 by 208 millimeters). When installed in a computer, the Mini-ATX motherboard still sits at the rear edge of the chassis so that port connectors can be mounted directly to it without cables. In most chassis, the left edge still aligns with the left side of the case to allow space for a full complement of expansion boards. Because of this placement, the smaller size of the Mini-ATX board cuts off one row of ATX mounting holes. As a result, the lower row of mounting holes is displaced on the Mini-ATX design, as shown in Figure 29.2.

microATX

The Mini-ATX design has one chief benefit. It reduces the materials costs for a motherboard by about 30 percent when compared to a full-size ATX board. But that wasn’t enough for the creators of the ATX specifications. They created another small-size motherboard called microATX, which took over the role of Mini-ATX. The current incarnation of the standard, version 1.0, was published in 1997.

The microATX specification represents more rethinking than a simple extra shrinking. The new design is actually a bit larger than Mini-ATX. A microATX motherboard measures 9.6 inches (244 millimeters) square.

The chief difference between Mini-ATX and microATX is how the two mate with the full-size ATX standard. Mini-ATX motherboards pay little heed to the larger standard; microATX boards fit neatly inside it. Although microATX motherboards are narrower than full-size ATX designs, they use nearly the same mounting scheme, so a microATX board will fit into a full-size chassis. Expansion slots appear in the same position, except microATX allows for fewer slots because of its reduced size—a maximum of four slots, typically three PCI and one AGP. (ISA slots are also allowed.) In addition, most of the screw holes used for mounting a microATX motherboard match the positions of those on a full-size ATX motherboard.

To mate with the conventional ATX chassis, the microATX motherboard reserves the same space for port connectors as does the full-size board, and the two boards use exactly the same power connectors. Figure 29.3 illustrates the microATX motherboard and compares it to its full-size sibling.

FlexATX

If small is good, even smaller is better. In 1999, Intel’s engineers unveiled yet another motherboard format, FlexATX, which shaves inches off the microATX design. The name reflects the design goal: FlexATX is meant to be flexible and allows for the development of devices beyond the traditional personal computer (but based on the traditional personal computer motherboard). In the classification system of its promoters, FlexATX is not a standalone standard but an addendum to the microATX specification.

A FlexATX motherboard measures no more than 9 inches wide and 7.5 inches deep. It retains the same area (in both size and location) for ports and connectors as that used by ATX and microATX motherboards.

Seeing a wide application of compact motherboards, however, the FlexATX design fits inside the conventional ATX chassis. The placement of its mounting holes matches those of an ATX motherboard, although (as with microATX) two added holes provide mounting security on the edge adjacent to where excess glass-epoxy was pared off.

The FlexATX design does not specify the number and location of expansion slots because the creators of the standard envisioned its use in systems that provide no board-style expansion (although the standard does permit engineers to use their imagination and add slots to the design).

Table 29.1 lists the dimensions for ATX motherboards and those derived from that standard. The complete ATX specifications and those related to it are available on the Web at www.formfactor.org.

NLX

Announced in September 1996, by Intel, the NLX motherboard design was a cooperative effort among several manufacturers, including IBM. At the time of its introduction, 12 computer-makers and one motherboard-only manufacturer (ASUStek) had announced support of the specification. The computer-makers included AST, Digital Equipment Corporation (now part of Compaq), Fujitsu, Gateway 2000, Hewlett-Packard, IBM, ICL Personal Computers, Micron Electronics, NEC Computer Systems, Sony, Toshiba, and Tulip Computers.

NLX is an improved low-profile layout. It was created specifically to overcome some shortcomings of earlier, no longer supported designs (LPX and its reduced-size version, Mini-LPX), in that those designs interfered with adapting to the latest technologies. NLX is meant to support all current Intel microprocessor designs and memory technologies as well as the advanced graphics port (AGP) for high-speed interconnections with video boards. In addition, the NLX design enhances the physical packaging of systems to allow greater mechanical integrity, better cooling, and more space for peripheral ports.

Key to the NLX design is the riser board. Although it’s similar to the riser boards of LPX systems, because you plug expansion boards into it, under the NLX design the riser takes on a greater role. In effect, it operates as a backplane and the NLX motherboard itself is a glorified processor board. The riser board attaches permanently to the chassis of its host computer while the NLX motherboard readily slides in and out of the case like a steroid-enhanced expansion board. In purest form of the NLX implementation, all cables in the system—including the power supply—attach to the riser, and none attach to the motherboard. Typically the cables for the floppy disk and hard disk drives in a computer will plug into the riser.

Although the design of the NLX motherboard is fixed by the specification, the riser board is not. Computer-makers can customize its design to accommodate different system designs. Although the typical riser board has four expansion connectors, the NLX specification allows a great deal of freedom to accommodate not only low-profile computers but also tower-style systems. The specification describes signals for up to five PCI expansion slots and an unlimited number of ISA slots on a single riser. All boards slide into the riser parallel to the motherboard.

The one aberration in the NLX motherboard/riser design is its accommodation for AGP video boards. The specification reserves a special area on the left side of the motherboard, opposite the riser board, for a single AGP slot. The AGP board slides into the motherboard slot parallel to the riser board. Microprocessors (the NLX design accommodates up to two) and memory reside on the right side of the motherboard so that they do not interfere with expansion boards. Figure 29.4 shows the basic layout of this two-board system.

An L-shaped integral rear panel of the board provides space for peripheral connectors. The higher part of the panel allows designers to stack multiple connectors, one over the other, on the right side of the chassis, away from expansion boards. The NLX design envisions the motherboard to be readily removable without removing an expansion board (except the AGP board) from inside the computer case. In a typical computer, the motherboard slides out the side of the chassis, guided by rails at the bottom of the chassis. Four screws inside the computer secure the board to the chassis and electrically ground the two together. A latch that’s part of the chassis and under the motherboard holds the board horizontally in place and also serves as a board ejector to aid in removing the motherboard. Spring-like contact fingers around the periphery of the rear panel shield electrically integrate the board with the rest of the chassis.

The mechanical specification of NLX motherboards allows for a small degree of freedom in their size. The specs allow any width between eight and nine inches (inclusive), except for systems integrating AGP video boards. These must be nine inches wide because the space reserved for the AGP connector is on the last inch of the board. NLX motherboards may be between 10.0 and 13.6 inches long. The smallest NLX motherboard is approximately the size of a mini-LPX motherboard; the largest about the size of an LPX board.

The NLX specifications provide for three different mounting screw patterns, depending on the length of the board. An NLX motherboard may use any of the three patterns, but each NLX chassis design must accommodate all three.

NLX uses a 340-pin connector between the motherboard and riser. This single connector carries signals for the ISA and PCI buses, IDE drives, and miscellaneous system functions. In addition, the specification reserves space for a larger connector to accommodate future, wider expansion buses and an optional connector for other system features.

WTX

Workstations require greater power and design flexibility from ordinary general-purpose computers, so in September, 1998, Intel introduced a new specification aimed at standardizing many aspects of workstation design. Under the specification, motherboards could be up to 14 by 16.75 inches or any size smaller. This design proved superfluous, however, and although the WTX specification is still published on the Web at www.wtx.org, its promoters no longer advocate its use.

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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.

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.”

Slot Number

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.

Slot Spacing

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.

Slot Layout

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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The final aspect of your computer’s infrastructure is what truly holds everything together—its case. Although it appears to be nothing more than a box, the simple case has several complex functions. It is both the foundation for your computer and a protective shell that guards against both physical and invisible dangers. More importantly, the case is the one part of your computer that you see all the time. It is your pride and joy. The case is the physical embodiment of your computer.

In fact, the case is the body of your computer. It’s a housing, vessel, and shield that provides the delicate electronics of the computer a secure environment in which to work. Cases come in various sizes, shapes, and effectiveness at their protective tasks to match your computer and the way you plan to use it.

In its protective role, the case guards the delicate circuitry inside your computer. Part of that protection is physical, guarding against physical dangers—forces that might act against its circuit boards, bending, stressing, even breaking them with deleterious results to their operation. It also prevents electrical short circuits that may be caused by the infall of the foreign objects that typically inhabit the office—paper clips, staples, letter openers, beer cans, and errant bridgework. The case also guards against invisible dangers, principally strong electrical fields that could induce noise that would interfere with the data handling of your system, potentially inducing errors that would crash your system.

The protective shield of the case works both ways. It also keeps what’s inside your computer inside your computer. Among the wonders of the workings of a computer, two in particular pose problems for the outside world. The electrical voltages inside the computer can be a shocking discovery if you accidentally encounter them. And the high-frequency electrical signals that course through the computer’s circuits can radiate like radio broadcasts and interfere with the reception of other transmissions—including everything from television to aircraft navigational beacons.

Your computer’s case also has a more mundane role. Its physical presence gives you a place to put the things that you want to connect to your computer. Drive bays allow you to put mass-storage devices within ready reach of your computer’s logic circuits while affording the case’s protection to your peripherals. In addition, your computer’s case provides the physical embodiment of the expansion slot, affording the boards that slide into the connectors of your computer’s expansion bus protection with the same mechanical and electrical shelter as the rest of the system.

The case can play a more mundane role, too. It also can serve as the world’s most expensive monitor stand, raising your screen to an appropriate viewing angle, elevated high above the clutter and confusion of your desktop. Or a tall, desk-side computer can be an impromptu stand for your coffee or cola cup or a more permanent residence for the papers that spillover from your desk. Appropriately chosen, the right notebook computer can be a gimmick pick-up device that can win you dates—providing you hang with the right (or wrong, depending on your perspective) crowd.

Compounding the function of your computer’s case is the need to be selective. Some of what’s inside your computer needs to get out—heat, for instance. And some of what’s outside needs to get in—such as signals from the keyboard and power from your electrical outlets. In addition, the computer case must form a solid foundation upon which your system can be built. It must give disk drives a firm base and hold electrical assemblies out of harm’s way. Overall, the simple case may not be as simple as you think.

The infrastructure that makes a city includes not only highways and mass transportation but also utilities, which supply businesses and homes with what they need in the modern world: water, sewers, gas, and electricity.

Computers, too, need utility supplies to keep them going. As mere electronic beasts, however, they need neither water nor gas, and whatever sewage they create usually gets stored on their hard disks rather than carted away. Electricity is the sole necessity of the computer.

Not just any electricity will do. Computers have a definite and strict requirement for the power they use, which fortunately for us corresponds to ordinary utility power that you get at your wall outlet. The circuits inside your computer would disappear in a flash if utility power got to them, however. They require a special diet, one that the infrastructure of your computer must create for them.

The power supply inside your computer converts utility power into a form that’s safe for computer circuitry. The infrastructure of your computer distributes this power, both to the motherboard and to the various disk drives you install inside your computer.

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You know an expansion board as soon as you see one. It has an edge connector that fits only one place: on the motherboard made to accommodate it. But the reality of the expansion board is confused by issues of nomenclature and standards.

Nomenclature

Even the name expansion board holds a bit of confusion. Some people call them expansion cards. Is it a card or a board? In fact, the computer industry uses the two terms indiscriminately. This confused usage has a good precedent—long before computers, the electronics industry used the same terms for printed circuit assemblies. If there was any distinction, board was the generalized term (as in printed circuit board). Cards typically were smaller and usually plugged into a connector. Boards were usually screwed or bolted in place. No one cared, because both terms were generally understood.

However, the expansion boards used by notebook computers truly are cards, even though their printed circuit cards (or boards) are safely sealed inside their metal cases. They are called cards because of their similarity to credit cards. The names of the standards for them explicitly make them cards: PC Card and CardBus. These names and the similarity to credit cards were not accidental—the little expansion boards were designed to be a familiar and friendly credit-card size.

In the computer industry, several terms describe the boards used for expanding computers, sometimes with subtle differences in design or technology. Some of these terms include the following:

* Expansion boards. The smaller printed circuit boards that plug into your computer’s motherboard are most often termed expansion boards because they provide you with the means of expanding the capabilities of your computer. As noted before, the expansion board is distinct from the expansion slot, the space inside the computer chassis the board occupies (or potentially occupies), and the expansion connector into which you plug the board.

Expansion boards are often distinguished by the standard followed by their interface or the connector at the bottom of the board. For example, an ISA board follows the Industry Standard Architecture bus standard, and a PCI board follows the Peripheral Component Interconnect standard. We’ll discuss these standards a bit later.

* Option boards. Some computer-makers prefer to describe expansion boards as option boards. You plug them into your system to add an optional feature. Strictly speaking, then, a standard equipment expansion board—for example, a graphics adapter—would not be an option board, but for consistency’s sake (or maybe inconsistency), most manufacturers include such standard equipment among their options boards, perhaps to give you the idea you’re getting options for free—just like that lunch.

* Daughter boards. Strictly speaking, any board that plugs into a motherboard should be a daughter board, but in the realm of the computer, the family relationship is not so straightforward. Many boards that plug into the motherboard of a computer have special names of their own—memory modules, microprocessor cartridges, and expansion boards are all daughter boards. However, most computer hardware–makers reserve the term daughter board for add-on circuit boards that attach as a second layer to their expansion boards.

This two-story form of packaging was prevalent when all the circuitry needed to build an expansion board just wouldn’t fit in the space available in a single slot. The daughter board bought added square inches for circuitry. Today’s circuits are so compact that this form of construction is rarely used. Most manufacturers don’t even use the entire allowable size for their expansion board products.

* Riser boards. As noted previously, low-profile computers reduce their size by providing horizontal slots for their expansion boards. To connect these boards to the motherboard, most use a special board called a riser board. As with an ordinary expansion board, the riser board plugs into the motherboard, but its circuit endowment comprises little more than a set of connectors to accommodate your expansion boards.

Although computer expansion boards can all be considered daughter boards, not all daughter boards are expansion boards. For example, some computer expansion boards can themselves be expanded by plugging a daughter board onto them. Because such boards plug only into their host board, they are not true computer expansion boards. Most people call the circuit boards that plug into the motherboard the system’s expansion boards. Circuit boards that plug into expansion boards are daughter boards. That convention at least relieves us of adding another generation and creating the granddaughter board.

Construction

As a concept, an expansion board might be almost anything as long as it can fulfill its purpose of enhancing the capabilities of the motherboard. As a practical matter, however, the expansion board has evolved to become a printed circuit board that fits into a connector in the motherboard and a space inside the computer chassis that makes up an expansion slot. This size and shape of an expansion board is entirely arbitrary. The expansion board could be as large as all creation or as small as a single chip.

Of course, expansion boards are not. Standards have been set on the size of expansion boards to make them interchangeable so that boards from different manufacturers will fit in as many computers as possible.

These standards are not entirely arbitrary. Several factors have influenced the choice of size. For example, an expansion board cannot be larger than the space provided by the expansion slot; otherwise, it would not fit and could hardly fulfill its expansion function. The board has to be large enough to hold the circuitry it needs to do what it has to do. Manufacturers prefer smaller boards because they cost less to make. However, the board can’t be too small or it cannot hold the required expansion connector.

Several components define the physical reality of the standard computer expansion board. The board proper is an ordinary printed circuit board fabricated with pin-in-hole or surface-mount technology or a combination of both. An expansion connector connects the board to the electronic circuitry of your computer. A retaining bracket secures the board inside your computer and provides a place to put peripheral connectors.

Substrate

The substrate is the board itself, a slice of glass-epoxy upon which the various circuit components and connection traces are bonded. Computer-makers fabricate expansion boards using exactly the same technologies as motherboards.

The original design for expansion boards envisioned one end of each board sliding into a card guide, a thin slot at one end of the expansion slot, to stabilize the board inside the computer and keep it from bending or flapping in the breeze. These expansion boards that stretch from one end of the slot to the other are often called full-length expansion boards.

Most modern expansion board designs don’t require all the area allowed for the substrate in the computer and are classed as short cards. Because of their diminutive dimensions and low mass, they are adequately secured in your computer by the expansion connector and their retaining brackets.

Retaining Bracket

Nearly all expansion boards have an L-shaped bracket attached at one end. Manufacturers use a number of terms for this bracket, perhaps the most colorful being “ORB,” an acronym for option retaining bracket. The current trend is to refer to this bracket as the bracket.

In a computer, the bracket serves two functions. It secures and stabilizes the expansion board in its slot. It provides a mounting space for port connectors that may be required for connecting peripherals to the expansion board, and it helps shield your computer, keeping electrical interference inside your computer’s case by plugging up the hole at the end of the expansion slot.

In most computers, a screw secures the bracket to the computer’s chassis. When installing an expansion board, you should always ensure that this screw tightly holds each expansion board in place. Properly installing each board with a screw will prevent you from accidentally pushing the board out of the expansion connector when you plug into the connector on the board. (Tilting the expansion board can cause the contacts on its edge connector to bridge across several pins of the expansion connector, thus shorting them out and possibly crashing or even damaging your computer.) In addition, firmly screw-mounting the bracket ensures electrical continuity between the bracket and computer chassis.

Connector

The card-edge connector on each expansion board is little more than an extension of the etched copper traces of the printed circuits on the board substrate. The chief difference is that the connector pads are gold plated during the fabrication of the expansion board. The gold does not tarnish or oxidize, so it ensures that the edge connector will make a clean contact with the expansion connector on the motherboard.

The chief current expansion board standard uses the placement of pad areas and slots to key the board so that expansion boards fit only in slots designed for them.

Nearly all expansion standards for desktop computers use edge connectors for one very good reason: They are cheap. The connector’s contacts get etched onto the board at the same time as the rest of its circuit traces. The only extra expense is the thin gold plating on the contact area to stave off the oxidation of the copper or lead-and-tin-coated traces.

After the pragmatic choice of an edge connector, the creator of an expansion standard still has a variety of choices. Most important is the spacing between contacts in the connector. The spacing, along with the number of contacts, determines the size. In a dream world—the one in which many designers operate—the size of the connector is no concern. In the real world, however, it has two dramatic effects. It determines how much space must be given up to connectors on the motherboard, and it governs the insertion force of an expansion board into the connector.

In true bus-style computers, the board space given up to the expansion bus is immaterial. The computer chassis is nothing but the bus, so there is no problem in devoting the whole back or bottom of the machine to the expansion bus. In the traditional computer design, however, the bus takes up space on the motherboard, which also has to provide the basic circuitry of the computer. The more motherboard space taken up by the bus, the less is available for building the basic computer. Consequently, the bus area must be as compact as possible to yield the largest possible circuit space on the motherboard.

The larger the connector, the more area of the expansion board that rubs against the contacts inside the socket when you plug the board in. To ensure a reliable electrical connection, the socket contacts must press forcefully against the contact tabs on the circuit board. Sliding an expansion board into a socket requires enough force to squeeze the board contacts between the socket contacts. The more area devoted to contacts, the greater the required insertion force. When a connector is too long, the insertion force may be greater than some people can comfortably apply, even greater than the automatic insertion machinery used by computer manufacturers can apply. Worse, if the insertion force is high enough, sliding in an expansion board may overly stress the motherboard, potentially cracking it or one of the conductive traces and putting it out of action.

Making the contact smaller shortens the connector, cutting down on the motherboard space required for the bus and reducing insertion force. It also requires greater precision in the manufacture of expansion boards. Nevertheless, newer expansion board standards are marked by closer spacing of their edge connector contacts—just compare an ISA board to a PCI board inside your computer.

More specialized bus standards, such as those for notebook computers and industrial computers, rely on pin connectors. These necessarily cost more because they add another part that must be soldered to each expansion board. But because the connector is a separate part, it can be manufactured with greater precision, and the greater precision allows for smaller, more compact connectors. Moreover, pin connectors can use more than two contact rows to further reduce the space that must be devoted to the bus. Pin connectors are also more reliable because they allow their contacts to mate on multiple sides. Pin connectors are also easier to shield, making them more desirable as concerns about emissions increase along with bus speeds.