Chapter 4. Motherboards and Buses

Motherboards have evolved over the years from the original Baby-AT form factor boards used in the original IBM PC and XT to the current BTX and ATX boards used in most full-size desktop and tower systems. ATX has a number of variants, including microATX (which is a smaller version of the ATX form factor used in the smaller systems) and FlexATX (an even smaller version for the lowest -cost home PCs and some smallform-factor corporate PCs). The newest form factor, BTX, relocates major components to improve system cooling and incorporates a thermal module. BTX also has smaller microBTX and picoBTX variations. Another small form factor called Mini-ITX is also available; it's really just a minimum-size version of FlexATX designed for very small systems. NLX is designed for corporate desktoptype systems but has largely been replaced by FlexAXT; WTX was designed for workstations and medium-duty servers, but never became popular. Table 4.1 shows the modern industry-standard form factors and their recommended uses.

Table 4.1. Common Industry-Standard Motherboard Form Factors

Although the Baby-AT, Full-size AT, and LPX boards were once popular, they have been replaced by more modern and interchangeable form factors. The modern form factors are true standards that provide improved interchangeability within each type. This means one brand of ATX boards can interchange with other brand ATX boards, BTX with other BTX, and so on. The additional features found on these boards as compared to the obsolete form factors, combined with true interchangeability, has made the migration to these newer form factors quick and easy. Today I recommend purchasing only systems with one of the modern industry-standard form factors. Each of these form factors, however, is discussed in more detail in the following sections.

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

Caution

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

You also need to watch out for systems that only appear to meet industry standards, such as certain Dell computer models built from 1996 to the presentespecially the XPS line of systems. These computers often use rewired versions of the ATX power supply (or even some that are completely nonstandard in size and shape) and modified motherboard power connectors, which makes both components completely incompatible with standard motherboards and power supplies. In some of the systems, the power supply has a completely proprietary shape as well and the motherboards are not fully standard ATX either. If you want to upgrade the power supply, you must use a special Dell-compatible power supply. And if you want to upgrade the motherboard ( assuming you can find one that fits), you must buy a standard power supply to match. The best alternative is to replace the motherboard, power supply, and possibly the case with industry-standard components simultaneously . For more details about how to determine whether your Dell computer uses nonstandard power connectors, see Chapter 19, "Power Supplies."

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

PC and XT

The first popular PC motherboard was, of course, the original IBM PC released in August 1981. Figure 4.1 shows how this board looked . IBM followed the PC with the XT motherboard in March 1983, which had the same size and shape as the PC board but had eight slots instead of five. Both the IBM PC and XT motherboards were 9"x13" in size. Also, the slots were spaced 0.8" apart in the XT instead of 1" apart as in the PC (see Figure 4.2). The XT also eliminated the little-used cassette port in the back, which was supposed to be used to save BASIC programs on cassette tape instead of the much more expensive (at the time) floppy drive.

Note

The Technical Reference section of the disc accompanying this book contains detailed information on the PC (5150) and XT (5160). All the information there is printable.

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

Full-Size AT

The full-size AT motherboard form factor matches the original IBM AT motherboard design. This allows for a very large board of up to 12" wide by 13.8" deep. The full-size AT board first debuted in August 1984, when IBM introduced the Personal Computer AT (advanced technology). To accommodate the 16-bit 286 processor and all the necessary support components at the time, IBM needed more room than the original PC/XT- sized boards could provide. So for the AT, IBM increased the size of the motherboard but retained the same screw hole and connector positions of the XT design. To accomplish this, IBM essentially started with a PC/XT-sized board and extended it in two directions (see Figure 4.3).

Note

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

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

The keyboard connector and slot connectors in the full-size AT boards still conformed to the same specific placement requirements to fit the holes in the XT cases already in use, but a larger case was still required to fit the larger board. Because of the larger size of the board, a full-size AT motherboard only fits into full-size AT desktop or tower cases. Because these motherboards do not fit into the smaller Baby-AT or minitower cases, and because of advances in component miniaturization, they are no longer being produced by most motherboard manufacturersexcept in some cases for dual processor server applications.

The important thing to note about the full-size AT systems is that you can always replace a full-size AT motherboard with a Baby-AT (or XT-size) board, but the opposite is not true unless the case is large enough to accommodate the full-size AT design.

Baby-AT

After IBM released the AT in August 1984, component consolidation allowed subsequent systems to be designed using far fewer chips and requiring much less in the way of motherboard real estate. Therefore, all the additional circuits on the 16-bit AT motherboard could fit into boards using the smaller XT form factor.

IBM was one of the first to use the smaller boards when it introduced a system called the XT-286 in September 1986. Unfortunately the "XT" designation in the name of that system caused a lot of confusion, and many people did not want to buy a system they thought used older and slower technology. Sales of the XT-286 were dismal. By this time, other companies had also developed XT-size AT class systems. However, they decided that rather than calling these boards XT-size, which seemed to make people think they were 8-bit designs, they would refer to them as "Baby-AT" designs. The intention was to make people understand that these new boards had AT technology in a smaller form factor and were not souped-up versions of older technology as was seemingly implied by IBM's XT-286 moniker.

Thus, the Baby-AT form factor is essentially the same form factor as the original IBM XT motherboard. The only difference is a slight modification in one of the screw hole positions to fit into an AT-style case. These motherboards also have specific placement of the keyboard and slot connectors to match the holes in the case. Note that virtually all full-size AT and Baby-AT motherboards use the standard 5-pin DIN type connector for the keyboard. Baby-AT motherboards can be used to replace full-size AT motherboards and will fit into several case designs. Because of its flexibility, from 1983 into early 1996, the Baby-AT form factor was the most popular motherboard type. Starting in mid-1996, Baby-AT was replaced by the superior ATX motherboard design, which is not directly interchangeable. Most systems sold since 1996 have used the improved ATX, microATX, or NLX design Baby-AT motherboards, power supplies, and cases are almost impossible to come by, even at surplus computer hardware outletsand in any event, they support outdated processors, memory, and other components that are equally difficult to find. Figure 4.5 shows the onboard features and layout of a late-model Baby-AT motherboard. Older Baby-AT motherboards have the same general layout but lack advanced features, such as USB connectors, DIMM memory sockets, and the AGP slot.

Any case that accepts a full-size AT motherboard will also accept a Baby-AT design. PC motherboards using the Baby-AT design have been manufactured to use virtually any processor from the original 8088 to the Pentium III or Athlon, although the pickings are slim where the newer processors are concerned . As such, systems with Baby-AT motherboards were the original upgradeable systems. Because any Baby-AT motherboard can be replaced with any other Baby-AT motherboard, this is an interchangeable design. Even though the Baby-AT design (shown in Figure 4.4) is now obsolete, ATX carries on its philosophy of interchangeability. Figure 4.5 shows a more modern Baby-AT motherboard, which includes USB compatibility, SIMM and DIMM sockets, and even a supplemental ATX power supply connection.

The easiest way to identify a Baby-AT form factor system without opening it is to look at the rear of the case. In a Baby-AT motherboard, the cards plug directly into the board at a 90° angle; in other words, the slots in the case for the cards are perpendicular to the motherboard. Also, the Baby-AT motherboard has only one visible connector directly attached to the board, which is the keyboard connector. Typically, this connector is the full-size 5-pin DIN type connector, although some Baby-AT systems use the smaller 6-pin mini-DIN connector (sometimes called a PS/2 type connector) and might even have a mouse connector. All other connectors are mounted on the case or on card edge brackets and are attached to the motherboard via cables. The keyboard connector is visible through an appropriately placed hole in the case.

Baby-AT boards all conform to specific widths and screw hole, slot, and keyboard connector locations, but one thing that can vary is the length of the board. Versions have been built that are smaller than the full 9"x13" size; these are often called mini-AT, micro-AT, or even things such as 2/3-Baby or 1/2-Baby. Even though they might not be the full size, they still bolt directly into the same case as a standard Baby-AT board and can be used as a direct replacement for one.

LPX

The LPX and mini-LPX form factor boards were a semiproprietary design that Western Digital originally developed in 1987 for some of its motherboards. The LP in LPX stands for Low Profile, which is so named because these boards incorporate slots that are parallel to the main board, enabling the expansion cards to install sideways . This allows for a slim or low-profile case design and overall a smaller system than the Baby-AT.

Although Western Digital no longer produces PC motherboards, the form factor lives on, and many other motherboard manufacturers have duplicated the general design. Unfortunately, because the specifications were never laid out in exact detailespecially with regard to the bus riser card portion of the designthese boards are termed semiproprietary and are not interchangeable between manufacturers. Some vendors , such as IBM and HP, for example, have built LPX systems that use a T-shaped riser card that allows expansion cards to be mounted at the normal 90° angle to the motherboard but still above the motherboard. This lack of standardization means that if you have a system with an LPX board, in most cases you can't replace the motherboard with a different LPX board later. You essentially have a system you can't upgrade or repair by replacing the motherboard with something better. In other words, you have what I call a disposable PC, something I would not normally recommend that anybody purchase.

Most people were not aware of the semiproprietary nature of the design of these boards, and they were extremely popular in what I call "retail store" PCs from the late 1980s through the late 1990s. This would include primarily Compaq and Packard Bell systems, as well as many others who used this form factor in their lower-cost systems. These boards were most often used in low-profile or Slimline case systems but were found in tower cases, too. These were often lower-cost systems such as those sold at retail electronics superstores. Although scarce even in retail chains today, because of their proprietary nature, I recommend staying away from any system that uses an LPX motherboard.

LPX boards are characterized by several distinctive features (see Figure 4.6). The most noticeable is that the expansion slots are mounted on a bus riser card that plugs into the motherboard. In most designs, expansion cards plug sideways into the riser card. This sideways placement allows for the low-profile case design. Slots are located on one or both sides of the riser card depending on the system and case design. Vendors who use LPX-type motherboards in tower cases sometimes use a T-shaped riser card instead, which puts the expansion slots at the normal right angle to the motherboard but on a raised shelf above the motherboard itself.

Another distinguishing feature of the LPX design is the standard placement of connectors on the back of the board. An LPX board has a row of connectors for video (VGA 15-pin), parallel (25-pin), two serial ports (9-pin each), and mini-DIN PS/2 style mouse and keyboard connectors. All these connectors are mounted across the rear of the motherboard and protrude through a slot in the case. Some LPX motherboards might have additional connectors for other internal ports, such as network or SCSI adapters. Because LPX systems use a high degree of motherboard port integration, many vendors of LPX motherboards, cases, and systems often refer to LPX products as having an "all-in-one" design.

The standard form factor used for LPX and mini-LPX motherboards in many typical low-cost systems is shown in Figure 4.7.

I am often asked, "How can I tell whether a system has an LPX board without opening the cover?" Because of the many variations in riser card design, and because newer motherboards such as NLX also use riser cards, the most reliable way to distinguish an LPX motherboard from other systems is to look at the connector signature (the layout and pattern of connectors on the back of the board). As you can see in Figure 4.8, all LPX motherboardsregardless of variations in riser card shape, size, or locationplace all external ports along the rear of the motherboard. By contrast, Baby-AT motherboards use case-mounted or expansion slotmounted connectors for serial, parallel, PS/2 mouse, and USB ports, whereas ATX-family and BTX-family motherboards group all external ports together to the left side of the expansion slots.

On an LPX board, the riser is placed in the middle of the motherboard, whereas NLX boards have the riser to the side (the motherboard actually plugs into the riser in NLX).

Figure 4.8 shows two typical examples of the connectors on the back of LPX boards. Note that not all LPX boards have the built-in audio, so those connectors might be missing. Other ports (such as USB) might be missing from what is shown in these diagrams, depending on exactly which options are included on a specific board; however, the general layout will be the same.

The connectors along the rear of the board would interfere with locating bus slots directly on the motherboard, which accounts for why riser cards are used for adding expansion boards.

Although the built-in connectors on the LPX boards were a good idea, unfortunately the LPX design was semiproprietary (not a fully interchangeable standard) and thus, not a good choice. Newer motherboard form factors such as ATX, microATX, and NLX have both built-in connectors and use a standard board design. The riser card design of LPX allowed system designers to create a low-profile desktop system, a feature now carried by the much more standardized NLX form factor. In fact, NLX was developed as the modern replacement for LPX.

ATX

The ATX form factor was the first of a dramatic evolution in motherboard form factors. ATX is a combination of the best features of the Baby-AT and LPX motherboard designs, with many new enhancements and features thrown in. The ATX form factor is essentially a Baby-AT motherboard turned sideways in the chassis, along with a modified power supply location and connector. The most important thing to know initially about the ATX form factor is that it is physically incompatible with either the previous Baby-AT or LPX design. In other words, a different case and power supply are required to match the ATX motherboard. These case and power supply designs have become common and are found in most new systems.

Intel initially released the official ATX specification in July 1995. It was written as an open specification for the industry. ATX boards didn't hit the market in force until mid-1996, when they rapidly began replacing Baby-AT boards in new systems. The ATX specification was updated to version 2.01 in February 1997, 2.03 in May 2000, 2.1 in June 2002, and 2.2 in February 2004. Intel publishes these detailed specifications so other manufacturers can use the interchangeable ATX design in their systems. The current specifications for ATX and other current motherboard types are available online from the Desktop Form Factors site: www.formfactors.org. ATX is the most popular motherboard form factor for new systems through 2006 and will continue to be popular in the future. An ATX system will be upgradeable for many years to come, exactly like Baby-AT was in the past.

ATX improved on the Baby-AT and LPX motherboard designs in several major areas:

• Built-in double high external I/O connector panel. The rear portion of the motherboard includes a stacked I/O connector area that is 6 1/4" wide by 1 3/4" tall. This enables external connectors to be located directly on the board and negates the need for cables running from internal connectors to the back of the case as with Baby-AT designs.

• Single main keyed internal power supply connector. This is a boon for the average end user who always had to worry about interchanging the Baby-AT power supply connectors and subsequently blowing the motherboard. The ATX specification includes a keyed and shrouded main power connector that is easy to plug in and can't be installed incorrectly. This connector also features pins for supplying 3.3V to the motherboard, so ATX motherboards do not require built-in voltage regulators that are susceptible to failure. The ATX specification was extended to include two additional optional keyed power connectors called the Auxiliary Power connector (3.3V and 5V) and the ATX12V connector for systems that require more power than the original specification would allow. The latest version of the ATX power connector specification uses a 24-pin connector rather than the original 20-pin connector.

  See "Motherboard Power Connectors," p. 1167.

  
  

• Relocated CPU and memory. The CPU and memory modules are relocated so they can't interfere with any bus expansion cards and can easily be accessed for upgrade without removing any of the installed bus adapters. The CPU and memory are relocated next to the power supply, which is where the primary system fan is located. Although the improved airflow over the processor helped to eliminate the need for extra-cost CPU fans in older and lower- powered systems, most current ATX systems today require active heatsinks on the processor because of how hot today's CPUs run. There is room for a CPU and a heatsink and fan combination of up to 2.8" in height, as well as more than adequate side clearance provided in that area.

• Relocated internal I/O connectors. The internal I/O connectors for the floppy and hard disk drives are relocated to be near the drive bays and out from under the expansion board slot and drive bay areas. Therefore, internal cables to the drives can be much shorter, and accessing the connectors does not require card or drive removal.

• Improved cooling. The CPU and main memory are designed and positioned to improve overall system cooling. This can decreasebut not necessarily eliminatethe need for separate case or CPU cooling fans. Most higher-speed systems still need additional cooling fans for the CPU and chassis. Note that the ATX specification originally specified that the ATX power supply fan blows into the system chassis instead of outward. This reverse flow, or positive pressure design, pressurizes the case and minimizes dust and dirt intrusion. More recently, the ATX specification was revised to allow the more normal standard flow, which negatively pressurizes the case by having the fan blow outward. Because the specification technically allows either type of airflow, and because some overall cooling efficiency is lost with the reverse flow design, most power supply manufacturers provide ATX power supplies with fans that exhaust air from the system, otherwise called a negative pressure design. See Chapter 21 for more detailed information.

• Lower cost to manufacture. The ATX specification eliminates the need for the rat's nest of cables to external port connectors found on Baby-AT motherboards, additional CPU or chassis cooling fans, or onboard 3.3V voltage regulators. Instead, ATX allows for shorter internal drive cables and no cables for standard external serial or parallel ports. These all conspire to greatly reduce the cost of the motherboard and the cost of a complete systemincluding the case and power supply.

Figure 4.9 shows the ATX system layout and chassis features, as you would see them looking in with the lid off on a desktop, or sideways in a tower with the side panel removed. Notice how virtually the entire motherboard is clear of the drive bays and how the devices such as CPU, memory, and internal drive connectors are easy to access and do not interfere with the bus slots. Also notice how the processor is positioned near the power supply.

The ATX motherboard shape is basically a Baby-AT design rotated sideways 90°. The expansion slots are now parallel to the shorter side dimension and do not interfere with the CPU, memory, or I/O connector sockets (see Figure 4.10). There are actually two basic sizes of standard ATX boards. In addition to a full-size ATX layout, Intel also specified a MiniATX design, which is a fully compatible subset of ATX that fits into the same case:

• A full-size ATX board is 12" wide x 9.6" deep (305mmx244mm).

• The Mini-ATX board is 11.2"x8.2" (284mmx208mm).

Mini-ATX is not an official standard; instead it is simply referenced as a slightly smaller version of ATX. In fact, all references to Mini-ATX were removed from the ATX 2.1 and later specifications. Two smaller official versions of ATX exist, called microATX and FlexATX. They are discussed in the following sections.

Although the case holes are similar to the Baby-AT case, cases for Baby-AT and ATX are generally incompatible. The ATX power supply design is identical in physical size to the standard Slimline power supply used with Baby-AT systems; however, they also use different connectors and supply different voltages.

The ATX form factor's design advantages have swept Baby-AT and LPX motherboards off the market. Although other form factors are now available, I have been recommending only ATX (or compatible variations such as microATX or FlexATX) systems for new system purchases since late 1996 and will probably continue to do so for the next several years.

The best way to tell whether your system has an ATX-family motherboard design without removing the lid is to look at the back of the system. Two distinguishing features identify ATX. One is that the expansion boards plug directly into the motherboard. There is usually no riser card as with LPX or NLX (except for certain slimline systems, such as rack-mounted servers), so the slots are usually perpendicular to the plane of the motherboard. Also, ATX boards have a unique double-high connector area for all the built-in connectors on the motherboard (see Figure 4.11 and Table 4.2). This is found just to the side of the bus slot area and can be used to easily identify an ATX board.

Table 4.2. Built-in Ports Usually Found on ATX Motherboards

Note

Most ATX motherboards feature connectors with industry-standardized color codes (shown in the previous table). This makes plugging in devices much easier and more foolproof: You merely match up the colors. For example, most keyboards have a cable with a purple plug, whereas most mice have a cable with a green plug. Even though the keyboard and mouse connectors on the motherboard appear the same (both are 6-pin Mini-DIN types), their color-coding matches the plugs on the respective devices. Thus, to plug them in properly, you merely insert the purple plug into the purple connector and the green plug into the green connector. This saves you from having to bend down to try to decipher small labels on the connectors to ensure you get them right.

The specification and related information about the ATX, Mini-ATX, microATX, FlexATX, or NLX form factor specifications are available from the Form Factors website at www.formfactors.org. The Form Factors site provides form factor specifications and design guides, as well as design considerations for new technologies, information on initiative supporters, vendor products, and a form factor discussion forum.

Note

Some motherboards, especially those used in server systems, come in nonstandard ATX variations collectively called extended ATX . This is a term applied to boards that are compatible with ATX but that are deeper. Standard ATX is 12"x9.6" (305mmx244mm), whereas extended ATX boards are up to 12"x13" (305mmx330mm). Because technically no official "extended ATX" standard exists, compatibility problems can exist with boards and chassis claiming to support extended ATX. When purchasing an extended ATX board, be sure it will fit in the chassis you intend to use. Dual Xeon processors fit in a standard ATX-size board, so choose a standard ATX-size board for maximum compatibility with the existing ATX chassis.

ATX Riser

In December 1999, Intel introduced a riser card design modification for ATX motherboards. The design includes the addition of a 22-pin (2x11) connector to one of the PCI slots on the motherboard, along with a two- or three-slot riser card that plugs in. The riser enables two or three PCI cards to be installed, but it does not support AGP.

ATX motherboards typically are found in vertically oriented tower-type cases, but often a horizontal desktop system is desired for a particular application. When ATX boards are installed in desktop cases, PCI cards can be as tall as 4.2", thus requiring a case that is at least 6"7" tall. For Slimline desktop systems, some manufacturers have used the NLX format, but the more complex design and lower popularity of NLX makes that a more expensive alternative. A low-cost way to use an industry-standard ATX form factor board in a Slimline desktop case is therefore needed. The best long-term solution to this problem is the eventual adoption of a lower-profile PCI card design that is shorter than the current 4.2". The PCI Low-Profile specification was released for engineering review by the Peripheral Component Interconnect Special Interest Group (PCI SIG) on February 14, 2000, and some PCI card products have been produced in this shorter (2.5") form factor. Until Low-Profile PCI becomes widespread, Intel has suggested a riser card approach to enable standard-height PCI cards to be used in Slimline and rack-mount (1U, 2U) systems.

By adding a small 22-pin extension connector to one of the PCI slots on a motherboard, the necessary additional signals for riser card support could be implemented. The current design enables the use of a two- or three-slot riser that is either 2" or 2.8" tall, respectively. To this riser, you can attach full-length cards sideways in the system, and the motherboard can be used with or without the riser. The only caveat is that, if a riser card is installed, the remaining PCI slots on the motherboard can't be used. You can have expansion cards plugged in to only the riser or the motherboard, but not both. Also, the riser card supports only PCI cardsnot AGP or ISA cards. A sample ATX board with a riser installed is shown in Figure 4.12.

The 22-pin extension connector usually is installed in line with PCI slot 6, which is the second one from the right; the slots are usually numbered from right to left ( facing the board) starting with 7 as the one closest to the processor. Some boards number the slots from right to left starting with 1; in that case, the extension connector is on PCI slot 2. The pinout of the ATX 22-pin riser extension connector is shown in Figure 4.13.

The PCI connector that is in line with the riser extension connector is just a standard PCI slot; none of the signals are changed.

Systems that use the riser generally are low-profile designs. Therefore, they don't fit normal PCI or AGP cards in the remaining (nonriser-bound) slots. Although the ATX riser standard originally was developed for use with low-end boardswhich have integrated video, sound, and network supportmany rack-mounted servers are also using the ATX riser because these boards also have most of their required components already integrated. In fact, the ATX riser appears to be more popular for rack-mounted servers than for the originally intended target market of Slimline desktop systems.

ATX riser cards, compatible cases, and compatible motherboards are available from a variety of vendors, allowing you to build your own Slimline ATX system.

microATX

microATX is a motherboard form factor Intel originally introduced in December 1997, as an evolution of the ATX form factor for smaller and lower-cost systems. The reduced size as compared to standard ATX allows for a smaller chassis, motherboard, and power supply, thereby reducing the cost of the entire system. The microATX form factor is also backward-compatible with the ATX form factor and can be used in full-size ATX cases. Of course, a microATX case doesn't take a full-size ATX board. This form factor has become popular in the low-cost PC market. Currently, mini-tower chassis systems dominate the low-cost PC market, although their small sizes and cramped interiors severely limit future upgradeability.

The main differences between microATX and standard or Mini-ATX are as follows :

• Reduced width motherboard (9.6" [244mm] instead of 12" [305mm] or 11.2" [284mm])

• Fewer I/O bus expansion slots (four maximum, although most boards feature only three)

• Smaller power supply optional (SFX/TFX form factors)

The microATX motherboard maximum size is only 9.6"x9.6" (244mmx244mm) as compared to the full-size ATX size of 12"x9.6" (305mmx244mm) or the Mini-ATX size of 11.2"x8.2" (284mmx208mm). Even smaller boards can be designed as long as they conform to the location of the mounting holes, connector positions, and so on, as defined by the standard. Fewer slots aren't a problem for typical home or small-business PC users because more components such as sound and video are usually integrated on the motherboard and therefore don't require separate slots. This higher integration reduces motherboard and system costs. External buses, such as USB, 10/100 Ethernet, and optionally SCSI or 1394 (FireWire), can provide additional expansion out of the box. The specifications for microATX motherboard dimensions are shown in Figure 4.14.

Smaller form factor (called SFX or TFX) power supplies have been defined for optional use with microATX systems, although the standard ATX supply also works fine because the connectors are the same. The smaller size SFX/TFX power supplies encourage flexibility in choosing mounting locations within the chassis and allows for smaller systems that consume less power overall. Although the smaller supplies can be used, they may lack sufficient power output for faster or more fully configured systems. Because of the high power demands of most modern systems, most third-party microATX chassis are designed to accept standard ATX power supplies, although microATX systems sold by vendors such as Compaq, HP, and eMachines typically use some type of SFX or TFX power supply to reduce costs.

The microATX form factor is similar to ATX for compatibility. The similarities include the following:

• Standard ATX 20-pin power connector

• Standard ATX I/O panel

• Mounting holes and dimensions are a subset of ATX

These similarities ensure that a microATX motherboard can easily work in a standard ATX chassis with a standard ATX power supply, as well as the smaller microATX chassis and SFX/TFX power supply.

The overall system size for a microATX is very small. A typical case is only 12"14" tall, about 7" wide, and 12" deep. This results in a kind of micro-tower or desktop size. A typical microATX motherboard is shown in Figure 4.15.

As with ATX, Intel released microATX to the public domain to facilitate adoption as a de facto standard. The specification and related information on microATX are available through the Desktop Form Factors site (www.formfactors.org).

FlexATX

In March 1999, Intel released the FlexATX addendum to the microATX specification. This added a new and even smaller variation of the ATX form factor to the motherboard scene. FlexATX's smaller design is intended to allow a variety of new PC designs, especially extremely inexpensive, smaller, consumer-oriented, appliance-type systems. Some of these designs might not even have expansion slots, allowing expansion only through USB or IEEE 1394/FireWire ports.

FlexATX defines a board that is up to 9"x7.5" (229mmx191mm) in size, which is the smallest of the ATX family boards. In all other ways, FlexATX is the same as ATX and microATX, making FlexATX fully backward compatible with ATX or microATX by using a subset of the mounting holes and the same I/O and power supply connector specifications (see Figure 4.16).

Most FlexATX systems likely use SFX/TFX (small or thin form factor) type power supplies (introduced in the microATX specification), although if the chassis allows it, a standard ATX power supply can also be used.

With the addition of FlexATX, the family of ATX boards has now grown to include four definitions of size (three are the official standards), as shown in Table 4.3.

Table 4.3. ATX Motherboard Form Factors

Note that these dimensions are the maximums allowed. Making a board smaller in any given dimension is always possible as long as it conforms to the mounting hole and connector placement requirements detailed in the respective specifications. Each board has the same basic screw hole and connector placement requirements, so if you have a case that fits a full-size ATX board, you could also mount a microATX, or FlexATX board in that same case. Obviously, if you have a smaller case designed for microATX or FlexATX, you won't be able to put the larger Mini-ATX or full-size ATX boards in that case.

ITX and Mini-ITX

FlexATX is the smallest industry-standard form factor specification, and it defines a board that is up to 9"x7.5" in size. Note the up to part of the dimensions, which means that, even though those dimensions are the maximums, less is also allowed. Therefore, a FlexATX board can be smaller than that, but how much smaller? By analyzing the FlexATX specificationand, in particular, studying the required mounting screw locationsyou can see that a FlexATX board could be made small enough to use only four mounting holes (C, F, H, and J). Refer to Figure 4.16 for the respective hole locations.

According to the FlexATX standard, the distance between holes H and J is 6.2", and the distance between hole J and the right edge of the board is 0.25". By leaving the same margin from hole H to the left edge, you could make a board with a minimum width of 6.7" (0.25" + 6.2" + 0.25") that would conform to the FlexATX specification. Similarly, the distance between holes C and H is 6.1", and the distance between hole C and the back edge of the board is 0.4". By leaving a minimum 0.2" margin from hole H to the front edge, you could make a board with a minimum depth of 6.7" (0.4" + 6.1" + 0.2") that would conform to the FlexATX specification. By combining the minimum width and depth, you can see that the minimum board size that would conform to the FlexATX specification is 6.7"x6.7" (170mmx170mm).

VIA Technologies Platform Solutions Division wanted to create a motherboard as small as possible, yet not define a completely new and incompatible form factor. To accomplish this, in March 2001 VIA created a board that was slightly narrower in width (8.5" instead of 9") but still the same depth as FlexATX, resulting in a board that was 6% smaller and yet still conformed to the FlexATX specification. VIA called this ITX but then realized that the size savings were simply too small to justify developing it further, so it was discontinued before any products were released.

In April 2002, VIA created an even smaller board that featured the absolute minimum width and depth dimensions allowed by FlexATX. It called it Mini-ITX. In essence, all Mini-ITX boards are simply FlexATX boards that are limited to the minimum allowable dimensions. All other aspects, including the I/O aperture size and location, screw hole locations, and power supply connections, are pure FlexATX. A Mini-ITX board fits in any chassis that accepts a FlexATX board; however, larger boards will not fit into a Mini-ITX chassis.

The Mini-ITX form factor was designed by VIA especially to support VIA's low-power embedded Eden ESP and C3 E-Series processors. Only a very small number of motherboards is available in this form factor, and only from VIA and one or two other manufacturers. Because the processors used on these boards are substantially less powerful than even the old Intel Celeron 4 or AMD Duron entry-level processors, the Mini-ITX form factor is intended for use mainly in nontraditional settings such as set-top boxes and computing appliances. The size of the ITX and Mini-ITX boards relate to FlexATX as shown in Table 4.4.

Table 4.4. Comparing the FlexATX, ITX, and Mini-ITX Form Factors

Again, I must point out that technically any ITX or Mini-ITX board conforms to the FlexATX specification. In particular, the Mini-ITX is the smallest board that can conform. Although the still-born ITX format was virtually the same as FlexATX in size (which is probably why it was discontinued before any were sold), Mini-ITX motherboards are 170mmx170mm (6.7"x6.7"), which is 34% smaller than the maximum allowed by FlexATX.

To take advantage of the smaller Mini-ITX format, several chassis makers are producing very small chassis to fit these boards. Most are the shape of a small cube, with one floppy and one optical drive bay visible from the front. The layout of a typical Mini-ITX motherboard, the VIA EPIA-V, is shown in Figure 4.17.

As Figure 4.17 makes clear, Mini-ITX motherboards can offer a full range of input-output ports. However, several differences exist between the Mini-ITX motherboards and other ATX designs:

• The processor on a Mini-ITX motherboard is usually permanently soldered to the board, making future processor upgrades or replacements impossible.

• Most Mini-ITX chassis use TFX power supplies, for which there are currently only a few suppliers. Consequently, replacements for them are more expensive and more difficult to find.

• The available TFX power supplies are rated for less output than larger supplies, typically up to 240 watts maximum.

• There is no provision for replacing onboard video with an AGP video card.

Because Mini-ITX boards and chassis are made by only a small number of suppliers, future upgrades or parts replacements are limited. However, Mini-ITX boards are actually FlexATX boards, so they can be installed in any standard FlexATX, microATX, or full-size ATX chassis and use the corresponding power supplies. The only caveats are that the smaller Mini-ITX chassis will not accept larger FlexATX, microATX, or full-size ATX boards and most Mini-ITX chassis accept only TFX power supplies. When you select a Mini-ITX system, you must be sure to select the appropriate processor type and speed necessary for the task you need it to perform because processor replacements or upgrades almost always require changing the entire motherboard.

Both the VIA C3 EBGA and the VIA Eden ESP are x86-compatible processors, so they are fully capable of running the same operating systems and applications as typical AMD and Intel processors, including Windows and Linux. However, as stated, you cannot expect systems using these processors to be anywhere near the bleeding edge with regards to overall performance.

VIA uses varying combinations of the following North Bridge and South Bridge chips in its Mini-ITX motherboards. The North Bridge uses either the PLE133 or CLE266 chips, whereas the South Bridge uses either the VT8231 or VT8235 chips.

The PLE133 North Bridge chip features built-in Trident AGP 4x video and support for PC100 and PC133 SDRAM memory. The CLE266, on the other hand, features built-in S3 Savage 4 4x AGP video, a built-in MPEG2 decoder for excellent DVD playback, and support for DDR266 SDRAM memory.

The VT8231 South Bridge chip features AC'97 audio, MC'97 modem, an ATA-100 host adapter, and four USB 1.1 ports. Additional capabilities can be added through optional chips. The VT8235 South Bridge chip features six-channel audio, an ATA-133 host adapter, USB 2.0 ports, 10/100 Ethernet, PCI controller, and MC'97 modem. It also supports the new 8X V-Link interface with the North Bridge.

VIA offers a number of Mini-ITX motherboards through its VIA Embedded Platform Division (VEPD), including the following:

• EPIA

• EPIA V (refer to Figure 4.17)

• EPIA M

• EPIA CL

• EPIA TC

• EPIA MII

• EPIA PD

• EPIA ML

• VT-310DP

• EPIA SP

Table 4.5 summarizes the differences in these motherboards.

Table 4.5. VPSD Mini-ITX Motherboards

^[*] EPIA PD has four serial ports; other models feature one serial port.

^[**] VT-310DP supports dual processors and Gigabit Ethernet.

The VT-310DP is the first VIA motherboard to feature dual Eden-N NanoBGA processors and Gigabit Ethernet. The VT-310DP and the single-processor EPIA SP also support SATA hard disks, making them suitable for low-demand NAS storage and multimedia server applications. The EPIA PD motherboard is a suitable choice for point-of-sale (POS) systems because it has four serial ports (often used for POS printers).

Obviously, with top performance only on par with 1GHz-class Celeron systems, Mini-ITX motherboards are not intended for power user applications. However, if you need a compact system for specialized low-end uses or for small-footprint computers for office suites and Internet access and don't mind investing in a small form factor that might make future upgrades or repairs extremely difficult, these tiny systems can be useful.

Note

The official site for ITX information is www.viaembedded.com/index.jsp. The site www.mini-itx.com is often mistaken for an official site, but it is actually a vendor that specializes in ITX systems and component sales.

The latest development in the ITX family is the Nano-ITX form factor, an ultra -compact (120mmx120mm) platform that uses the mini-PCI slot familiar from notebook computers for expansion. Nano-ITX motherboards are made possible by VIA's development of the Luke CoreFusion processor, which integrates the VIA Eden-N CPU and the CN400 North Bridge, which includes the S3 Graphics UniChrome Pro graphics processor. The VIA VT8237R South Bridge provides onboard ATA/IDE and Serial ATA host adapters and support for USB 2.0 ports, 10/100 Ethernet, six-channel AC'97 audio, TV-out (S-video/composite/component), and a serial port. Nano-ITX motherboards from VIA include the EPIA NL and EPIA N.

BTX

Balanced Technology Extended (BTX) is a motherboard form factor specification Intel originally released in September 2003. A 1.0a update was released in February 2004. BTX was designed to eventually replace the venerable ATX form factor while addressing ever-increasing component power and cooling requirements, as well as enabling improved circuit routing and more flexible chassis designs. However, the recent trend toward more power efficient dual- core processor designs has slowed the need for the benefits inherent in the BTX standard, which has in turn slowed the adoption of BTX. BTX might eventually supplant ATX as the most common system form factor, but at this point that's not yet a certainty .

BTX represents a completely new form factor that is not backward-compatible with ATX or other designs. A full-size BTX board is 17% larger than ATX, allowing room for more integrated components onboard. The I/O connectors, slots, and mounting holes are in different locations than with ATX, requiring new chassis designs. However, the power supply interface connectors are the same as in the latest ATX12V specifications, and newer ATX, TFX, SFX, CFX, and LFX power supplies can be used. The latter two power supply form factors were specifically created to support compact and low-profile BTX systems.

The primary advantages to BTX include

• Optimized inline component layout and routing. Signals are aligned front to back, allowing connections between components and I/O connectors to run unobstructed.

• Optimized airflow path . Allows for a condensed system design and an optimized, unobstructed airflow path for efficient system cooling with fewer fans and lower acoustics.

• Support and retention module (SRM). Offers mechanical support for heavy heatsinks. It also helps to prevent board flexing or damaging board components and traces during shipping and handling.

• Scalable board dimensions. Flexible board sizes enable developers to use the same components for a variety of system sizes and configurations.

• Low-profile options. Component keep-out specifications enable lower profiles, making it easier to design slim-line or small form factor systems.

• Flexible, compatible power supply designs. Connectors are shared with recent ATX designs; smaller, more efficient power supply form factors can be used for small form factor systems, whereas standard ATX12V power supplies can be used for larger tower configurations.

BTX includes three definitions of motherboard size, as shown in Table 4.6.

Table 4.6. BTX Motherboard Form Factors

Each board has the same basic screw hole and connector placement requirements. So, if you have a case that fits a full-size BTX board, you can also mount a microBTX or picoBTX board in that same case (see Figure 4.18). Obviously, if you have a smaller case designed for MicroBTX or picoBTX, you won't be able to put the larger microBTX or BTX boards in that case.

BTX requires up to 10 mounting holes and supports up to seven slots, depending on the size, as shown in Table 4.7.

Table 4.7. BTX Motherboard Mounting Holes

BTX also clearly specifies volumetric zones around the motherboard to prevent any interference from the chassis or internal components such as drives, which allows for maximum interchangeability without physical interference or fit problems.

With processors exceeding 100W in thermal output, as well as voltage regulators, motherboard chipsets, and video cards adding to the thermal load in a system, BTX was designed to allow all the high-heat-producing core components to be mounted inline from front to back, so that a single high-efficiency thermal module (heatsink) can cool the system. This eliminates the need for an excessive number of fans. The thermal module includes a heatsink for the processor, a high-efficiency fan, and a duct to direct airflow through the system. Extra support for the thermal module is provided under the board via a support and retention module (SRM), which provides structural support for heatsinks that are much heavier than allowed in ATX designs (see Figure 4.19).

BTX uses the same power connectors as in the latest power supply form factor specifications, including a 24-pin main connector for the board and a 4-pin ATX12V connector for the CPU voltage regulator module. The particular power supply form factor used depends mostly on the chassis selected.

A typical tower system has components arranged as shown in Figure 4.20.

From Figure 4.20, you can see that the main heat-producing core components are centrally located inline from front to rear, allowing the most efficient thermal design. Air flows from front to rear through the center, cooling the processor, motherboard chipset, memory, and video card.

To support the heavy processor heatsink and thermal module assembly, an SRM is mounted under the board. The SRM is essentially a metal plate affixed to the chassis under the board, and the thermal module is bolted directly to the SRM instead of to the motherboard. This helps carry the weight of the module and prevents excessive loads from being applied to the processor and motherboard, especially during the shipping and handling of the system.

The BTX I/O connector area is similar to ATX, except that it is at the opposite side of the rear of the board. The size of the area is slightly shorter but wider than ATX, allowing a large number of interfaces and connectors to be built in to the motherboard.

Although BTX offers improved thermal characteristics over ATX, industry analysts expect that ATX will continue to be the dominant motherboard and chassis design into 2007. Gateway is the first major vendor to offer systems based on BTX.

NLX

NLX is a low-profile form factor designed to replace the nonstandard LPX design used in previous low-profile systems. First introduced in November 1996 by Intel, NLX was a popular form factor in the late 1990s for Slimline corporate desktop systems from vendors such as Compaq, HP, Toshiba, and others. Since 2000, many Slimline systems have used variations on the FlexATX motherboard instead.

NLX is similar in initial appearance to LPX, but with numerous improvements designed to enable full integration of the latest technologies. NLX is basically an improved version of the proprietary LPX design, but, unlike LPX, NLX is fully standardized, which means you should be able to replace one NLX board with another from a different manufacturersomething that was not possible with LPX.

Another limitation of LPX boards is the difficulty in handling the larger physical size of the newer processors and their larger heatsinks, as well as newer bus structures such as AGP for video. The NLX form factor has been designed specifically to address these problems (see Figure 4.21). In fact, NLX provides enough room for some vendors to support dual Slot 1 Pentium III processors in this form factor.

The main characteristic of an NLX system is that the motherboard plugs into the riser, unlike LPX where the riser plugs into the motherboard. Therefore, the motherboard can be removed from the system without disturbing the riser or any of the expansion cards plugged into it. In addition, the motherboard in a typical NLX system literally has no internal cables or connectors attached to it! All devices that normally plug into the motherboardsuch as drive cables, the power supply, the front panel light, switch connectors, and so onplug into the riser instead (see Figure 4.21). By using the riser card as a connector concentration point, you can remove the lid on an NLX system and literally slide the motherboard out the left side of the system without unplugging a single cable or connector on the inside. This allows for unbelievably quick motherboard changes; in fact, I have swapped motherboards in less than 30 seconds on NLX systems!

As Figure 4.22 shows, by using different sizes and types of riser cards, a system designer can customize the features of a given NLX system.

Such a design is a boon for the corporate market, where ease and swiftness of servicing is a major feature. Not only can components be replaced with lightning speed, but because of the industry-standard design, motherboards, power supplies, and other components can be interchanged even among different systems.

Specific advantages of the NLX form factor include

• Support for all desktop system processor technologies. This is especially important because, since the NLX form factor was developed, both AMD and Intel adopted and then abandoned bulkier slot-based processors and returned to more compact socketed processors. NLX can handle both types of processors.

• Flexibility in the face of rapidly changing processor technologies. Backplane-like flexibility has been built in to the form by allowing a new motherboard to be easily and quickly installed without tearing your entire system to pieces. But unlike traditional backplane systems, many industry leaders , such as Compaq, Toshiba, and HP, have sold NLX-based systems.

• Support for newer technologies. This includes Accelerated Graphics Port (AGP) high-performance graphic solutions, Universal Serial Bus (USB), and memory modules in DIMM or RIMM form.

• Ease and speed of servicing and repair. Compared to other industry-standard interchangeable form factors, NLX systems are by far the easiest to work on and allow component swaps or other servicing in the shortest amount of time.

Furthermore, with the importance of multimedia applications, connectivity support for such things as video playback, enhanced graphics, and extended audio has been built in to the motherboard. This should represent a good cost savings over expensive daughterboard arrangements that have been necessary for many advanced multimedia uses in the past. Although ATX also has this support, LPX and Baby-AT don't have the room for these additional connectors.

Figure 4.23 shows the basic NLX system layout. Notice that, similar to ATX, the motherboard is clear of the drive bays and other chassis-mounted components. Also, the motherboard and I/O cards (which, like the LPX form factor, are mounted parallel to the motherboard) can easily be slid in to and out of the side of the chassis, leaving the riser card and other cards in place. The processor can be easily accessed and enjoys greater cooling than in a more closed-in layout.

Note the position of the optional AGP slot shown in Figure 4.23. It is mounted on the motherboard itself, not on the riser card as with PCI or ISA slots. This location was necessary because AGP was developed well after the NLX form factor was introduced. Most NLX motherboards use chipset-integrated or motherboard-based video instead of a separate AGP card, but you must remove an AGP card installed in an NLX system before you can remove the motherboard for servicing. Also, the AGP card used in an NLX system must have a different form factor to enable it to clear the rear connector shield at the back of the NLX motherboard (see Figure 4.24).

The NLX motherboard is specified in three lengths, front to back: 13.6", 11.2", or 10" total (see Figure 4.25). With proper bracketry, the shorter boards can go into a case designed for a longer board.

As with most of the form factors, you can identify NLX via the unique I/O shield or connector area at the back of the board (see Figure 4.26). You only need a quick look at the rear of any given system to determine which type of board is contained within. Figure 4.26 shows the unique stepped design of the NLX I/O connector area. This allows for a row of connectors all along the bottom and has room for double-stacked connectors on one side.

As you can see, the NLX form factor has been designed for maximum flexibility and space efficiency. Even extremely long I/O cards will fit easily without getting in the way of other system componentsa problem with Baby-AT form factor systems.

Although NLX is a standard form factorjust as the ATX family ismost NLX products have been sold as part of complete systems aimed at the corporate market. Very few aftermarket motherboards have been developed in this form factor. Thus, although NLX makes swapping motherboards easy, you are more likely to encounter it in a corporate environment than in a home or small-business computer. The microATX and FlexATX form factors have largely superseded NLX in the markets formerly dominated by LPX. Overall, one of the ATX variants is still the best choice for most new systems where expandability, upgradeability, low cost, and ease of service are of prime importance. In the future, picoBTX is likely to eventually replace both the NLX and FlexATX form factors.

WTX

WTX was a board and system form factor developed for the mid-range workstation market; however, most vendors making workstations and servers have used the ATX form factor. WTX went beyond ATX and defined the size and shape of the board and the interface between the board and chassis, as well as required chassis features.

WTX was first released in September 1998 (1.0) and updated in February 1999 (1.1). The specification and other information on WTX used to be available at www.wtx.org; however, WTX has been officially discontinued and there will be no further updates.

Figure 4.27 shows a typical WTX system with the cover removed. Note that easy access is provided to internal components via pull-out drawers and swinging side panels.

WTX motherboards have a maximum width of 14" (356mm) and a maximum length of 16.75" (425 mm), which is significantly larger than ATX. There are no minimum dimensions, so board designers are free to design smaller boards as long as they meet the mounting criteria. The additional space provided by the WTX form factor provides room for two or more processors and other onboard equipment needed in a workstation or server design. Although WTX is no longer an official form factor, a number of server and workstation motherboard vendors, such as Tyan, MSI, and SuperMicro, continue to build products that use it.

The WTX specification offers flexibility by leaving motherboard mounting features and locations undefined. Instead of defining exact screw hole positions, WTX motherboards must mount to a standard mounting adapter plate, which must be supplied with the board. The WTX chassis is designed to accept the mounting plate with attached motherboard and not just a bare board alone.

WTX motherboards use different power connectors than ATX motherboards. Originally, WTX motherboards used a 24-pin power connector that supplied only 5V and 3.3V power to the motherboard and a separate 22-pin power connector that supplied 12V power. Modern WTX motherboards still use a 24-pin primary power connector, but the connector might use the EPS12V (also known as the Superset ATX or SSI) standard or the older ATX-GES standard. Both ATX-GES and EPS12V provide 3.3V, 5V, and 12V power to the motherboard, but the pinouts are completely different. EPS12V motherboards also use an 8-pin power connector to provide additional 12V power to the processor(s). Table 4.8 compares the pinouts of the ATX-GES and EPS12V 24-pin primary power connectors.

Table 4.8. ATX-GES and EPS12V 24-Pin Primary Power Connector Pinouts

Caution

Keep in mind that motherboards using the WTX, ATX-GES, and EPS12V power supply standards all use the same connector (the 24-pin Molex 39-01-2240 connector, a longer version of the 20-pin Molex connector used by ATX power supplies). However, they use different voltages on almost every wire. If you mismatch the motherboard and power supply, you will destroy one or both of them!

Proprietary Designs

Motherboards that are not one of the industry standard form factors, such as AT/Baby-AT, NLX, or any of the ATX formats, are deemed proprietary or semiproprietary . LPX, Mini-ITX, and Nano-ITX systems fall into the semiproprietary category for example, while other companies have fully proprietary systems that only they manufacture. Most people purchasing PCs should avoid proprietary designs because they do not allow for a future motherboard, power supply, or case upgrade, which limits future use and serviceability of the system. To me, proprietary systems are disposable PCs because you can neither upgrade them nor easily repair them. The problem is that the proprietary parts can come only from the original system manufacturer, and they usually cost much more than nonproprietary parts. Therefore, after your proprietary system goes out of warranty, not only is it not upgradeable, but it is also essentially no longer worth repairing. If the motherboard or any component on it goes bad, you will be better off purchasing a completely new standard system than paying five times the normal price for a new proprietary motherboard. In addition, a new motherboard in a standard form factor system would be one or more generations newer and faster than the one you would be replacing. In a proprietary system, the replacement board would not only cost way too much, but it would be the same as the one that failed.

Note that you might be able to perform limited upgrades to older systems with proprietary motherboards, in the form of custom (non-OEM) processor replacements with attached voltage regulators, usually called "overdrive" or "turbo" chips. Unfortunately, these often don't perform up to the standards of a less expensive new processor and motherboard combination. Of course, I usually recommend upgrading the motherboard and processor togetherbut that is something that can't be done with a proprietary system.

Until the late 1990s, the LPX motherboard design was at the heart of most proprietary systems. These systems were sold primarily in the retail store channel by Compaq, IBM's Aptiva line, HP's Vectra line, and Packard Bell (no longer in business in North America). As such, virtually all their systems have problems inherent with their proprietary designs.

If the motherboard in your current ATX form factor system dies, you can find any number of replacement boards that will bolt directly inwith your choice of processors and clock speedsat great prices. However, if the motherboard dies in a proprietary form factor system, you'll pay for a replacement available only from the original manufacturer, and you have little or no opportunity to select a board with a faster or better processor than the one that failed. In other words, upgrading or repairing one of these systems via a motherboard replacement is difficult and usually not cost-effective .

Systems sold by the leading mail-order suppliers, such as Gateway, Micron, and others, are available in industry-standard form factors such as ATX, microATX, FlexATX, NLX, and even BTX in the case of Gateway. This allows for easy upgrading and system expansion in the future. These standard factors allow you to replace your own motherboards, power supplies, and other components easily and select components from any number of suppliers other than where you originally bought the system.

Remember that, even though Dell uses industry-standard form factors for some of its desktop systems, many of its systems are based on proprietary components, including power supplies with nonstandard pinouts. For this reason, Dell computers can be difficult to upgrade with aftermarket motherboard and power supply components. In many situations, if you want to replace a Dell motherboard, you must also replace the power supply to avoid damage to your new industry-standard motherboard. Learn more in Chapter 19.

Backplane Systems

One type of design that has been used in some systems over the years is the backplane system . These systems do not have a motherboard in the true sense of the word. In a backplane system, the components typically found on a motherboard are located instead on an expansion adapter card plugged into a slot.

In these systems, the board with the slots is called a backplane, rather than a motherboard. Systems using this type of construction are called backplane systems.

Backplane systems come in two main typespassive and active. A passive backplane means the main backplane board does not contain any circuitry at all except for the bus connectors and maybe some buffer and driver circuits. All the circuitry found on a conventional motherboard is contained on one or more expansion cards installed in slots on the backplane. Some backplane systems use a passive design that incorporates the entire system circuitry into a single mothercard. The mothercard is essentially a complete motherboard designed to plug into a slot in the passive backplane. The passive backplane/mothercard concept enables the entire system to be easily upgraded by changing one or more cards. Because of the expense of the high-function mothercard, this type of system design is rarely found in standard PC systems today, although it was once favored by a few early 286/386 vendors such as Zenith Data Systems. The passive backplane design does enjoy popularity in industrial systems, which are often rack-mounted. Some high-end fileservers also feature this design. Figure 4.28 shows a typical Pentium 4 single-board computer used in PICMG passive backplane systems. Figure 4.29 shows a rack-mount chassis with a passive backplane.

Passive backplane systems with mothercards (often called single-board computers or SBCs) are by far the most popular backplane design. They are used in industrial or laboratory-type systems and are rack-mountable. They usually have a large number of slots and extremely heavy-duty power supplies; they also feature high-capacity, reverse flow cooling designed to pressurize the chassis with cool, filtered air. Many passive backplane systems, such as the one pictured in Figure 4.28, adhere to the PCI/ISA passive backplane and CompactPCI form factor standards set forth by the PCI Industrial Computer Manufacturers Group (PICMG). You can get more information about these standards from PICMG's website at www.picmg.org.

Note

Another popular standard for SBCs is the PISA standard developed by JUMPtec and Kontron. The PISA standard uses a half-length SBC that plugs in to a backplane slot similar to the old EISA slot. PISA backplanes also support PCI and ISA cards. Learn more at www.kontron-em.com/?lang=e.

An active backplane means the main backplane board contains bus control and usually other circuitry as well. Most active backplane systems contain all the circuitry found on a typical motherboard except for what is then called the processor complex . The processor complex is the name of the circuit board that contains the main system processor and any other circuitry directly related to it, such as clock control, cache, and so forth. The processor's complex design enables the user to easily upgrade the system later to a new processor type by changing one card. In effect, it amounts to a modular motherboard with a replaceable processor section.

Many large PC manufacturers have built systems with an active backplane/processor complex. Both IBM and Compaq, for example, have used this type of design in some of their high-end (server class) systems. ALR (now owned by Gateway) once made a series of desktop and server PCs that also featured this design. This allows an easier and generally more affordable upgrade than the passive backplane/mothercard design because the processor complex board is usually much cheaper than a mothercard. Unfortunately, because no standards exist for the processor complex interface to the system, these boards are proprietary and can be purchased only from the system manufacturer. This limited market and availability causes the prices of these boards to be higher than most complete motherboards from other manufacturers.

The motherboard system design and the backplane system design have advantages and disadvantages. Most original PCs were designed as backplanes in the late 1970s. Apple and IBM shifted the market to the now traditional motherboard with a slot-type design because this kind of system generally is cheaper to mass-produce than one with the backplane design. In the late 1980s, Zenith Data manufactured a line of backplane-based 8088, 286, and 386-based systems but later abandoned this for a standard motherboard design similar to other vendors. The theoretical advantage of a backplane system, however, is that you can easily upgrade it to a new processor and level of performance by changing a single card. For example, you can upgrade a system's processor just by changing the card. In a motherboard-design system, you often must change the motherboard, a seemingly more formidable task. Unfortunately, the reality of the situation is that a backplane design is frequently much more expensive to upgrade. For example, because the bus remains fixed on the backplane, the backplane design precludes more comprehensive upgrades that involve adding local bus slots.

Another nail in the coffin of backplane designs is the upgradeable processor. Starting with the 486, Intel and AMD began standardizing the sockets or slots in which processors were to be installed, allowing a single motherboard to support a wider variety of processors and system speeds. Because board designs could be made more flexible, changing only the processor chip for a faster standard OEM type (not one of the kludgy "overdrive" chips) is the easiest and most cost-effective way to upgrade without changing the entire motherboard.

Because of the limited availability of the processor-complex boards or mothercards, they usually end up being more expensive than a complete new motherboard that uses an industry-standard form factor. The bottom line is that unless you have a requirement for a large-capacity industrial or laboratory-type system, especially one that would be rack-mounted, you are better off sticking with standard ATX form factor PCs. They will certainly be far less expensive.

Note

Some companies offer plug-in processor cards that essentially turn your existing motherboard into an active backplane, shutting down the main CPU and memory and having the card's processor and memory essentially take over. These are, unfortunately, much more expensive than a new motherboard and processor alone, use the more expensive SO-DIMM memory, don't provide AGP video, and are generally not recommended.