PC Repair Bench Book

John Wiley & Sons

Chapter One

IN THIS CHAPTER

The motherboard, also known as the system board, main board, or planar board, is a large printed circuit board that includes or provides an interconnect to most of the essential components of the PC:

* Microprocessor (see Chapter 2)

* Expansion bus (see Chapter 2)

* Chipset (see Chapter 3)

* Memory sockets and RAM modules (see Chapter 6)

* Cache memory (see Chapter 7)

* Integrated Drive Electronics (IDE), Enhanced IDE (EIDE), or Small Computer System Interface (SCSI) controllers (see Chapter 11)

* Mouse and keyboard connectors (see Part VI)

* Parallel and serial ports (see Parts V and VI)

As this list shows, there is more to working with a motherboard than I can cover in just this one chapter. Motherboards are the glue that binds the PC's components together. I can safely say that virtually every component, internal or peripheral, that's installed on or connected to a PC has some connection (no pun intended!) to the motherboard.

Motherboard manufacturers attempt to differentiate their products and increase their value by integrating a varying combination of devices and controllers into their boards. The upside of including more on the motherboard is a wider compatibility to a wider range of systems and potentially a deeper list of features. The downside is that unless you're very careful when selecting a new motherboard, you might not get the combination or quality of processor or peripheral support that you intended.

Although I assume that you have some background in working with PCs and their components, I want to be sure that you and I are on the same page when it comes to motherboards. In the following sections, I cover what is likely some fundamental material. However, when it comes to motherboards, I'd rather be safe than sorry.

Differentiating Motherboard Designs

If PCs had only a single type and style of motherboards, the task of working with them would be greatly simplified. However, even though most of today's PCs use the ATX (see "Creating the new standard: The ATX" later in this chapter), you can expect to encounter different motherboard form factors on the job. If, after all else has failed, you decide to replace a PC's motherboard, you must match the form factor of the motherboard to the case and its mountings.

Laying out the mainboard

Essentially, the two basic design approaches to PC motherboards are the mainboard (or the true mother-of-all-boards) design and the backplane design.

A mainboard design, like the one in Figure 1-1, incorporates the PC's primary system components on a single circuit board. This type of motherboard contains most of the circuitry of a PC and acts as the conduit through which all the PC's operations flow.

On a typical motherboard (see Figure 1-1), you will find the microprocessor, the Basic Input/Output System (BIOS) ROM, the chipset, RAM, expansion cards, perhaps some serial and parallel ports, disk controllers, connectors for the mouse and the keyboard, and possibly a few other components as well.

TIP

Mainboard motherboard designs, although somewhat standard, do vary in the inclusion and placement of system components and interfaces. Before you charge down the road to diagnose, troubleshoot, or replace any motherboard, be very sure that you can at least identify the components indicated in Figure 1-1 on your PC's mainboard.

Connecting to the backplane

There are actually two types of backplane mainboards: passive and active. A passive backplane mainboard is only a receiver card with open slots into which a processor card (which contains a central processing unit [CPU] and its support chips) and input/output (I/O) cards that provide bus and device interfaces are plugged. These add-in cards are referred to as daughterboards.

The backplane interconnects the system components through a bus structure and provides some basic data buffering services. The backplane design is popular with server-type computers because it can be quickly upgraded or repaired. The backplane design provides the advantage of getting a server back online with only the replacement of a single slotted card, instead of replacing an entire mainboard!

An active backplane design, also called an intelligent backplane, adds some CPU or controller-driven circuitry to the backplane board, which can speed up the processing speed of the system. Even on an active backplane, the CPU is on its own card to provide for easy replacement.

The utility of the backplane design is being challenged by newer motherboards that incorporate the slot-style mountings of Pentium-class processors. The advantage of the active backplane is that the processor can be easily accessed and replaced, but the slot-style motherboards also offer this same advantage.

NOTE

For purposes of clarity and because they are the most commonly used in PCs, when I refer to a motherboard, I am referring to the mainboard design. When referring to a backplane design, I will specifically say so.

Factoring in the motherboard form

When the original IBM PC was introduced in 1981, it had a simple motherboard designed to hold an 8-bit processor (the Intel 8088), five expansion cards, a keyboard connector, 64-256K RAM (from individual memory chips mounted on the motherboard), a chipset, BIOS ROM, and a cassette tape I/O adapter for permanent storage. The PC was designed to be a desktop computer, and its system case layout dictated the first of what are now called motherboard form factors. Simply, a form factor defines a motherboard's size, shape, and how it is mounted to the case. However, form factors have been extended over time to include the system case, the placement and size of the power supply, the power requirements of the system, external connector placements and specifications, and case airflow and cooling guidelines.

Table 1-1 lists the common form factors that have been and are being used in PCs.

SETTING THE STANDARD: THE IBM AT

When IBM released its first 16-bit computer, the PC AT, the circuitry added to the motherboard of its predecessor (the PC XT) increased the size of its motherboard and case to 12 inches wide by 13 inches deep. During this time, many clone (non-IBM) manufacturers also began releasing XT-compatible motherboards, which included keyboard connectors, expansion slots, and mounting holes to fit into AT cases. The AT's size, shape, and mounting placements became the first motherboard form factor standard, a standard that has essentially continued through today. Nearly all present-day motherboard form factors are a derivative of the early AT standard.

BRINGING UP THE BABY AT

It wasn't long before clone manufacturers began releasing their own 16-bit PCs and motherboards with higher integration in the supporting chipsets that allowed their motherboard to take a smaller form. This smaller form was called the Baby AT, shown in Figure 1-2, a more compact motherboard that was compatible with AT cases. The Baby AT became very popular because of its size and flexibility and joined the AT motherboard as a de facto standard.

TAKING THE STANDARD ONE STEP SMALLER

Most of the PC cases manufactured between 1984 and 1996 were made to house a Baby AT motherboard. However, with still higher integration and further miniaturization of the processor, chipset, and other support components, it became possible to produce an even smaller version of the AT form factor. The Micro-AT motherboard (see Figure 1-3), which is nearly half the size of the Baby AT mainboard, is also compatible with the motherboard mountings in AT and Baby AT cases.

WORKING WITH A LOW PROFILE: LPX AND MINI-LPX

Originally created by Western Digital to provide slimline cases to the consumer market, the LPX and Mini-LPX form factors have produced many variations. Actually, the LPX and Mini-LPX specifications are more of a general motherboard category than a specific form factor with a standard specification, like that of the AT and its derivatives. Manufacturers such as Packard Bell and Compaq used their own proprietary configurations for LPX motherboards in their PCs. Unfortunately, this practice guarantees that their customers cannot typically upgrade their computers without swapping the motherboard.

TIP

One quick note on the meaning of form factor names: There aren't any. If the form factor names ever had meanings, they are lost to time.

The LPX style is characterized by a riser card that has plugs into a slot in the middle of the motherboard. LPX riser cards typically have two or three expansion slot sockets on them, but the number of sockets available depends on the size of the riser card and whether it has expansion slots on both sides. The motherboard is mounted flat in the LPX case, and the riser card is inserted perpendicularly. This arrangement allows the expansion cards mounted in the riser card to be placed parallel to the motherboard, which allows for a much slimmer case design.

CREATING THE NEW STANDARD: THE ATX

In 1995, Intel released its "next best thing" with the ATX form factor. The ATX is an improvement over preceding form factors because of its published and continuously maintained standard, which guarantees compatibility among all ATX motherboards and cases.

The ATX form factor, shown in Figure 1-4, is based on the Baby AT but is rotated 90 degrees and incorporates unique mounting locations and power supply connections. Unlike many of the previous motherboard form factors, ATX locates its I/O connections so that they're accessible through the back of an ATX case.

The ATX form factor specification incorporates solutions to the performance issues associated with Baby AT and LPX forms. ATX places the CPU and RAM slots out of the way of expansion cards and near the power supply fan, which improves the airflow over the CPU and RAM chips.

The ATX specification also defines the Mini-ATX sub-specification, which has a board size of 11.2 inches by 8.2 inches. Other sub-specifications of the ATX form factor that you might encounter are the Micro-ATX and the Flex-ATX.

SLIMMING DOWN WITH NLX

NLX is a newer format and standardized low-profile motherboard form factor. It is designed to support a number of current and emerging microprocessor technologies along with many newer developments, including support for Accelerated Graphics Port (AGP) video adapters and tall memory modules (such as dual inline memory modules, or DIMMs). The NLX form provides more flexibility for the system-level design and for easy removal and replacement of the motherboard, allegedly without tools. The NLX motherboard measures about 8 inches by 13.6 inches and uses a plug-in riser board for its expansion bus support. The riser board attaches to the edge of the mainboard, as shown in Figure 1-5.

Three primary influences were behind the development of the NLX standard: processor and system cooling requirements, the number of connectors needed by multimedia hardware, and a further reduction of interior cable clutter. The size and thermal characteristics of newer microprocessors, especially those configured into multiple processor sets, along with the addition of high-performance (and high-heat) graphics adaptors, forced a new look at the airflow in slimline cases. As multimedia systems became more common, the need for more connectors from the motherboard to the outside world also increased. As more internal adapters and controllers were added to the motherboard, the interior of the system case was cluttered with cabling, which impeded repair or upgrade activities.

Working with the Motherboard

In the vast majority of situations, the problem that you're trying to track down on a PC is not likely to be specifically caused by the motherboard itself. Actually, if the problem is a bad motherboard (not a common event), your only course of action is to replace it. However, sometimes maybe - just maybe - you can check out the motherboard and isolate the problem.

TIP

If you do remove an allegedly bad motherboard, you really should test it in a test bed PC before throwing it out. It could actually still be good. And even if a new motherboard fixed its PC's problems, the solution might be more coincidental than anything else.

Using the right tools

The following is a list of the tools that you should have in your toolkit for removing or installing a motherboard:

* Dental mirror: A dental mirror-like tool can be purchased from most tool suppliers, so you don't have to beg your dentist for one. A dental mirror is perfect for seeing around corners in an assembled system, like when you need to see a detail being blocked by a disk drive cage. It can also come in handy when you're trying to attach a connector or a power cord to the back of a PC.

* Digital multimeter: If the motherboard is running strangely, some of the first places to look are its power connections. A multimeter or a digital voltmeter is a good tool to have for testing the continuity of power cables and the power supply's output.

* Electrostatic discharge (ESD) mat and wrist (or ankle) strap: If you don't have access to an ESD mat on which you can set any static-sensitive parts that you remove (such as expansion cards or a motherboard), by all means wear an ESD wrist or ankle strap and have plenty of anti-static bags available. Even with an ESD strap in use, never stack unprotected cards or parts on top of one another and always ground yourself to the system case's metal as often as possible.

* Penlight or mini flex-type flashlight: Having some light to help you see small identifying marks on the motherboard, its chips, and expansion cards can prevent a serious error and save the time removing and reinserting the wrong parts. You might want to consider spare batteries as well.

* Screwdrivers: Your toolkit should include a collection of screwdrivers that has at least one of each of the following screwdrivers: a standard (slot), a mini-head Phillips (cross-head recess), a standard-size Phillips (magnetic tip optional), and a Torx. Magnetic screwdrivers can be potentially dangerous if used incorrectly, such as gouging the motherboard or blowing an integrated circuit (IC) chip. However, they can come in handy for retrieving a dropped screw or for starting a screw in an inaccessible place.

* Software system testing utilities: As long as you are able to boot into some operating system, a set of diagnostic utilities (like Norton Utilities) can be among the best tools in your kit. Use these software aids to diagnose a number of suspected motherboard or system performance problems, such as system slow-downs and inexplicable crashes.

* Your eyes, ears, and nose: Your senses are among your best tools. As corny as that might sound, your senses are probably the tools most often used when you first begin your troubleshooting.

(Continues...)

Excerpted from PC Repair Bench Bookby Ron Gilster Copyright © 2003 by Ron Gilster. Excerpted by permission.
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