In order to upgrade or troubleshoot a PC effectively, a technician must be familiar with the general mechanical and physical aspects of the PC. They must be able to disassemble the unit quickly (without causing damage to the case or internal assemblies in the process), then accurately identify each sub-assembly, expansion board, and connector. Once a diagnosis and repair has been completed, the technician must be able to re-assemble the PC and its enclosures (again without damaging assemblies or enclosures). This chapter is designed to provide you with a guided tour of the typical desktop and tower PC (Fig. 2-1), point out the various operating sub-assemblies, and offer a series of assembly guidelines.

The first step will be to take a look at the generic components you’ll expect to find in a desktop or tower system. Figure 2-2 illustrates an exposed view of a desktop PC. Although it may look crowded at first glance, you will see that there are actually only a handful of sub-assemblies to deal with. With a little practice, identifying various assemblies should become almost automatic. An average tower system is illustrated in Fig. 2-3. With few exceptions, desktop and tower PCs incorporate seven key items; the enclosure, the power supply, the motherboard, a floppy disk drive, a hard disk drive, a video adapter, and a drive controller. The following sections detail each item. Feel free to skip directly to related chapters later in the book for more detailed definitions and discussion.

The enclosure is the most obvious and least glamorous element of a PC. Yet, the enclosure serves some very important functions. First, the enclosure (such as the Olson Baby AT case of Fig. 2-4) forms the mechanical foundation (or chassis) of every PC. Every other sub-assembly is bolted securely to this chassis. Second, the chassis is electrically grounded through the power supply. Grounding prevents the buildup or discharge of static electricity from damaging other sub-assemblies. Whenever you work inside of a PC, be sure to use a properly-grounded anti-static wrist strap to prevent electrostatic discharge from your body from accidentally damaging circuitry inside the system. If you do not have an anti-static wrist strap handy, you can discharge yourself on the PC's metal chassis as long as the power supply is plugged in. However, since you are strongly urged to protect yourself by unplugging the power supply AC, do not rely on the chassis to discharge you. Grounding also prevents a serious shock or fire hazard if AC should come in contact with the metal case.

The enclosure also limits the PC's expansion capacity. Average-sized desktop enclosures typically offer room for motherboards with 6 to 8 expansion slots, and provide space for 3 or 4 drives - two drives mounted in front slots (or external drive bays), and one or two drives mounted inside the PC (in internal drive bays). An average-sized enclosure such as this allows a fair amount of space to expand the system as your customer's needs change. Unfortunately, the push toward smaller PCs has led to the use of smaller, more confined enclosures. Small (or low-profile) enclosures (such as the Olson Slimline Chassis in Fig. 2-5) restrict the size of the motherboard which results in fewer expansion slots (usually 4 to 6), and allows room for only 1 to 3 drives.

The great advantage to tower enclosures is their larger physical size. Towers usually offer 4 or 5 external drive bays, as well as 3 or 4 internal bays (Fig. 2-6). To accommodate such expandability, a large power supply (250 to 300 watts) is often included. Tower cases can also fit larger motherboards which tend to support a greater number of expansion slots. The higher power demands of a tower system result in greater heat generation. Towers compensate for heat by providing one or more internal fans to force air into the enclosure. If a second internal fan is included, it generally works in conjunction with the first fan to exhaust heated air. For example, you’ll often find tower systems with two fans - one in the lower front to force in cooler air, and one in the upper rear to exhaust heated air. If only one fan is used, it will usually be located in the upper rear of the chassis to exhaust heated air.

The power supply is the silver box that is usually located in the rear right quarter of the enclosure (Fig. 2-7). AC enters the supply through the AC line cord connected at the rear of the enclosure. A supply then produces a series of DC outputs which power the motherboard and drives. The importance of a power supply is easy enough to understand, but its implications for system integrity and expandability may not be as obvious.

Power supplies sustain a great deal of electrical stress in normal everyday operation. The conversion of AC into DC results in substantial heat, which is why so many power supplies are equipped with a cooling fan. Surges, spikes, and other anomalies which plague AC power distribution (especially in underdeveloped regions of the world) also find their way into PC power supplies where damage can occur. The quality of a power supply's design and components and design dictate how long it will last in operation. A quality supply will resist power problems and tolerate the rigors of normal operation, but a sub-standard supply can fail spontaneously after only a few months of operation. When replacing or upgrading a power supply, be sure to choose a reliable model.

Power supplies also limit a system's expandability. Every element used in the PC requires a certain amount of power (marked "W" or watts). The supply must be capable of producing enough power to adequately meet the system's demand. An under-powered supply (typical in low-profile systems) or a supply overloaded by excessive expansion (which frequently occurs in tower systems) may not be able to support the power needs of the system. Inadequate power results in very strange system behavior such as unpredictable system lockups, random memory faults, or disk access problems. When replacing a power supply, be certain that the new supply can provide at least as much power as the supply being replaced. When upgrading a supply, choose a supply that offers at least 50 watts more than the original supply.

NOTE: Power supply assemblies are generally regarded as extremely safe because it is virtually impossible to come into contact with exposed high-energy circuitry. Still, exercise care and common sense whenever working with a running power supply.

The motherboard (also known as the main board, system board, backplane board, or planar board) holds the majority of a computer's processing power. As a minimum, a motherboard contains the system CPU, math co-processor (now routinely built into the CPU), clock/timing circuits, RAM, cache, BIOS ROM, serial port(s), parallel port, and expansion slots. Each portion of the motherboard is tied together with interconnecting logic circuitry. Some advanced motherboards also include circuitry to handle drive and video interfaces. You can identify the motherboard easily as shown in Fig. 2-8 - it is the single large printed circuit board located just off of the enclosure's base.

As you might expect, it is the motherboard more than any other element of the PC which defines the performance (and performance limitations) of any given computer system. This is the reason why motherboard upgrades are so popular, and often provide such stunning improvements to a PC. Let’s break motherboard limitations down into the following nine categories:

CPU type - a CPU is responsible for processing each instruction and virtually all of the data needed by the computer (whether the instruction is for BIOS, the operating system, or an application). The type of CPU limits the PC's overall processing power. For example, a PC with a Pentium Pro CPU runs Windows much better than a PC with an i486 CPU. Also, a Pentium MMX CPU will generally handle graphics-intensive applications better than a "classic" Pentium CPU.

CPU speed - even when CPUs are the same, clock speed (measured in MHz) effects performance. For example, a PC with a "classic" Pentium 166MHz CPU will run faster than a PC with a "classic" Pentium 120MHz CPU.

CPU upgrade potential - since CPUs have a finite processing limit, it follows that upgrading the CPU will improve system processing. While this is great in theory, you can’t just place any old CPU in the CPU socket and expect the motherboard to work. Any motherboard is limited to using a handful of current CPU versions. For example, Intel’s recent AN430TX motherboard supports Pentium processors at 90, 100, 120, 133, 150, 166 and 200MHz, as well as Pentium MMX processors running at 166, 200 and 233MHz. By comparison, Intel’s new NX440LX motherboard supports Pentium II microprocessors operating at 233, 266, and 300MHz. Changing the processor type and speed requires changes in several jumper settings.

Memory slots - the sheer amount of memory which can be added to the motherboard will indirectly effect system performance because of a reduced dependence on virtual memory (a swap file on the hard drive). Memory is added in the form of SIMMs (Single In-line Memory Modules) or DIMMs (Dual In-line Memory Modules). Motherboards which can accept more or larger-capacity memory modules will support more memory. It is not uncommon today to find motherboards which will support 512MB of RAM (equal to the storage capacity of older hard drives).

Memory types - the type of memory will also have an effect on motherboard (and system) performance. Faster memory will improve system performance. DRAM remains the slowest type of PC memory, and is usually employed in older systems or video boards. EDO RAM is faster than ordinary DRAM, and is now commonplace in PCs. SDRAM is measurably faster then EDO RAM, and is appearing in high-to-mid-range PC applications. By the time you read this book, SDRAM should be common. RDRAM is an emerging memory type which should gain broad acceptance in the next few years. It is not necessary for you to understand what these memory types are yet - just understand that memory performance and system performance are related.

Cache memory - traditional RAM is much slower than a CPU - so slow that the CPU must insert pauses (or "wait states") in order for memory to catch up. Cache is a technique of improving memory performance by keeping a limited amount of frequently-used information in VERY fast cache RAM. If the needed information is found, the CPU reads the cache at full speed (and performance is improved because less time is wasted). By making the cache larger, it is possible to hold more "frequently-used" data. Older motherboards employed from 128KB to 256KB of cache. Current motherboards use 512KB to 1MB of cache RAM.

Chipsets - a chipset is a set of highly optimized, tightly inter-related ICs which, taken together, handle virtually all of the support functions for a motherboard. As new CPUs and hardware features are crammed into a PC, new chipsets must be developed to implement those functions. For example, the Intel 430HX chipset supports the Pentium CPU and EDO RAM. Their 430VX chipset supports use of the Pentium CPU, the Universal Serial Bus, and SDRAM. By comparison, the Intel 440LX chipset supports the Pentium II CPU, an Accelerated Graphics Port, SDRAM, and an Ultra DMA-33 drive interface.

System BIOS - the BIOS ROM contained on the motherboard also limits the system's capabilities - although such limits are not always drastic or obvious. BIOS is a set of small programs recorded onto ROM ICs that allow the operating system (such as MS-DOS or Windows) to interact with memory and the various drives and devices in the system. Although the BIOS versions produced today are generally quite uniform, older BIOS ICs may not support some of the new features we now expect from computers. For example, many systems using i286-based motherboards do not support the format process for 3.5" 1.44MB floppy disk drives directly as newer systems do, or your BIOS may not support new bootable CD-ROM drives (using the "El Torito" standard). Overcoming BIOS limitations is often a matter of upgrading the BIOS program, or upgrading the motherboard entirely.

Expansion slots - each motherboard offers a fixed number of expansion slots. The number of expansion slots limits the number of features and devices that can be added to the system. Internal modems, scanner boards, video boards, drive controller boards, sound boards, network cards, and SCSI controllers are only some of the devices competing for expansion space in your PC. The fewer slots that are available, the less a system can be expanded. The type of expansion slots also influences expandability and performance. Classical motherboard designs offer a mix of 8-bit XT and 16-bit ISA slots. More recent motherboards have added one or two slots to accommodate enhanced expansion technologies such as the VL bus for improved video boards, and a second VL slot may be available for an improved drive or network adapter). Today, motherboards typically incorporate two or three PCI slots for high-performance network, video, or drive controller boards (the remainder of available expansion slots will generally be 16-bit ISA slots).

The modern PC would be entirely useless without long-term, high-volume storage, as well as the ability to transfer files between PCs. Drives represent a variety of devices used for storing or retrieving relatively large amounts of information. Floppy disk drives (FDDs), hard disk drives (HDDs), and CD-ROM drives are the three most popular drive types for desktop and tower PCs, although Iomega Zip drives (resembling 3.5" floppy drives) and tape drives are occasionally found. CD recorders and DVD-ROM drives are now common in current PCs. Even PC Card "drives" are finding their way into desktop/tower systems. Figure 2-9 illustrates the standard profile for each drive.

Floppy disk drives (FDD) - floppy disks have gone through several incarnations (5.25" 360KB, 5.25" 1.2MB, 3.5" 720KB, and 3.5" 1.44MB) since they first appeared in the IBM PC. But in spite of its limited storage capacity, the floppy drive remains the traditional PC drive which is universally accepted in virtually every PC manufactured since 1982. Floppy disks use only one light to indicate drive activity.

Iomega "Zip" drives - combining magnetic and optical storage technologies, Iomega has developed a 3.5" drive capable of storing up to 100MB on a single "Zip" disk. Over the last few years, the Zip drive has proven to be an inexpensive and handy storage system - so much so that some PC manufacturers now include Zip drives as standard equipment in their new systems. At first glance, the Zip disk and drive could easily be mistaken for a floppy disk drive.

CD-ROM drives - originally developed for the music industry as a digital replacement for aging phonographs, the CD-ROM quickly found a place in modern PCs where a laser is used to read an optical disc. One optical disc can store up to 650MB of programs, data, or other media such as Kodak photos and digital audio. Although their storage capacity is now dated when compared to multi-gigabyte hard drives, CD-ROM drives are standard equipment in all modern PCs. CD-ROM drives use a "load/eject" button, a volume control (to adjust CD audio), and a single activity light.

Tape drives - tape drives offer a significant amount of storage capacity using relatively inexpensive tape media. However, tape devices are slow, hot, and noisy, so they have largely been relegated to occasional system backup chores. Tape drives use two lights; one as a power indicator, and another as a drive activity light.

Hard disk drives (HDD) - the hard drive is truly the icon of the personal computer. Magnetic storage technology has evolved at a staggering pace, and the slow 100-200MB hard drives of ten years ago have been replaced by lightning-fast 4 and 5GB hard drives. The hard drive has provided the PC industry with huge, fast, reliable, and quiet storage mechanisms for just pennies per megabyte. This upward spiral of hard drive capacity shows no signs of slowing. Hard drives are standard equipment on all PCs, and are the preferred boot device for quick loading of even the largest operating systems. The hard drive typically uses only one light to indicate drive activity.

CD-R drives - CDs have been around for years, but recording your own CD has been prohibited by proprietary (and hideously expensive) hardware and software. In the last few years, CD-R drives have plummeted in price. While hardly standard equipment, the combination of falling prices, improved reliability, more extensive PC resources (i.e. RAM, hard drive space, and faster CPUs) and good intuitive Windows 95 authoring software has made CD-R drives an attractive option for such tasks as software backups, file archiving, and software product prototyping. CD-R drives use a "load/eject" button, drive activity light, and volume control. They also sport a second activity light to show when the drive is writing.

DVD-ROM drives - the DVD drive represents the next step in the evolution of optical storage for the PC. DVD discs can offer up to 17GB of storage on a single disc the size of a CD (yet are backward-compatible with almost all existing CD-ROM standards). They are an ideal medium for the distribution of audio and video multimedia (when combined with a PCI MPEG-2 decoder board), as well as unimaginable volumes of data. The first generations of DVD drives, which appeared in mid-1997, only read DVD discs, but future iterations of DVD (known as DVD-R and DVD-RAM) will be able to record blank DVD discs. DVD-ROM drives use a "load/eject" button, a volume control (to adjust CD audio), and a single activity light.

PC Card drives - with the explosive growth of portable computers, the use of desktop PC Card (formerly referred to as PCMCIA cards) "drives" is increasing to support the easy transfer of files between laptop and desktop systems. At first glance, the PC Card drive appears much like a 3.5" floppy drive, though the card opening is thicker and narrower. The term "drive" is a bit of a misnomer here because the PC Card drive is entirely electronic - there are no moving parts except for the electrical card connector and a simple card ejection mechanism. PC Card drives are relatively rare, and are most often encountered on PC platforms used for data acquisition or post-processing from remote data-gathering PCs. A mechanical lever ejects the PC Card, and a single light is used to indicate drive activity.

Drives are typically located in the front right quarter of the desktop enclosure. Each drive is secured into an available drive bay within the enclosure. There are two types of drive bays that you should be familiar with; internal and external. The external drive bay allows a drive to be mounted facing the outside world. Floppy, CD-ROM, CD-R, DVD-ROM, PC Card, and tape drives rely on the availability of external drive bays. After all, what good is the drive if you can't insert or remove the media? On the other hand, hard disk drives use non-removable media. This means the drive can be mounted in an internal (or non-accessible) bay. A typical desktop PC offers two external and two internal bays. The external bays usually hold a 3.5" FDD and a CD-ROM. The internal drive bay(s) are typically reserved for one or two hard drives. Larger desktop cases may offer additional external bays. Tower cases can easily support a full range of external drives mounted along the upper front of the enclosure. A tower's internal drive bays can handle another three or four hard drives.

While many PCs today incorporate video, sound, and FDD/HDD controller circuitry directly on the motherboard, those circuits can often be disabled when expansion boards are used. In fact, many such "integrated" controllers are eventually disabled so that video and drive systems can be upgraded with more advanced expansion boards. In most cases, you should expect to find at least a video board plugged into an expansion slot. The video board will often be accompanied by an FDD/HDD controller board. Of course, there will probably be additional boards in the system as well. This part of the chapter is intended to help you identify each category of expansion board on-sight.

Video boards - video adapter circuits (whether implemented on the motherboard or on an expansion board) are designed to convert raw graphic data traveling over the system bus into pixel data that can be displayed by a monitor. Without the monitor attached, however, the video adapter can only be identified through its video port connector. Figure 2-10 compares the four major generations of video adapters; MDA, CGA, EGA, and VGA. Keep in mind that the illustrations shown in Fig. 2-10 are typical examples - some video board designs may not follow these layouts exactly. Chapter 49 describes video adapter standards and service in more detail.

The Monochrome Display Adapter (MDA) is the oldest video adapter board, and few are still in service. MDA boards are noted for their use of a 25-pin parallel port included with the 9-pin video connector. You might find MDA boards used in IBM PC/XTs or compatible systems. The Color Graphics Adapter (CGA) is roughly the same vintage as MDA, and is the first graphics adapter to introduce color to PC displays. A CGA board can often be identified by a round RCA-type feature connector located just above a 9-pin video connector. Like the MDA boards, CGA is long-since obsolete, and many of the older systems that used CGA boards have been scrapped, or have been upgraded to later video systems. The Enhanced Graphics Adapter (EGA) offers more colors and higher display resolution than CGA. You can identify an EGA board by its small bank of DIP switches located above two RCA-type feature connectors and a 9-pin video connector.

The Video Graphics Array (VGA) board marked a departure from previous video systems. VGA abandoned logic-level video signals (on/off signaling) in favor of analog video levels. Thus, primary colors could be "mixed" together to provide many more color combinations than ever before - up to 262144 possible colors for ordinary VGA. You can easily identify a VGA connector as a 15-pin high-density connector (15 pins stuffed into a 9-pin shell). SVGA (or Super VGA) extends the capabilities of VGA by adding more resolutions and color depths allowing as many as 16 million colors (known as "true color" mode) to be displayed at one time. Table 2-1 compares the common resolutions and color depths for a typical SVGA video board.

Drive adapters - the term "drive adapter" is usually applied to floppy drive controller, EIDE/IDE drive controller, and tape drive accelerator boards. As a rule, SCSI adapters are not classified as "drive adapters" because a SCSI adapter can handle other peripherals besides drives. Drive controllers are easily identified by tracing drive signal cables from the particular drive back to the supporting controller. Figure 2-11 illustrates one typical drive controller board which handles two hard drive ports, and one floppy drive port.

Note the signal "headers" which connect to the individual ribbon cables. The 34-pin header is marked "FDD" or "floppy", and is always connects to the floppy drive(s). The 40-pin headers are marked "EIDE" and "IDE" respectively, or "HDD1" and "HDD2", and are always connected to EIDE/IDE hard drives. The first hard drive port is often designed for EIDE drives, and the second hard drive port is designed for older IDE drives (though many new controllers support EIDE drives on both drive ports). Chapter 17 outlines drive adapters in more detail.

NOTE: If there is only ONE hard drive port on the controller (marked "HDD" or "hard"), chances are VERY good that the controller is an older IDE-only controller board. Do not attempt to use EIDE drives on IDE ports. The 40-pin interface will work (and you won’t damage the drive), but you cannot partition and format the full capacity of the drive without drive overlay software such as EZ-Drive.

You should also be able to identify "proprietary" drive adapters - most notably for early (non-IDE) CD-ROM drives. The earliest CD-ROM adapters used stand-alone controller boards, but these were quickly replaced by one or more proprietary controller ports built right into popular sound boards like the Creative Labs "Sound Blaster" series (Fig. 2-12). Note that there are three connectors; a 44-pin connector for a Mitsumi CD-ROM, a 36-pin connector for a Sony CD-ROM, and a 44-pin interface for a Creative-brand CD-ROM. You would then use a jumper to select the desired port, depending on which CD-ROM came packaged with the sound board.

The real trick comes with 40-pin CD-ROM interfaces - it’s impossible to tell by sight if the port is IDE or proprietary. Still, you can employ the following rule. Older sound boards with a 40-pin CD-ROM interface are almost always proprietary, and the port will often share space with other proprietary interfaces nearby. Newer sound boards with a 40-pin CD-ROM interface sitting by itself (with no other proprietary ports nearby) are almost always standard IDE.

NOTE: An IDE interface on a sound board is a "true" IDE port, and should support any other IDE drives (i.e. older IDE hard drives or IDE tape backups) without problems.

SCSI adapters - the Small-Computer System Interface (SCSI) offers impressive expandability by allowing SCSI-compatible devices (SCSI hard drives, SCSI CD-ROMs, SCSI tape drives, SCSI scanners, and so on) to all be connected together over the same daisy-chained cable. There are several ways to detect the presence of a SCSI adapter. First, you will see a screen message generated by the SCSI adapter BIOS when the PC initializes. You can also confirm the presence of a SCSI adapter by identifying the interconnecting cables. Internally, SCSI cables are 50-pin or 68-pin ribbon cables. Figure 2-13 illustrates a SCSI adapter with a 50-pin SCSI header. Since many SCSI adapters can handle both internal and external devices, the adapter will have an external 50-pin D-type connector available. If the SCSI adapter also includes a 34-pin header, the adapter is providing a standard floppy drive port. You may need to disable any such floppy port since there is probably a working floppy port elsewhere in the system (i.e. on the motherboard or drive controller board). Chapters 17 and 45 cover SCSI concepts and troubleshooting in great detail.

Ports and modems - PCs are rarely any use in a vacuum - they must be able to communicate with devices in the "outside world". PC communication is accomplished through the use of parallel or serial ports, and you should recognize such ports on sight. Traditionally, parallel ports allow the PC to drive printers, but with improvements in parallel port performance, new peripherals are available which can operate through a parallel port (i.e. parallel port tape drives, hard drives, and CD-ROM drives). Such devices are particularly handy when they must be moved between several machines. A parallel port is implemented as a 25-pin (female) connector (Fig. 2-14). While older PCs included parallel ports as part of the MDA video board (Fig. 2-10) or as a stand-alone expansion board, virtually all current PCs incorporate at least one parallel port directly on the motherboard. You’ll find much more about parallel ports in Chapter 37.

Given the tremendous appeal of inexpensive online resources such as AOL and the Internet, serial communication has evolved substantially over the last decade. As a result, you will likely find one or two serial ports located on the PC as shown in Fig. 2-14. Older PCs typically implement a single serial (or RS-232) port as a 25-pin D-type (male) connector. Do not confuse this with 25-pin D-type female connectors which are used for parallel ports! Since most serial communication can be accomplished with far fewer than 25 pins, most PC manufacturers now use a 9-pin D-type male connector instead of the 25-pin D-type male connector. Newer systems offer two 9-pin D-type male serial ports directly on the motherboard. When implemented on a stand-alone expansion board, you will often find a 9-pin D-type male serial port combined with a 25-pin D-type female parallel port. Chapter 46 covers serial ports in detail.

To communicate over a telephone line, serial signals must be translated into tones which can be carried within the frequency bandwidth of an ordinary voice telephone line. Returning signals must also be decoded into serial signals. The device which performs this PC-telephone line interface is called a modem. External modems are stand-alone devices which attach to an available serial port. Internal modems, however, are quite popular, and combine the circuitry for a serial port and modem on a single expansion board. You can usually identify an internal modem board by its two RJ11 (phone jack) connectors. Note that one jack is for the telephone line itself, while the second connector is a "feed-through" which can be connected to any standard telephone. Modems are covered in Chapter 32.

Sound boards - the acceptance of sound boards in everyday PCs has been simply staggering. What started as a novel means of moving beyond the limitations of PC speakers has quickly evolved into a low-cost, CD-quality stereo playback/recording system. Even business applications are embracing sound cards for presentations and simple speech recognition tasks. Sound cards are firmly established as an essential part of every PC used for educational, game, and multimedia applications. Fortunately, sound boards are relatively easy to recognize as shown in Fig. 2-15.

The giveaway here is the volume control knob. Sound cards are the only devices that currently require such manual adjustments. Three miniature jacks are also included. The Line Input jack allows pre-recorded sound (i.e. output from tape player, CD player, or synthesizer) to be digitized and recorded by the sound board. The Microphone Input supports recording from an ordinary 600W microphone. The Stereo Output is the main output for the board where digitized voice and music files are reproduced. An output can drive amplified speakers or an interim stereo amplifier deck. Keep in mind that your particular sound board may have slightly different features. You will also note that the sound board shown in Fig. 2-15 offers a 15-pin D-type female connector. This feature connector is designed to serve double-duty as either a joystick port or a MIDI interface. Chapter 47 covers sound board concepts and troubleshooting in detail.

MPEG decoder boards - although a DVD-ROM drive requires a "standard" interface (i.e. SCSI or EIDE) for normal programs and data, DVD video and audio do not use this data path. There are two reasons for this. First, the data required to reproduce real-time video and audio would bog-down even the fastest PC when transferred across a standard drive interface. Second, video and audio data are highly compressed using MPEG standards, so even of the PC wasn’t bogged down by the compressed data, the decompression process would load-down the system with processing overhead. In order to play DVD audio and video, DVD-ROM drives require a stand-alone, hardware-based MPEG-2 decoder board such as the one in Fig. 2-16. The MPEG-2 decoder board works independently of the drive controller system, video system, and sound system.

There are five major connections on the MPEG-2 decoder board; an Analog Input jack, an Analog Output jack, a Digital Output jack, a Monitor connector, and a Video Input connector. The Analog Input is rarely (if ever) used in normal operations, but it may be handy for mixing in an auxiliary audio signal to the decoder board. The Analog Output signal provides the master audio signal which is fed to the Line Input of your existing sound board. The advantage of using a Line Input is that you don’t need a volume control on the decoder board. Instead, you can set the Line Input volume through your sound board’s "mixer" applet. When you play a DVD video, any audio will continue to play through your sound board and speakers. The Digital Output is intended to drive an external Dolby Digital device, so you will probably not be using the Digital Output in most basic PC setups.

The MPEG-2 decoder board will now drive your VGA/SVGA monitor through the Monitor connector. This is important because the decoded video stream is converted to RGB information, and fed to the monitor directly - this avoids having to pass the video data across the PCI bus to your video card. The normal output from your video card is looped from your video board to the decoder card, so while the decoder board is idle, your normal video signal is just "passed through" the MPEG-2 board to the monitor.

Joystick adapters - the use of PC games and simulators often requires the use of an analog joystick. Joysticks are connected to one of two 15-pin D-type female connectors on the joystick adapter (also called the game port). Figure 2-17 illustrates the typical layout for a joystick adapter. Since two connectors are usually included, an adapter can support two analog joysticks. Another hallmark of a joystick adapter is its small size - typically an 8-bit (or half-slot) board. Chapter 24 outlines joystick and game port installation and troubleshooting.

All to often, the mechanics of PC repair - taking the system apart and putting it back together again - are overlooked or treated as an afterthought. As you saw in the first part of this chapter, PC assemblies are not terribly complicated, but a careless or rushed approach to the repair can do more harm than good. Lost parts and collateral damage to the system are certain ways to loose a customer (and perhaps open yourself to legal recourse). The following section outlines a set of considerations that can help ensure a speedy, top-quality repair effort.

It is a fact of modern computing that the data contained on a customer's hard drive(s) is usually more valuable than the PC hardware itself. If your customer is an entrepreneur or corporate client, you can expect that the system contains valuable accounting, technical, reference, design, or operations information that is vital to their business. As a consequence, you should make it a priority to protect yourself from any potential liability issues connected with your customer's data. Even if the drives are causing the problem, a customer may hold you responsible if you are unable to restore or recover their precious information. Start a consistent regimen of written and oral precautions. Such precautions should include (but are not limited to):

• Always advise your customers to backup their systems regularly. Before the customer brings in their system, advise them to perform a complete backup of their drives if possible.
• Always advise your customers to check their backups - a backup is useless if it can’t be restored.
• When a customer delivers a system for repair, be sure that they sign a work order. Work orders should expressly give you authority and permission to work on the customer's system, outline such things as your hourly rate, labor minimums for evaluation and service, and show all applicable disclaimers. Your work order should include a strong disclaimer expressly relieving you of any and all liability for the contents of any magnetic media (i.e. hard drives) in the system. If you attempt data recovery, the disclaimer should also disclaim any warranty or guarantee of results - that way, you're not liable if you are unable to recover vital files. Since liability issues vary from state to state and country to country, a local attorney can advise you on specific wording.

Most desktop and tower systems use a metal chassis covered by a painted metal cover or shroud that is secured with a series of screws. There are often nine screws - two on either side of the enclosure, and five at the rear of the chassis. While this pattern covers many of the desktop PCs in service, you are likely to encounter a number of variations. You may find that instead of bolting screws in from the sides, the screws may be bolted in from the bottom. There may also be more or fewer screws in the rear of the chassis. Only on very rare occasions will you find screws used to secure the enclosure at its front - the molded plastic housing found on most desktop PCs does not accommodate screws without spoiling the finished "look".

Tower cases are a bit different. The metal shroud also uses about nine screws - all secured from the rear. The bottom and front edges of the enclosure are typically bent inward to "interlock" with the chassis when seated properly. This approach allows the entire enclosure to fit securely along the whole chassis while using only a minimum of screws. Enclosures that do not interlock, however, may require screws along the bottom and front edges. As a general rule, PC enclosure manufacturers tend to minimize the use of visible screws in order to enhance a "seamless" appearance - this is why most screws are relegated to the back chassis.

There are three factors to keep in mind when removing screws and other mounting hardware. First, be extremely careful not to mark or gouge the painted metal enclosure. Customers are rightfully possessive of their PC investment, and putting a scratch or dent in an enclosure is tantamount to dinging their new car (a careless reputation is very bad for business). Be equally careful of the enclosure after removing and setting it aside. Second, store the screws in a safe, organized place. The old "egg carton" trick may seem cliché, but it really does work. Of course, you are free to use plastic bags or organizer boxes as well - the idea here is to keep screws and other hardware off the work surface (unless you enjoy picking them up off the floor). Third, take note of each screw as you remove it, and keep groups of screws separated. This allows you put the right screws back into the corresponding locations. Since most enclosures use screws of equal size and length, this is rarely an issue at this phase of disassembly. But as you dismantle other sub-assemblies for upgrade or repair, keeping track of hardware becomes an important concern.

Use care when sliding the enclosure away. Metal inserts or reinforcements welded to the cover can easily catch on ribbon cables or other wiring. This can result in damage to the cable, and damage to whatever the cable is attached to. The rule here is simple: FORCE NOTHING! If you encounter any resistance at all, stop and search for the obstruction carefully - it's faster to clear an obstruction than to replace a damaged cable.

After your repair or upgrade is complete, you will need to close the system. Before sliding the enclosure back into place, however, make it a point to check the PC carefully. Make sure that every sub-assembly is installed and secured into place with the proper screws and hardware - leftover parts are unacceptable. A little care in organizing and sorting hardware during disassembly really pay off here. Remember to re-attach power and signal cables as required. Each cable must be installed properly and completely (in its correct orientation). Take time to route each signal cable with care and avoid jamming them into the system haphazardly. Careless cable runs stand a good chance of being caught and damaged by the enclosure during re-assembly, or the next time the system needs to be disassembled. Properly routed cables also reduce the chance of signal problems (such as noise or crosstalk) that can result in unstable long-term operation. Also check the installation of any auxiliary cables such as CD-ROM sound cables, the speaker cable, and the keylock cable.

Once the system components are re-assembled securely, you can apply power to the PC and run final diagnostics to test the system. When the system checks properly, you can slide the enclosure into place (being careful not to damage any cables or wiring) and secure the enclosure with its full complement of screws.

Whether you’re troubleshooting, upgrading, or building your own PC from scratch, there’s no doubt that you’ll get plenty of hands-on time inside desktop and tower PCs. Unfortunately, many potential problems can be overlooked (or even caused) while working inside a PC. The following tips should help you make the most of your PC experience, and minimize the chances of collateral problems:

• Be extremely careful of any sharp edges along the metal cover, or inside the metal chassis itself. Case manufacturers often save costs by omitting such production steps as burr removal and dulling sharp edges.
• Make sure that the chassis assembly is tight. All chassis are NOT created equal - some stand solid as a house, while others can seem to sway freely. Take note of the chassis condition, and tighten the chassis if necessary.
• Watch your vents and fans for good air flow. Make sure that the fan blades, grills, and any intake and exhaust filters are kept clean. Check to see that all fans are working.
• Watch for dust and debris. When you’re examining the enclosure, check for accumulations of dust or other debris. Dust is generally a thermal insulator and electrical conductor, and can easily block the flow of air inside a chassis, so it is important to avoid accumulations of dust and debris wherever possible.
• Choose new chassis with care. Replacing a chassis (or building a new PC from scratch) is an exciting but time-consuming effort, so plan for adequate expansion in terms of drive bays, expansion slot openings, power supply capacity, and drive power cables.
• Go with standardized cases, power supplies, and motherboards. New PC systems have largely abandoned the use of "AT"-style cases (i.e. Baby AT or Full AT) in favor of ATX or NLX versions. As you’ll see in the following sections, standard dimensioning ensures that cases, motherboards, and power supplies will ALL fit together.
• Keep drives mounted snugly. All PC drives (whether in an internal or external drive bay) should be mounted with at least four screws. Fewer screws can allow the drive to vibrate, and this can shorten the drive’s working life. Make sure that all four screws are in place and secure, but do not over-tighten the screws. Over-tightening can actually warp a drive’s internal frame and cause premature failures as well.
• Careful when mounting the motherboard. Under no circumstances should you ever flex a motherboard, or install it in such a way that it is uneven. See that no metal edges or standoffs touch the motherboard, and the motherboard should not sit flush against any part of the PC chassis.
• Check your cables closely. There are a myriad of cables inside a PC. Make it a point to check the installation and routing of each cable. Each end of a cable should be installed evenly and completely. Cables should be run (where possible) to minimize any interruptions to air flow.
• Check your expansion boards. Whenever you’re working inside a PC, make sure that any expansion boards inside the system are inserted evenly and completely into their bus slots. Often, exchanging external cables can accidentally wiggle a card loose - resulting in possible system problems. Also see that each expansion board is secured with a screw in the PC chassis.
• Check your memory devices. While you’re in the system, take a look at the memory devices. Make sure that each SIMM or DIMM module is clipped securely into place (especially if you’re replacing or upgrading memory). If your motherboard uses COAST (cache-on-a-stick) modules for cache RAM, also see that the COAST module is installed properly.
• Check the CPU heat sink/fan. Chances are that your CPU is fitted with a heat sink/fan assembly. Check to see that the heat sink is attached securely to the CPU, and verify that the fan portion of the assembly is working once the system is powered up.

Traditional PC chassis have always been somewhat of a "hit or miss" proposition. You’d choose cases, power supplies, and motherboards, and hope that everything would fit properly. All too often, screw holes won’t line up, and you’d be forced to return assemblies, or "kluge" the assemblies together - aligning as many screw holes as possible, and ignoring, clipping, or removing standoffs outright. Over the last few years, the PC industry has come together to develop a set of standard dimensions for key PC components (cases, motherboards, and power supplies). The two current standards are known as ATX and NLX. This part of the chapter looks at both of these standards in more detail.

NOTE: The use of new "form factors" does not have any bearing on the capabilities or performance of any new PC - only the dimensions of the motherboard, case, and power supply are effected.

The version 2.01 ATX form factor (Fig. 2-18) is the first true effort to standardize the major assemblies of a PC. In addition to the use of well-established mounting holes, the ATX approach makes several key improvements to the layout of a system. The CPU is relocated to a position on the motherboard which will not interfere with the use of full-length expansion boards (a common complaint of baby/full AT motherboard users). Since full-length cards can now be used in all the slots, it won’t be necessary to shuffle expansion cards around to avoid interfering with the CPU. The CPU itself can also be upgraded without having to remove expansion cards. SIMM and DIMM connectors are also located away from drive bays and expansion slots for easier access. The use of rear I/O ports (Fig. 2-19) and front panel connections have been standardized on the ATX motherboard which simplifies case design, and reduces the wiring on the motherboard. Integrated drive controller connections are now located closer to the drive bays to reduce drive cable lengths and reduce clutter. The ATX power supply provides power (including a native 3.3 volts) through a single 20-pin cable rather than the two 6-pin cables used in traditional baby/full AT systems. Finally, the ATX case design is configured to be cooled by a SINGLE fan located in the ATX power supply. This not only simplifies the case and reduces power demands, but it makes the system quieter.

ATX motherboard sizes - a full size ATX board is 12" wide by 9.6" deep (305mm x 244mm). The Mini-ATX board is 11.2" x 8.2" (284mm x 208mm). Designers have attempted to use as many mounting holes as possible from older baby/full AT style motherboards to allow existing chassis to use ATX motherboards with a minimum of modification. Figure 2-20 illustrates a comparison between a full AT motherboard, a baby AT motherboard, and a full size ATX motherboard.

ATX motherboard connectors - aside from the board size and placement of mounting holes, an ATX motherboard is also characterized by the general placement of various connectors. The listing below outlines the major connectors:

• Expansion slots (PCI/ISA) are located at the rear left of the motherboard.
• The power input connector is placed along the right edge of the board (near CPU).
• Drive signal connectors are located along the front edge of the board near the drive bays.
• Front panel I/O connectors (i.e. power switch and LED) are located along the front edge of the board - usually to the right of the expansion slots.
• Back panel I/O connectors (i.e. COM ports, parallel port, USB port, and so on) are all located on a single panel to the right rear of the motherboard.
• Memory module connectors are located between the CPU and expansion slots, or between the CPU and drive signal connectors (usually visible on inspection).
• The CPU is usually located on the right side of the motherboard in front of the back panel I/O connectors.

ATX power supply - an ATX power supply is about 6.1" long, 5.7" wide, and 3.5" deep - roughly equivalent to a PS/2 power supply footprint. The supply must generate the four traditional PC voltage levels (+5V. -5V, +12V, -12V), as well as a 3.3V level to better support low-voltage logic being used in modern PCs. Power is provided to the motherboard through a single 20-pin connector. A single exhaust fan assembly located in the supply must be capable of maintaining a minimum air flow of 23CFM.

ATX case - the only real distinguishing characteristic of an ATX case is the rear opening corresponding to the motherboard’s back panel I/O connector plate.

The version 1.2 NLX form factor (Fig. 2-21) is one of the newest dimensioning specifications for modern PCs. NLX is specifically designed to accommodate "low-profile" PC systems, while providing superior management for heat control, and easy maintainability. The key to the NLX configuration is not the motherboard, but a riser board. The vertical riser board connects directly to the power supply (not the motherboard), and holds all of the expansion boards horizontally. The riser board also holds the drive cable connectors (i.e. floppy connectors and hard drive connectors) which previously resided on the motherboard. This means that the NLX motherboard has no cables to be attached or removed when servicing the NLS system. An NLX motherboard can simply be "undocked" from the system’s riser card, and another one can be installed in a matter of moments. A wide area for back panel I/O connectors is provided on the rear of the NLX motherboard which allows for a large variety of high-end ports such as TV, sound, gameports, and so on. NLX motherboards are also some of the first to support the AGP (or Accelerated Graphics Port) for better graphics performance on PCs. The CPU is placed toward the front of the NLX motherboard (close to the fan) to ensure better system cooling. You can get a better view of the NLX riser, motherboard, and back panel in Fig. 2-22.

NLX motherboard sizes - NLX motherboards are not as straightforward as ATX units. The NLX specification defines motherboards of 9.0" x 13.6" (maximum) and 8.0" x 10.0" (minimum) - this means an NLX motherboard might run anywhere between these two sizes, and an NLX case must be able to support all possible sizes, though the typical NLX motherboard dimensions will be as follows (Fig. 2-23 illustrates a typical NLX motherboard and mounting):

• 8.0" x 10.0"
• 9.0" x 10.0"
• 8.0" x 11.2"
• 9.0" x 11.2"
• 8.0" x 13.6"
• 9.0" x 13.6"

NLX motherboard connectors - Perhaps the most noticeable difference between an NLX motherboard and other motherboards is the apparent lack of expansion board connectors and drive port connectors which have been implemented on the riser card. You’ll also note the presence of a 340-pin card edge connector which interfaces to the riser card. The list below outlines the disposition of important connections:

• Expansion slots (PCI/ISA) are located on the riser card in a horizontal orientation.
• The power input connector is attached to the riser card.
• Drive signal connectors are attached to the riser card.
• Back panel I/O connectors (i.e. COM ports, parallel port, USB port, and so on) are all located on a single panel to the right rear of the motherboard. This back panel occupies the entire rear of the motherboard.
• Memory module connectors are typically located somewhere between the CPU and expansion slots, or behind the CPU toward the rear of the motherboard.
• The CPU is usually located on the left front side of the motherboard in direct proximity to an NLX case intake fan.
• The AGP (Accelerated Graphics Port) connector is located along the left side of the motherboard several inches from the left rear corner of the motherboard.

NLX power supply - an NLX power supply uses the same dimensions as an ATX power supply (about 6.1" long, 5.7" wide, and 3.5" deep). The supply must generate the four traditional PC voltage levels (+5V. -5V, +12V, -12V), as well as a 3.3V level to better support low-voltage logic being used in modern PCs. Power is provided to the riser card through a single 20-pin connector. A single exhaust fan assembly located in the supply must be capable of maintaining a minimum air flow of 23CFM.

NLX case - the only real distinguishing characteristic of an NLX case is the long rear opening corresponding to the motherboard’s back panel I/O connector plate. There may also be hinged access or other provision to ease the installation or replacement of NLX motherboards. An additional inlet fan is located in the front left part of the chassis to aid in cooling the CPU.

That’s it for Chapter 2. Be sure to review the glossary and chapter questions on the accompanying CD. If you have access to the Internet, point your web browser to some of the contacts below:

AGP Implementers' Forum: http://www.agpforum.org/

Amtrade Products: www.amtrade.com

ATX information: www.teleport.com/~atx/

Enlight: www.enlightcorp.com.tw

Fong Kai Industrial: www.fkusa.com

Intel chipsets: http://developer.intel.com/design/pcisets/

Intel’s AN430TX motherboard: http://developer.intel.com/design/motherbd/an/index.htm

Intel’s NX440LX motherboard: http://developer.intel.com/design/motherbd/nx/index.htm

InWin Development: www.in-win.com

Iomega: http://www.iomega.com

NLX information: www.teleport.com/~nlx/

ProCase: www.procase.com.tw/68.htm

The Intel AGP web site: http://developer.intel.com/pc-supp/platform/agfxport/