NETWORKING

If one has more than one computer under his/her disposal then those computers can be connected together using a network. A network allows scanner, CD-writer, Hard Disk Drive, printer etc. Connected to one computer on the networks to be shared by all other computers connected to the network.

="color: red;">Setting Up a Network

Some times back, network installation was a complex job, but with the arrival of plug and play (PnP) devices, PnP operating systems and simple network such as 10Base-T, this process has become quite easy.

With the hardware setup, software setup is a very important part of the network installation. Nearly all networks use software drivers to enable their networking features, give access to shared resources, passing data through the network adapter, and accessing data from remote computers etc.

when using a network, hardware and software at the sending and receiving end must match. The two ends of the network must use the same networking protocol.

The protocol defines

• The size of the blocks of bytes transferred between computers,
• The addressed added to each block of bytes
• The means of controlling network access and routing the data, and 
• ways to recover when an error appears in the data.

The software for setting up a network ranges from simple to very complex.

For a simple home or small business network, a low end computer network will be more then enough.

If your use is simple sharing of data between computers and sharing of hardware devices, such as printer, scanner, CD-ROM drive etc. Then use windows operating system (windows-98 or higher) as your networking software, It will meet all these requirements.

Preparation For Network Installation


The first step in installing any network is planning-deciding exactly what you want the network to do-then you need to find out how to do it.

The hardware for putting together a peer to peer 10Base-T network comprises three parts

1. network/host adapter in each computer
2. a hub and 
3. wire/cable that links them together.

Network/Host Adapter
You''ll need one 10Base-T network adapter for every computer you want to connect to the network. The network adapter provides the connection between computer and the network cable.

All 10Base-T network adapters provide the same basic functions, although some have more feature than others. Any 10base-T adapter can be used, provided it us supported directly by the windows or it has a windows driver.

Some network adapters have facility for placing optional boot ROMs, which allows computers to boot up from a remote disk drive, but this feature is more applicable to client-server type installation, rather that to the peer to peer type networks.

10base-T network is a peer to peer network system, in this network system every PC is equal. Each computer can share files and other resources (such as printers) among one another. They are equally, each as the peer (equal) of the others, so this scheme is called peer-to-peer networking.

In this peer-to-peer scheme there is not dedicated files server, all computers can have their own, local storage, and each computer can be granted access to the drives and printers connected to the others.

Wiring 


A simple 10Base-T network use unshielded twisted pair (UTP) cabling. Ordinary modular telephone cables are unsuitable for 10base-T because these cables are flat and lack the needed twists.

The twist in the twisted pair cable minimize noise and interference.

Ordinary modular telephone cables use for-wire RJ-11 connectors that will fit into 10Base-T connectors but won't connect with all of the necessary signals. A 10base-T jack is designed to accept eight-wire RJ-45 connectors even though only four of the connections are active.

If all your PCs are in one room or reasonably close to each other then the best choice is to buy-made RJ-45 cables, which are available in standard length (for example 10, 25, 50 and 100 feet). After making connection, any extra cable can be loosely coiled up without any problem.

The only restriction imposed by 10Base-T is that the length of cable between PC and hub cannot exceed 100 meters (328 feet).

HUBS

If you want to network more than two computers then you'll need at least one hub for your network.

A hub is simply a box with circuitry inside and number of jacks for RJ-45 plugs on the back. The circuitry inside links the 10Base-T cables together.

Hubs are distinguished by the number of features that offer. But most of those features are designed to make the network administrator's job easier and are unnecessary in a small (five or less computers) peer to peer network. For such smaller system, and simple hub will be all one needs.

The first step in wiring your network is to determine the mist convenient location to put the 10Base-T hub. One can always set the hub on or under a desk, adjacent to a PC, or wherever is convenient.

The important consideration with locating the hub is that it should be put in a convenient location, out of the way but easy to reach. Ideally, It should be kept at the exact center of the computers it serves. This will minimize wiring hassles.

As long as one does not violate the 10Base-T wiring limitations, the hub can be placed anywhere.

Cable Installation 

Once you've set a location for each computers and the hub, you can start the wiring. You need to take one cable from each computer to the hub. One can use ready-made cables with modular connectors crimped on each ends, or using special crimping tool to crimp modular connectors on the cables, one can make required cables.

Network Configuration

Once the wiring is done, prepare the network adapter for each computer. Typically, a network adapter requires an interrupt number and base address from your system resources.

If the adapter you choose doesn't follow the plug-and-play standard, you can assign resources using DIP switches or jumps on each network adapter or through the setup software accompanying the adapter.

If you are using a plug and play (PnP) network host adapter then windows will automatically assign these resource value to the adapter. In a non-PnP adapter card, configure the card to match the settings you've chosen. This could be done with jumpers or switches provided on the card. 

• After configuring the card, shut down the computer and install the network adapter in your computer. unplug the computers power cable, remove the cover, and remove a blank retaining bracket. fro the empty expansion slot you want to use for the adapter board.
• now simply slide the adapter board into empty expansion slot. Be sure to screw the card tightly to retaining bracket. The screw will prevent board from dislodging when plugging in the network cable, it will also provide a better ground which will improve network reliability.

Before reinstalling system cover, plug it in and switch it on to make sure that the network adapter card doesn't   accidentally interfere with some other system function. Once you're sure all is well, shut down and turn off the system and reinstall its cover.

Repeat this process for each computer in your network.

Next step is network software/driver installation.

Under Windows 98, you can install the network adapter in two ways.

1. You can step through the Add New Hardware Wizard from Control Panel
2. Or you can use the Network properties sheet, available from the control Panel

To use the Network properties sheet, double click on the network icon under Control Panel. From the next screen, click on Add, ad you'll see a "Select Network Component" menu.

Click on the Adapter and click the Add button. Windows will then let you choose a manufacturer and model of network adapter to install. Select the board you want to install or choose a suitable compatible board.

Once you've selected an adapter, click OK. Windows will immediately install all the drivers required not only by the adapter but also all the services and protocols required to get your network working.

When windows finishes with the installation, you'll see a revised Network Sheet listing everything that the operating system has installed.

If Windows doesn't prompt you for the names you want to use for your computer and the working group, then select the Identification tab and enter the names.

Choose descriptive names such as Jeff Computer, Finance, Accounts, DTP Section, and so on.

Once you've finished, click OK. Click OK in the next screen as well, and Windows will tell you to reboot/restart your system.

Finally you are ready to link your network together. Slide the RJ-45 plug at one end of the cable into the RJ-45 jack at the back of each computer, in your network card then plug the other end of each cable into the hub.

Switch on the hub, and all the computers are now a part of the network.

Now you need to setup the Windows operating system to share printer, disk drives and other resources among the computers connected to the network.

How to Troubleshoot a Dead Motherboard/Computer

A. Try these steps:


Disconnect the power cord form the power supply, be sure the power supply is set for 110 or 220 volts depending on your location and power and double check it (most have a 110/220 switch)

Feel/look at the back of the power supply to determine if the fan is working.

If not, trouble-shoot the power supply and replace it if necessary.

If the fan is working, try another power supply anyway.

Plug the computer directly into a known-good (a lamp works) power outlet

Check to be sure you do not have a motherboard stand-off in the wrong position and shorting-out the bottom of the motherboard.

Inspect/shake and listen for loose metallic objects (loose screws) on top of and under the motherboard and in expansion board slots

Look carefully at the ISA and PCI slots, see if any of the contacts got bent/shorted-out. Sometimes an expansion board will dislodge one and it will be pushed into the bottom of the slot.

Inspect the motherboard for broken or burnt components.

Carefully inspect the motherboard for black soot from bad bearings and clean and replace the culprit.

Look for bent/shorted pins on the motherboard headers and straighten.

Be sure the speaker is plugged into the motherboard. If you hear beeps. Decode the beep code.

If not, double-check all jumpers.

Push down on all chips that have sockets in attempt to reseat them.

What CPU are you using? Be sure the core voltage is correct.

Be sure the CMOS battery jumper is in the correct position. Some distributors purposely ship motherboards with the jumper in the wrong position.

Find the jumper that clears the CMOS, put it into the clear position for several minutes, put it back in the normal position, plug-in the power cord, and push the power-on button. If you apply power to the motherboard with the jumper in the clear position you may damage the motherboard.

Pull all boards except video.

Disconnect all cables going to all drives, pull all cables except power, power on, and speaker, connect the power supply to the motherboard (the black wires go in the middle on AT power supply connectors--"Black OK, red your dead"), reseat the memory, plug-in and screw-down the display adapter and nothing else (push down on the top, front of the adapter and make sure it is properly seated by looking at it all along the PCI or AGP connector), connect the power-on switch and the speaker.

Check the monitor plug for bent or pushed-in pins, connect the monitor and nothing else. Check the monitor power cable.

Reseat/replace the memory.

Check for Motherboard Electrolytic Capacitor Failures

Check the CPU for bent pins.

Try another processor. Note: If you apply power to a motherboard with an Athlon or Duron processor without the CPU fan connected, even for a few seconds, you will fry it (see http://duxcw.com/digest/guides/cpu/socketa/heattip.html).

Check the CMOS battery with a multi-meter. Should be around 3 volts (2.8 is ok).

Try a different video board.

See if the CPU and memory will work with another motherboard.

Pull the motherboard, set it on the box it came in, install video, memory, CPU, power, and power on. See if it boots. I have seen several instances where this works when the board will not work in the case. And, when reinstalled in the case, it continues to work. I have also seen where it didn't work out of the case immediately, but did work the next day and continued to work. One of those mysteries.

Replace the motherboard.

Video Troubleshooting

Symptom 2: There is no display, and you hear a series of beeps when the PC initializes. The video adapter failed to initialize during the system's POST.

• Check the video board installation. Make sure that the video board is installed securely and completely in the expansion slot. Often, flexing of the motherboard can allow the video board to "pop out" of the bus slot.
• Replace the video board. If problem persist, try a known-good video board in the system.

Symptom 3: You see large blank bands at the top and bottom of the display in some screen modes, but not in others. Multifrequency and multimode monitors are not auto-sizing correctly. This is not necessarily a defect, but it can cause some confusion.

• Check the monitor. If the monitor is not auto-sizing, It will not compensate for change in screen resolution. Try a different monitor.
• Replace the video board. Your particular video board may not be providing the proper sync polarity signals to drive the monitor's auto-sizing features. Try a known-good video board instead.

Symptom 4: The display image rolls. Vertical synchronization is not keeping the image steady (horizontal sync may also be affected). This problem is typical of a monitor that cannot display a particular screen mode.

• Check the monitor. If the monitor is not capable of supporting a particular video mode, try a different monitor instead.
• Reconfigure the software. Set your application or windows to use a different screen resolution.
• Replace the video adapter. If the monitor is supposed to operate properly but does not, the video adapter sync circuits may be defective.

Symptom 5: An error message indicating an invalid system configuration appears on system startup. The system CMOS backup battery has probably failed. This is typically a symptom that occurs in older systems.

• Check your CMOS setup. Look for the entry marked " Video System" and make sure that it is set EGA/VGA. If there are any other video setup entries, make sure they are all set properly. Replace the CMOS backup battery if necessary.

Video and Sound Troubleshooting

Video and sound are certainly two of the most exciting aspects of the PC. New video systems promise high performance, with resolutions and color depths that simply weren't practical a few years ago. Enhanced video systems even provide support for features like a video conferencing and MPEG video playback. Today's sound boards offer symphonic sound with MIDI and wave table synthesis. Taken together, video and sound supply the backbone of a computer's multimedia capabilities. Still, the advances in video and sound have not come without price-namely, the demand for computing power. In addition, advanced chipsets and their drivers are sometimes incompatible with PC configurations. This chapter looks at background and troubleshooting for video and sound system.

Video Adapters


The conventional frame buffer is the oldest and best-established type of video adapter. The term frame buffer refers to the adapter's operation-image data is loaded and stored in video memory one "frame" at a time. Frame buffer architecture has changed very little since PCs first started displaying text and graphics. A typical frame buffer video board is composed of four major seconds:

• CRTC (cathode-ray-tube controller). A highly integrated controller which generates control signal and manages the video adapter's operation. The functions of the CRTC are now normally handled by video chipsets.
• VIRAM (video RAM). An amount of RAM used to hold to store image data which is processed and passed to the monitor.
• Character ROM. A relatively small ROM used to hold the dot patterns of text characters. The character ROM is disabled while the video adapter is in the graphics mode.
• Video BIOS. A ROM used to hold the low-level instructions needed to operate the video board. All video adapter from EGA onward use video BIOS in one form or another.

Video troubleshooting

Symptom 1: The computer is on, but there is no display (the PC seems to initialized properly). In most cases, there is a problem with your monitor or video board instillation.

• Check the beep codes. If there are a beep codes during initialization, the video adapter may be defective or installed incorrectly. Now what is beep codes? Beep Codes - each time PC initializes, BIOS executes a series of test collectively known as the power-on self-test (or POST). The POST checks each main area of the system, including the motherboard, video system, drive, drive system, and keyboard. This ensures that all elements of the PC can be used safely. If a fault is detected, POST reports the faults as either an audible beeps (or beep code) are typically used to indicate faults that are detected prior to the video system initialization. By matching the beep code to your particular BIOS maker and version, you can usually determine the fault with great accuracy.
• Check the monitor. Make sure that the monitor is plugged in, turned on, set for proper brightness, and connected to the video adapter securely.
• Check the hardware conflicts. Video boards do not use interrupts or DMA channels, but they do use memory addresses. If the video board's memory addressed conflict with those of other devices in the system, the video board may not work.
• Check for exclusions. you may need to add an "exclude" switch to your memory manager's command line to preview other devices from using the memory spaces required by the video board.

Hard drive troubleshooting

NOTE : Drive troubleshooting has the potential to destroy any data on the drive(s). Before attempting to troubleshoot hard disk drive problems, be sure to back up as much of the drive as possible. If there is no backup available, do not repartition or reformate the drive unless it is absolutely necessary and all other possible alternatives have been exhausted.

Symptom 1: The Hard drive is completely dead. If the drive does not spin up, the drive light doesn't illuminate during power-up, or you see and error message indicating that the drive is not found or not ready, follow the steps below:

• Check the power connector. Make sure the 4-pin power connector is inserted properly and completely. If the drive is being powered by a Y-connector, make sure any interim connections are secure. Use a voltmeter and measure the +5 volt (pin4) and +12-volt (pin 1) levels. If either voltage (especially the +12-volt supply) is unsually low or absent, replace the power supply.
• Check the signal connector. Make sure the drive's signal interface cable is connected securely at both the drive and the controller. If the cable is visible worn or damaged, try a new one.
• Check the CMOS Setup. Enter the CMOS setup routine and make sure that all the parameters entered for the drive are correct. Heads, cylinders, sectors per track, landing zone, and write precompensation must all be correct-otherwise, POST will not recognize the drive.
• Replace the hard drive. Try a known-good hard drive. If a known-good drive works as expected, your original drive is probably defective.
• Replace the drive controller. If problem persist with a known-good hard drive, replace the drive controller.

Hard Drives

Hard drives have quick become one of the most powerful and progressive parts of the PC. Tremendous storage capacities and outstanding speed allow unmatched performance-including use as virtual memory. However, PC also depend on the hard drive. Drive problems can prevent a system from booting, and data loss can render weeks (or months) of work inaccessible.

Hard drive interfaces
There are four major interfaces used with hard drives: ST506/412, ESDI, IDE/EIDE, and SCSI. These are older interfaces, and you will almost never encounter them unless working on an early-model i386 or older system.EIDE (Enhanced IDE) drives and interfaces appered in 1949 and have now largely replaced IDE in new systems, though the 40-pin interface is exactly the same. SCSI (Small Computer System Interface) drives are outstanding in server or multitasking systems.

Drive Troubleshooting

Computer serve little purpose without some means of long-term storage, and over the years, a large number of drives have been developed to provide access to vast amounts of information. This chapter explains the constuctuon and troubleshooting procedures for floppy disk, hard drives, CD-ROM drives, tape drives, and a series of other popular drives alternatives.

Floppy Drives
Floppy disk drives remain the standard removable-media mass-storage device. Although they are remarkably slow and offer punny-storage capacities when compared to hard drives and other mass-storage devices.

Floppy drive interface
Floppy drives use a 34-pin signal interface and a 4-pin "mate-n-lock" power connector.

Floppy disk troubleshooting
Symptom 1: The floppy drive is completely dead (the disk does not even initialize when inserted). If the drive light doesn't illuminate during power-up, or you see and error message indicating that the drive is not ready, follow the steps below:

• Check the power connector. Make sure the 4-pin power connector in inserted properly and completely. If the drive is being powered by a Y-connector, make sure any interim connections are secure. Use a voltmeter and measure the +5 volt (pin 4) and +12-volt (pin 1) levels. If either voltage (especially the +12-volt supply) is unusually low or absent, replace the power supply.
•  Check the signal connector. Make sure the 34-pin IDC header is connected securely at both the drive and the controller. If the cable is visibly worn or damage, try a new one.
• Check the CMOS setup. Enter the CMOS Setup routine and make sure that the drive is listed and selected properly. For example, if the drive is listed in CMOS as an A: drive but is physically cabled as a B: drive that controller won't recognize it. Pay particular attention to the drive type (1.44 Mbytes, 1.2 Mbytes, and so on)
• Replace the floppy drive, replace the floppy controller.
• Replace the drive controller. If problems persist with a known good floppy drive, replace the floppy controller.

Using the virus work disks

NOTE: This step ask you to create 10 copies of the virus checking software. Even though the disks are exclusively for your use, and you will use only one disk at a time, this kind of "multiple duplication" may violate the license agreement for your antivirus software. Be sure your license allows multiple copies of the software before proceeding.

Using the virus work disk. Whenever a PC comes in for service, use one of your antivirus work disk. Professionals always create antivirus diskettes in batches because the diskettes are disposable. That is, if a virus is detected and cleaned, the diskette that detected the infection should be destroyed, and you should boot the system with a new work disk to locate any other instances of the same virus, or any different viruses. This may seem radical, but it is cheap insurance against cross-contamination of the diskette. Once a system is booted with a work disk and checks clean, you can put that work disk away, and boot the system again with a diagnostic or boot disk as required. it is also advisable to check the PC for viruses again once the repair is complete.

Problem with antivirus tools. The protocol outlined above should help to protect you (and your customer) from virus attacks. Still there are two situations where trouble can occur:

• Virus checkers get obsolete fast. Viruses are prolifierating with the aid of powerful new programming languages and vast avenues of distribution such as the Internet. You will need to update your virus work disks regularly with the very latest antivirus software. Too often, technicians buy an antivirus package and continue to use it for years. The software certainly remain adept at detecting the viruses it was designed for, but it does not take into account the many new strains that crop up regularly. As a result, older virus checkers may allow newer viruses to pass undectected.
• Technicians get cheap with their floppy disks. If a work disk detects and eliminates a virus, It should be considered contaminated, and you should throw it away. Start again with a fresh work diskette. Continue checking and eradicting viruses until the system checks clean. The 40 cents or so that the diskette costs is not worth the risk of contracting the virus.

Virus disks

Computer viruses are a serious concern for any PC troubleshooter.
You will almost always employ some type of diagnostic software during the course of your troubleshooting. Often, the same diagnostic disk is reused on system after system, and even take along when making service calls in the field. Unfortunately, if a PC with a computer virus manages to infect your diagnostic disk, you will wind up spreading the virus to any subsequent system. As you might imageine, the consequences for your customer's data can be immeasurable.
Before you employ a boot disk or any form of diagnostic to troubleshoot the system, you should "sterlize" your shop by checking the system for viruses first.

What is Antivirus?

Antivirus (or anti-virus) software is used to prevent, detect, and remove malware, including computer viruses, worms, and torjan horses. Such programs may also prevent and remove adware, spyware, and other forms of malware.

Anti-virus software now scans for rootkits; a rootkit is a type of malware that is designed to gain administrative-level control over a computer system without being detected. Rootkits can change how to operating system functions and in some cases, rootkits can tamper with the anti-virus program and render it ineffective. Rootkits are also very difficult to remove, in some cases requiring a complete re-installation of the operating system.

There are a huge number of security companies offering antivirus programs these days and some are better than others.

Here are my eleven favourates:

• Kaspersky Antivirus                         
• BitDefender Antivirus
• ESET NOD32 Antivirus
• AVG Antivirus
• Avast! Antivirus
• McAfee Antivirus Plus
• Norton Antivirus Software
• Panda Antivirus
• Trend Micro Antivirus
• F-Secure Antivirus
• Avira Antivirus

What is Virus?

Viruse - A virus is a small peace of software that piggybacks on real programs. For example, a virus might attach itself to program such as a spreedsheet program. Each time the spreadsheet program runs, the virus runs, too, and it has the chance to reproduce (by attaching to the programs of wreak havoc.

E-mail viruses - An e-mail virus travels as an attachment to e-mail messages, as usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail address book. Some e-mail viruses don't even require a double click -- they lunch when you view the infected message in the preview pane of your e-mail software.

Torjan horse - A Torjan horse is simply a computer program. The program claims to do one thing (it may claim to be a game) but instead does damage when you run it (it may erase your (hard disk). Torjan horse have no way to replicate automatically.

Worms - A worms is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worms scan the network for another machine that has specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well.

CPU symptoms

Symptom : The system boots with no problem, but crashes or freezes when certain applications are run. It may appear that the application is corrupt.

• Test the CPU. Try a diagnostic such as TouchStone Software's checkIt. Run repeatitive tests on the CPU. While the CPU may work in real mode, diagnostics can detect errors running protected-mode instructions and perform through register checking.
• Replace the CPU. When an error code suggesting a CPU fault is returned, try a known-good CPU.
• Expand the diagnostic. If a CPU fault is not detected, expand the diagnostic to test other portions of the motherboard. If the entire system checks properly, you may indeed have a corrupt file in your application. 

A keyboard error

Symptom : A keyboard error is reported, but a new keyboard has no effect. If a known-good keyboard refuses to work, there are several items you can check.

• Check the keyboard connection. Make sure that the new keyboard is installed correctly. If there is a cable connecting the keyboard panel connector to the motherboard, make sure that that wiring is attached properly.
• Check the pico fuse. The keyboard fuse on the motherboard may have failed. Many motherboard designs incorporate a small fuse (called a picofuse) in the +5-volt dc line that drives the keyboard. If this fuse fails, the keyboard will appear dead. Replace the fuse.
• Replace the motherboard. If problems persist, or if you cannot replace the keyboard fuse, replace the motherboard outright.

Motherboard symptoms

Symptom 1: A motherboard failure is reported, but it goes away when the PC's out cover is removed. There is probably an intermittent connection or short circuit between the motherboard and chassis. This often happens during an upgrade or new assembly.

• Check for short circuits. Examine any metal standoffs securing the motherboard, and see if any are touching metal traces on the motherboard. You may be able to eliminate such shorts by inserting a thin spacer made from a manila folder.
• Try reinforcing the chassis. If the chassis is warping a bit, make sure that the entire chassis assembly is tight.
• Try a new motherboard. If the chassis is not warping, there may simply be a fault somewhere in the motherboard itself. If the motherboard erboard is under warranty, replace it outright.

CPU Issues

The CPU is the single most complex and expensive IC on the motherboard. It is responsible for processing every instruction and virtually all the data in memory at one time or another. While CPU failures are somewhat rare, there are some possible symtoms that you should be familiar with. When you suspect a CPU problem, make the following checks before proceeding:

• Check the socket. Make sure that the CPU is inserted completely into its socke. For a ZIF socket, make sure that the lever is closed completely and latched.
• Check the motherboard jumpers. When you are upgrading a CPU, make sure that the motherboard is configured properly for the specific CPU type and clock setting.
• Check for heat. Make sure that the CPU's heat sink (or heatsink/fan assembly) is attached securely to the CPU. If there is a fan on the heat sink, make sure that the fan is running. You may need to reattach the heat sink using thermal grease. Thermal grease (also called thermal gel, thermal compound, thermal paste, heat paste, heat sink paste, heat transfer compound, or heat sink compound) is a fluid substance, originally with properties akin to grease, which increases the thermal conductivity of a thermal interface by compensating for the irregular surfaces of the components. In electronics, it is often used to aid a component's thermal dissipation via a heat sink.

Motherboard Troubleshooting

The motherboard is the heart of any personal computer. It is the motherboard that handles system resources (IRQ lines, DMA channels, I/O locations) as well as "core" components such as the CPU, math coprocessor, and all system memory, including DRAM, BIOS ROM, cache, and CMOS RAM. Indeed, most of a PC's capabilities are defined by motherhboard components. This chapter shows you how to recognize symptoms and translate error information into motherboard repair. You will typically find the following elements on your motherboard:

• CPU (central processing unit). The CPU is a programmable logic device that performs all of the instruction, logic, and mathematical processing in the PC. This is the single most important IC in the computer CPU failure can disable the entire PC.
• MPC (math coprocessor). The MPC is a programmable logic device (closely related to CPU) which is tailored for handling floatingpoint math operations. Math-intensive application software which is written to take advantage for the MCP can realize substantial improvements in performance. All contemporary CPIs now integrate math coprocessor functions without the need for a seperate MCP.
• BIOS (basic input /output system). This is the code used to control the motherboard's hardware and perform low-level motherboard operations. Traditionally, BIOS has been stored on DIP ICs which can be easily removed and replaced as needed. Such ICs starts with a "27" prefix, them have two or three digits indicating the number fo kilobits (divide by 8 for kilobytes). For example, the 2764 is a ROM providing 8 kbytes of storage. If flash BIOS is used, you will find the BIOS ROM in a PLCC holder.  
• CMOS / RTC. This is a dual-function IC which maintains system setup variables in up to 128 bytes of low-power CMOS RAM. It also supplies a real-time clock which keeps track of the date, day and time. Traditional PCs used the Motorola MC146818, but many other variations have come into use. You can often locate the CMOS/RTC IC by its large, rectangular shape. IT will also locate the COMS/RTC IC by its large, rectangular shape. It will also be located near the system backup battery.
• Clocks. Proper synchronization and signaling of the motherboard requires the use of precision oscillating ICs. There are typically two clock ICs, one for the 14.318-MHz OSC signal on the ISA bus, and one driving the CPU clock (and other processing ICs). The CPU clock IC accepts the Power Good signal from the powersupply and generates the system Reset signal. The OSC clock IC also produces a 1.19-MHz signal for the PIT.
•  PIT (programmable interval timer). The interval timer provides three channels for the PC, Channel 0 is set to produce an interrupt every 54.94 ms (the RTC time base). Channel 1 is programmed to produce an interrupt every 15.12 us to signal the start of a memory refresh cycle, which must be performed at least once every few milliseconds. Channel 2 is a noncritical timer which serves to support speaker signals.
• DMAC (direct memory access controllers). DMA is critical to operation of an XT or AT system Normally, the CPU must handle each bytes of data transferred in the system-a slow process when large volimes of data must be transferred. DMA allows data transfer without the intervention or the CPU. A single DMA controller provides four channels (an AT uses two DMACs to provide eight channels).
• DMA page registers. These are relatively simple buffers that point to the 64-kbyte area (page) that DMA transfers will take place from (or to). Early PCs used one page register, but i386 and later systems often use two. Remember that is a common for newer systems to indicate a fault in the second page register because the page registers are usually integrated into chipsets.
• PIC (programmable interrupt controller). The PIC recognizes and prioritizes hardware interrupts, then passes the interrupt signal to the CPU along with a vector which points to the location of the interrupt handler routine. The XT used one PIC which supplied eight channels, but ATs use two PICs for sixteen channels (IRQ o-15).
• KBC (keyboard controller). The KBC is a very specialized single-chip microcontroller (including its own small amount of BIOS) designed as an interface between the system and the AT serial keyboard. On system initialization, the POST will cause a KBC self-test. If the self-test passes, the system can proceed to test the keyboard. Another vital part of the keyboard controller is that it handles the A20 gate. The A20 gate handles access to extended/ expanded memory (protected-mode addressing). If the KBC fails, the system will be unable to work in the protected mode.
• Gate A20. This is the simple gate that controls address line A20, and thus controls protected-mode operation by accessing memory over 1 Mbyte. The A20 gate is operated from the KBC.
• BUS controller. This device accepts control and timing signals from the system and generates the I/O and memory read and write signals needed to transfer data among system components also manages the translation of 16-bit words into 8-bit words for single-byte data transfers, and then the translation back again.
• UART (universal asynchronous receiver/transmitter). UTARTs are the key components in seral communications ports. They translate parallel data into framed serial data for transmission, and reverse the process to convert framed serial data back into parallel data during reception. Current systems use 16550A UTRATs. When a serialport error is reported, the UART has probably failed. UARTs incorporated in the motherboard are typically provided with jumpers that will set the port I/O address and IRQ line, and allow you to disable the UART in favour of expansion board communication devices.
•  DRAM. General storage and main system memory is priovided by dynamic RAM. Modern DRAM ICs can hold a great deal of data, but it must be refreshed every few milliseconds-otherwise, the data will be lost. A key purpose of the DMAC and PIC is to manage refresh. Some DRAM is fabricated on the motherboard, but much more DRAM can be added in the form of SIMMs.
•  Cache. Cache has become an important element of PC performance improvement. If data and instructions are loaded into fast static RAM in advance of the CPU's need, memory wait states can be eliminated. Many systems are equipped with 128 to 256 kbytes of externated. Many systems are equipped with 128 to 256 kbytes of external cache, but some systems can have up to 512 kbytes of cache. To boost performance every further, newer CPUs are designed with a small amount of internal cache. If external chane fails, you may be able to circumvent teh error by disabling the cache through the CMOS setup. If internal cache fails, the CPU will have to be replaced.

Conflict troubleshooting

The PC provides only a limited number of interrupts (IRQs), DMA channels, and I/O addresses for devices to use. No two devices can use the same resources-if the same resource is assigned to two devices, they will compete for control. Conflicts can result in problems ranging for erratic device behaviour to system lockups and crashes. The procedure below offers a reliable method for locating and eliminating device conflict:

• Power the computer down and remove the new expansion device.
• Start the machine to the DOS mode and run the MSD.EXE program that is in your \WINDOWS directory (you can also try any number of shareware or commercial diagnostics that detect resource assignments).
• The program will let you look at which interrupts, DMA channels, and I/O addresses are currently in use on your system. Record those on a sheet of paper (or print the report to a printer) and exit the program.
• Examine the new device and check its resource assignments against the resource already in use. Chances are that the new device will be using an IRQ, DMA, or I/O assignment already shown in your MSD report.
• Change the conflicting resource. For example, if you find and IRQ conflict, change the IRQ on your new device to an IRQ that is not in use. If the device's resources are set through software, simply proceed to the next step.
• Turn the system off again and place the device back in your computer.
• Run any setup software for the new device. It should be recognized properly.
• If you cannot find any available resources, you will have to disable at least one other device in order to free up the resources for your new device.
• If the new device works under Windows, be sure to run any Windows installation software. If the device is running under Windows 95, run the Add New Hardware wizard.

When a system starts but crashes/reboots intermittently

1. The system randomly crashes or reboots for no apparent reason:

• Check for viruses. Some viruses (especially memory-resident viruses) can cause the PC to crash or reboot unexpectedly. If you haven't run your virus checker yet, do so now.
• Check the power supply cables. Verify that the power supply cables are attached properly and securely to the motherboard.
• Check the powersupply voltage. As I have said earlier use a voltmeter to verify that each output from the power supply is correct. If any output is low (especially the +5-volt output), replace the power supply.
• Check the CPU. with all power off, check to see that the CPU is cool, that the heat-sink/fan assembly is fitted on correctly, and that the CPU itself is inserted properly and completely into its socket. If the CPU overheats, it will stall, taking the entire system with it. 
• Check the CPU socket. If the Cpu is seated in a ZIF (sero insertion force) socket, make sure that the socket's tension lever is closed and locked into place.
• Check all SIMMs. With all power off, make sure that all SIMMs are seated properly in their holders and locked into place. you may try removing each SIMM, clean the contacts, and reinstalling the SIMMs.
• Check the expansion boards. Make sure that all expansion boards are seated properly. Any boards that are not secured properly, or that are inserted unevenly, can short bus signals and cause spurious reboots. If you have recently installed now expansion hardware, make sure that there are no hardware conflicts between interrupts. DMA channels, or I/Q addresses. 
• Check the motherboard for shorts. Inspect the motherboard at every metal standoff and make sure that no metal traces are being shorted against a standoff or screw. You may want to free the motherboard and see if the crashes or reboots go away. If so, use nonconductive spacers (such as a small piece of manila folder to insulate the motherboard from each metal standoff. If the system continues to crash or reboot (and all voltages from the power supply are correct). replace the motherboard.   

Start up Problem

If there is no power light, but you do hear the cooling fan running:

• Check the ac voltage. Use a voltmeter and conform that there is adequate ac voltage at the wall outlet. Unusually low ac voltages (such as during "brownout" conditions) can cause the power supply to malfunction.
• Check the power supply cables. Verify that the power supply cables are attached properly and secure to the motherboard. A typical connection scheme is shown in figure above.
• Check the power supply voltages. Use a voltmeter to verify that each output from the power supply is correct. If any output is very low or absent (especially the +5-volt output), replace the power supply.
• Check the "Power Good" signal. Use a voltmeter and verify that the "Power Good" signal is +5 volts. If this signal is below 1.0 volt, It may inhibit the CPU from running by forcing a continous Rest condition. Since the "Power Good" signal is generated by the power supply, replace the power supply. 

Evaluating Startup Problems

There are many problems that can plague the PC, but perhaps the most troubling problems occur during startup-when computer fails to start at all or does not start completely. Startup problems make it most impossible to use diagnostics or other utilities that we depend on to help isolate problems. With the advent of windows 95, there are even more difficulties that can develop. This part of the chapter offers you a series of possible explanations for full and partial system failures.

When a system doesn't start at all

1. If there is no power light, and you cannot hear any cooling fan:

• Check the ac voltage. Use a voltmeter and confirm that there is adequate ac voltage at the wall outlet.
• Check the ac cord. The ac cord may be loose or disconnected.
• Check the power supply fuse(s). The main fuse may have opened. Replace any failed fuse.  

1           1   Orange     Pwr Good
             2   Red          +5
             3   Yellow       +12
             4   Blue          -12
             5   Black        Gnd
 6          6   Black        Gnd

                                                                   Motherboard power connector
1           1   Black        Gnd
             2   Black        Gnd
             3   White        -5
             4   Red          +5
             5   Red          +5
6           6   Red          +5

NOTE: If you replace a main fuse and the fuse continues to fail, you may have a serious fault in the power supply. Try replacing the power supply.

Guard and shielding

Modern PCs and peripherals often employ an assortment of metal and plastic shields or guards within the device. Shields and guards serve a variety of purposes, but all should be replaced when service is complete.

1. Replace EMI shields. PCs operate at very high frequencies, and the signals they generate can sometimes be transmitted to nearby recivers such as radios and televisions. Ideally, the PCs design should prevent such EMI (electromagnetic interference), but it may also be necessary to add metal shields to attenuate excessive interference. Whenever you remove metal housings or shields from a PC, be sure to replace them before returning the system to service. 
2. Replace x-ray shields. Monitors use extremely high voltages at the CRT, which in turn can liberate x-rays through the CRT glass. The lead contained in CRT glass is usually a sufficient shield, but larger CRT funnel. When you remove x-ray shields from a monitor, be sure to replace them before running the monitor or returning it to the service. 
3. Replace all guards and other mechanical assemblies. Printers typically employ a large assortment of guards and covers (both plastic and metal) to protect delicate mechanical assemblies from dust and accidental contact. You can usually operate a printer for short periods without guards in place, but you should always make in a point to replace any protective assemblies before returning the device to service.   

Electricity Control

PCs and their peripherals use raw alternating current as a power source. While the myriad of plugs, outlets, and line cords used today are generally regarded as quite sage for end users, technicians must often work in close proximity to exposed circuitry. In reality, the odds of electrocution are quite slim, but electricity can injure or kill when handled carelessly.

• Keep the PC unplugged when working inside. As a rule, unplug the PC (don't just turn it off) during upgrades or repairs.
• Use only one hand for "hot" measurements. If you make measurements or probe around inside a powered system (especially inside the power supply), keep one hand behind your back. If you should contact a live wire, there is no pathway through your heart.
• Use properly rated test probe's. If you attemp to measure high through commercial test probes, you can be electrocuted right through the probe's insulation. Make sure the prob you're using is rated for the expected voltage levels.

Rules

• Use wrist straps. Grounding wrist straps are the first line of defence against ESD. They attach to your wirst and connect to a grounded surface or outlet through a wire. When properly connected, a wrist strap "bleeds off" any charge on your body and clothing, make it safer to handle delicate electronics.
• Use antistatic containers. You have probably noticed that all delicate electronics comes packaged in blue or pink bag. These act as "Faraday cages" which dissipate charges before thay can reach the device contained inside. Always keep devices inside antistatic containers until you are ready to actually install them, then place any removed device into an antistatic container immediately.
• Use and antistatic mat. A mat works like a wirst strap by connecting to ground and bleeding off any accumilated charges. You can place boards, ICs or SIMMs sagely on a properly connected anti static mat without having to place them in container. Antistatic mats are very popular on PC repair workbenches where sensitive items are regularly installed and removed.
• Use antistatic chemicals. Monitor screens, most synthetic surfaces, and virtually all plastic enclosures are major sources of ESD. when properly and regularly applied, antistatic chemicals can go a long way towards preventing ESD damage from accidental or casual contact with sensitive electronics.
• Manage temperature and humidity. Static builds up easily in cool, dry environments. work in a warm area with adequate relative humidity (RH). Use a humidifier if necessary to maintain adequate RH.

Troubleshooting Guidelines

Static Control
Modern PCs depend on extermely complex integrated circuits, and those ICs are very sensitive to ESD (electrostatic discharge). Unfortunately, static electricity means as passing a comb through our hair or putting on a sweater. When ESD is allowed to discharge through an IC, the IC is destroyed. They are no outware signs of ESD damage-no smoke, no fire, and rarely and shock or other physical sensation. Still, the damage is quite real. ESD is controlled by combination of grounding, protective materials, and environmental management.

Disclaimer & Cautions

It is IMPORTANT that you read and understand the following information. Please read it carefully!

PERSONAL RISK & LIMITS OF LIABILITY

The repair of personal computer and their peripherals involves some amount of personal risk. Use extreme caution when working with ac and high-voltage power sources. Every resonable effort has been made to identify and reduce areas of personal risk. You are instructed to read this carefully before attemping the procedures disussed. If you are uncomfortable followin the procedures do not attempt them - refer your service to qualified service personnel.

PREPARING FOR SERVICES

Troubleshooting is a lot like taking a test-there is a problem that needs to be solved, and you are "graded" on the speed and success with which you solve that problem. As with any test, a certain amount of preparation is required, and you often have to make some basic assumptions about the problem in order to solve it. Unfortunately, many less-experienced technicians are quick to jump at a solution without performing essential checks and observations. Too often, this results in lost time and wasted materials. We present you with a set of checklists and guidelines that can help to speed your diagnosis of a problem and keep your troubleshooting on track.