Hard disk drive

Hard disk drive:

A hard disk drive (often shortened as “hard disk” or “hard drive”), is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, “drive” efers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air
pressure) with fixed media.


Disk formatting is the initial part of the process for preparing a hard disk or other storage medium for its first use. The disk formatting includes setting up an empty file system. A disk formatting may set up multiple file systems by formatting partitions for each file system. Disk formatting is also part of a process involving rebuilding an entire disk from scratch. The formatting’s are two types

  1. Low level formatting
  2. High level formatting

Low level formatting:

To find out if a drive is low level formatted or not the DOS FDISK program is used. If the FDISK program recognizes the hard disk drive, then the drive has been already low level formatted.

       To level format a drive the easiest method is to call the BIOS interrupt INT 13h, function 05h. The BIOS  will  convert this function call into proper CCB(Command Control Block), i.e.set of commands for the drive controller, and send these command code bytes to the proper I/O port connected to the disk controller.

High-level formatting:

       After the low level formatting and partitioning, the final step for preparing the hard disk drive for use is to high level format the drive. The drive is already divided into tracks and sectors by the low level formatting procedure, so the high level format program need to only create File Allocation Table(FAT), directory system, etc. so that the DOS can use the hard disk drive to store and read files. During the high level format the format program verifies all the tracks and sectors in that particular DOS partition.The following DOS command is used for formatting the hard disk:

       The /S switch transfers the system files to the hard disk drive, primarily the two hidden DOS files (I/O.SYS,MSDOS.SYS) and the file COMMAND.COM.

Components of HDD:

  Many types of hard disks are on the market, but nearly all drives share the same basic physical components. Some differences may exist in the implementation of these components (and in the quality of materials used to make them), but the operational characteristics of most drives are similar.

Hard Disk Platters (Disks):

       A typical hard disk has one or more platters, or disks. Hard disks for PC systems have been available in a number of form factors over the years. Normally, the physical size of a drive is expressed as the size of the platters. Following are the most common platter sizes used in PC hard disks today:
•  5 1/4 inch (actually 130mm or 5.12 inches)
•  3 1/2 inch (actually 95mm or 3.74 inches)
•  2 1/2 inch
•  1.8 inch

      Larger hard drives that have 8-inch, 14-inch, or even larger platters are available, but these drives typically have not been associated with PC systems. Currently, the 3 1/2-inch drives are the most popular for desktop and some portable systems, whereas the 2 1/2-inch and smaller drives are very popular in portable or notebook systems. These little drives are fairly amazing, with current capacities of up to 1GB or more, and capacities of 20GB are expected by the year 2000. Imagine carrying a notebook computer around with a built-in 20GB drive. It will happen sooner than you think! Due to their small size, these drives are extremely rugged; they can withstand rough treatment that would have destroyed most desktop drives a few years ago.

      Most hard drives have two or more platters, although some of the smaller drives have only one. The number of platters that a drive can have is limited by the drive’s physical size vertically. So far, the maximum number of platters that I have seen in any 3 1/2-inch drive is 11.
Platters traditionally have been made from an aluminum alloy for strength and light weight. With manufacturers’ desire for higher and higher densities and smaller drives, many drives now use platters made of glass (or, more technically, a glass-ceramic composite). One such material is called MemCor, which is produced by the Dow Corning Corporation. MemCor is composed of glass with ceramic implants, which resists cracking better than pure glass.

      Glass platters offer greater rigidity and, therefore, can be machined to one-half the thickness of conventional aluminum disks, or less. Glass platters also are much more thermally stable than aluminum platters, which means that they do not change dimensions (expand or contract) very much with any changes in temperature. Several hard disks made by companies such as Seagate, Toshiba, Areal Technology, Maxtor, and Hewlett-Packard currently use glass or glass-ceramic platters.
Read Write Head:

      The RW head is the key component that performs the reading and writing functions. It is placed on a slider which is in term connected to an actuator arm which allow the RW head to access various parts of the platter during data IO functions by sliding across the spinning platter. The sliding motion is derived by passing a current through the coil which is part of the actuator-assembly. As the coil is placed between two magnets, the forward or backward sliding motion is hence derived by simple current reversal. This location of the platter (just like the landmark along the road) is identified and made possible by the embedded servo code written on the platter.

Recording Media:

      No matter what substrate is used, the platters are covered with a thin layer of a magnetically retentive substance called media in which magnetic information is stored. Two popular types of media are used on hard disk platters:
•  Oxide media
•  Thin-film media

      Oxide media is made of various compounds, containing iron oxide as the active ingredient. A magnetic layer is created by coating the aluminum platter with a syrup containing iron-oxide particles. This media is spread across the disk by spinning the platters at high speed. Centrifugal force causes the material to flow from the center of the platter to the outside, creating an even coating of media material on the platter. The surface then is cured and polished. Finally, a layer of material that protects and lubricates the surface is added and burnished smooth. The oxide media coating normally is about 30 millionths of an inch thick.

      As drive density increases, the media needs to be thinner and more perfectly formed. The capabilities of oxide coatings have been exceeded by most higher-capacity drives. Because oxide media is very soft, disks that use this type of media are subject to head-crash damage if the drive is jolted during operation. Most older drives, especially those sold as low-end models, have oxide media on the drive platters. Oxide media, which has been used since 1955, remained popular because of its relatively low cost and ease of application. Today, however, very few drives use oxide media.

      Thin-film media is thinner, harder, and more perfectly formed than oxide media. Thin film was developed as a high-performance media that enabled a new generation of drives to have lower head floating heights, which in turn made possible increases in drive density. Originally, thin-film media was used only in higher-capacity or higher-quality drive systems, but today, virtually all drives have thin-film media.

Head Actuator Mechanisms:

      Possibly more important than the heads themselves is the mechanical system that moves them: the head actuator. This mechanism moves the heads across the disk and positions them accurately above the desired cylinder. Many variations on head actuator mechanisms are in use, but all of them can be categorized as being one of two basic types:
•  Stepper motor actuators
•  Voice-coil actuators

     The use of one or the other type of positioner has profound effects on a drive’s performance and reliability. The effect is not limited to speed; it also includes accuracy, sensitivity to temperature, position, vibration, and overall reliability. To put it bluntly, a drive equipped with a stepper motor actuator is much less reliable (by a large factor) than a drive equipped with a voice-coil actuator.
The head actuator is the single most important specification in the drive. The type of head actuator mechanism in a drive tells you a great deal about the drive’s performance and reliability characteristics.

Voice coil actuator:

     A voice-coil actuator with servo control is not affected by temperature changes, as a stepper motor is. When the temperature is cold and the platters have shrunk (or when the temperature is hot and the platters have expanded), the voice-coil system compensates because it never positions the heads in predetermined track positions. Rather, the voice-coil system searches for the specific track, guided by the prewritten servo information and can position the head rack precisely above the desired track at that track’s current position, regardless of the temperature. Because of the continuous feedback of servo information, the heads adjust to the current position of the track at all times.
Two main types of voice-coil positioner mechanisms are available:
•  Linear voice-coil actuators
•  Rotary voice-coil actuators
The types differ only in the physical arrangement of the magnets and coils.

Troubleshoot Hard Disk Drive Problems:

General Information:

     If your computer is operating but has a problem such as “cannot read sectors” or in general shows a “retry , abort ,ignore” error while reading the drive, it is an indication that there are some bad spots on the drive. These bad spots normally are fixed by reformatting the hard drive. This is a big hassle since all the programs will have to be reloaded and unless you back up and restore your data, you will loose ALL the data stored on the hard drive.

Meanwhile, here are some troubleshooting instructions

1. There is some type of electrical connection problem
2. The hard drive controller has failed
3. The hard drive has failed physically
4. The hard drive has failed electronically
5. There is a problem with the recording on the hard drive
6. The CMOS settings are not correctly set
7. There is a conflict with the IRQ settings
8. There is a conflict with the jumper settings
9. The drive is unable to boot.
10. Fdisk reports wrong size when using drives larger then 64GB.

1. There is some type of electrical connection problem

    Make sure the cable connections are correct.
Check the 4-wire connector that carries power and make sure it is properly plugged in. This connector has a taper on one end and cannot be put on backwards.

    When power is first applied to the computer, the hard drive light will momentarily come on which is a good indication that the drive is getting power. Also the vibration of the spinning platter and the slight hum will verify that the drive is plugged in.

    Next check the data ribbon cable. This cable is a flat cable with a one edge colored red or blue to indicate the location of pin 1. Some of these cables are also keyed by having a small tab in the center of the connector’s edge. On many hard drives pin 1 is the pin closest to the power supply connection, but not always, so check the hard drive documentation or look on this site in Hard Drives and locate your model.

    If all the cables are connected properly, and power is applied, you should be able to hear and feel the drive spinning. If the drive is not spinning, turn off the power and try using a different power plug (maybe the one that the CD-ROM is connected). If the drive is not spinning then it is probably bad.

2. The hard drive controller has failed

    A controller failure is usually indicated by an error at boot up. There is not much that can be done except to replace the hard drive. See hard drive error codes

3. The hard drive has failed physically
There can be two indications for this condition.

    1) The drive is not spinning. To troubleshoot this condition you need to physically access the drive while the computer is on. With the cover off, look at the drive and find the side which has NO components. With your hand touch that side and try to feel the spinning of the hard drive platter. A typical hard drive has a small amount of vibration and a slight whine.

    2) The hard drive head has crashed onto the platter. This usually causes the drive to emit unusual sounds sometimes grinding and many times repeating on a regular basis. A normal hard drive has a smooth whine so its should be easy to identify the bad drive by just listening.

4. The hard drive has failed electronically

    This will be indicated by an error message during the computer boot cycle. Not much can be done in this condition other then replace the drive.

5. There is a problem with the recording on the hard drive (read or write)
There are two conditions that can cause this problem.
1) The hard drive is unable to read a sector on the platter.

    This problem can be identified by running a program that is capable in performing a hard drive surface test. In Windows 95 you can use the scandisk which is found in the Start/Programs/Accessories/System Tools folder. Another way is to use a utility program like Norton Utilities to perform the surface scan.

    This problem can also be seen when you are formatting the hard drive and is indicated as “bad sectors” during the formatting. These bad sectors are normally recorded as such by the format program and the computer knows not to use them but more bad sectors can be created as the hard drive ages.

    2) One or more files have been damaged by some process.
These type or problems are caused when the computer is unexpectedly rebooted after a lock-up or perhaps a power failure. They are easy to troubleshoot and repair. Simply run the scandisk program which is found in the Start/Programs/Accessories/System Tools folder and allow the computer to repair any errors found.

    After such a repair it is very possible that one of more files were corrupted and are now unusable. It is impossible to tell which files will be affected in advance but if you write down the bad file names shown during the scan disk operation you can try to find the application which loaded them and re-install that application.

 6. The CMOS settings are not correctly set
Check the CMOS settings. These settings must match the required settings of the manufacturer.
a) older computers

    On these computers you have to go into the CMOS/BIOS during boot and change the setting by selecting a number from 1 to 48, by selecting a TYPE number of 1 or 2, or by selected the setting “User defined” and manually entering the hard drive parameters of “head”, “cylinders”, “spt”, “WP”, and “LZ”. These settings can be found on your hard drive users manual, on the manufacturer’s web site, or on this site by looking for the company, then the hard drive’s model number.

    After entering these parameters you will normally save them before exiting the BIOS program and then reboot the computer.
    b) newer computers

    On these computers you can almost always find a selection that allows the computer to “Automatically” find IDE style hard drives. There are two methods in use. First you can select “Auto” from the main BIOS screen for the drive C: D: E: or F:. After rebooting the drive will be automatically detected. Second, some Bios types have a selection called “Detect hard drive” which allows you to initiate a detection process which looks for a drive, presents you with the drive found and gives you the option of accepting or rejecting the detected drive. This process is repeated for each of the available drive assignments C D E and F.
Again you must save the BIOS changes and reboot the computer.

    Very critical also is the LBA setting which can cause the drive to operate but not be able to see all the data. This comes into play with drives larger than 500 megabytes and is found by entering the computer BIOS at boot up and looking in the area where the hard drive is configured.

“Wrong LBA setting” Symptoms:

* Computer comes on, but you get many read errors
* Windows comes up sometimes other times it hangs
* you can save short files but larger ones don’t seem to save
* Scandisk is not able to fix the problems it finds

The LBA setting in the BIOS is not correct. Most likely on drives that are more than 528MB, the LBA setting is not enabled. Enter the BIOS and enable the LBA.

    This can happen very easily when a drive is on a computer and works fine but then the motherboard is changed. The old BIOS had LBA enabled but the new one might not. After the drive is installed it seems to work .

7. There is a conflict with the IRQ settings

    a) Normally the primary hard drive controller uses Interrupt Request Line (IRQ) number 14 which allows the hard drive C and D to operate correctly.. The secondary hard drive controller uses IRQ number 15 which allows the hard drives E and F to work properly.

    What happens is some times a different device such as a sound card will use the IRQ 15 by default or because the settings was changed by a user. This causes the computer to not see the secondary hard drives immediately after the installation of this device using IRQ 15. The only way to fix this problem is to change this device so that it uses a different IRQ setting.

    b) Another problem can be introduced in Windows 95 by CD-ROM device drivers which are loaded by the autoexec.bat and config.sys files at boot up. If windows 95 sees a conflict with these drivers it will switch itself into the DOS compatibility mode. This can be seen by going to Control Panel/System/Performance/File system.

    A normal windows 95 installation uses 32 bit file access. When there is a conflict you will see that the system is switched to the DOS compatibility mode.
8. There is a conflict with the jumper settings

    All IDE hard drives must be properly setup using jumpers found on the hard drive. The users guide for each drive has instruction for these settings. Each drive can either be a Master or a Slave. Since there can be as many as two separate controllers on each computer the each controller can have a Master and a Slave.

    A typical computer with 4 IDE hard drives would setup the primary channel as Drives C (master) and D (slave), and the secondary channel as Drive E (master) and Drive F (slave).
On 2 drive systems, the first drive should be setup as Master and the second as Slave and the secondary channel is ignored.

    On many motherboards you must go into the BIOS and actually either enable or disable the secondary drive controller and save the changes. So if your computer came with 2 drives and you’ve added two more, before the new drives are detected you will need to go into the BIOS and enable the secondary IDE controller, save the changes and reboot.

9. The drive is unable to boot

    To troubleshoot this condition boot the computer with a bootable DOS disk. After the computer has booted with the disk try to access drive C: by issuing the standard directory command
DIR C: <enter>

    If the C: drive is working and you can see the directory listing then you might be able to make the drive bootable again by issuing the system command which transfers the system files from the floppy drive to the hard drive as follows:

        sys a: c: <enter>
The sys file has to be on the floppy disk. If it is not then find a disk that has the file or use another computer to copy the file to the floppy disk. You can also copy the command.com file from the floppy to the hard drive by typing…

        copy a:\command.com c:\command.com <enter>
…the computer will ask you to verify the operation.
Note: A drive must also have the BOOT partition activated before it can boot properly. This is done using FDISK.exe ( of course you must take precautions doing this since fdisk can make your drive useless if mis-used).

    To troubleshoot problems where the hard drive hangs at boot and the computer never responds, turn off the computer and disconnect the hard drive from the ribbon cable that connects it to the motherboard. When you turn the computer back on, you should at least get an error message about the drive being bad, and perhaps go into the BIOS. Once in BIOS you can change the hard drive type to AUTO and after shutting off the computer and reconnecting the hard drive, try again to see if it now works.

10. Fdisk reports wrong size when using drives larger then 64GB

    According to Microsoft KB article Q263044, “When you use Fdisk.exe to partition a hard disk that is larger than 64 GB (64 gigabytes, or 68,719,476,736 bytes) in size, Fdisk does not report the correct size of the hard disk.

    The size that Fdisk reports is the full size of the hard disk minus 64 GB. For example, if the physical drive is 70.3 GB (75,484,122,112 bytes) in size, Fdisk reports the drive as being 6.3 GB (6,764,579,840 bytes) in size.”

Hard drive error codes
Typically a hard drive failure will be indicated by an error code while the computer is booting.

        * 1701 – hard drive failure. …BIOS Post Codes
This BIOS error code is displayed during the computer boot process when the hard drive has failed.

          Also could be a cable connection problems as described above. IRQ conflicts and bad jumper settings could cause this problem as well.
One more possibility is that the CMOS battery has died. This can be verified by entering the BIOS during boot, then setting the hard drive settings and rebooting. If the hard drive error goes away then the battery is dead.

Parallel Port

This interface is found on the back of older PCs and is used for connecting external devices such as printers or a scanners. It uses a 25-pin connector (DB-25) and is rather large compared to most new interfaces. The parallel port is sometimes called a Centronics interface, since Centronics was the company that designed the original parallel port standard. It is sometimes also referred to as a printer port because the printer is the device most commonly attached to the parallel port. The latest parallel port standard, which supports the same connectors as the Centronics interface, is called the Enhanced Parallel Port (EPP). This standard supports bi-directional communication and can transfer data up to ten times faster than the original Centronics port. However, since the parallel port is a rather dated technology, don’t be surprised to see USB or Firewire interfaces completely replace parallel ports in the future.

Figure – Parallel port connector and signals

Case Cable Connections

Apart from the audio connetions (dealt with above) the other case connections all link to essentially the same are of my motherboard.

PC Build: Case connection headersThe image on the left shows the remaining motherbaord headers (boxed in red). On the left lies the main motherboard header whilst on the right is the blue header for the front panel USB sockets. First I’ll attach the USB cables (mostly as these are easier).

Front Panel USB Cables

PC Build: USB cables and connectosThe image to the left may be a little busy, but in concept it’s fairly simple. The image to the bottom left shows the USB headers on the motherboard and the image top left shows a schematic of these connectors indicating what each pin does. Basically the top row is for one USB connectoin and the bottom row is for another. The image top right shows one of the two USB connectors that connect to the front of the case. You will see that each of the little plugs is labelled with GROUND (corresponding to GND on the pin diagram), VCC (corresponding to USB_PWR on the diagram), and +D and -D, respectively (corresponding to P+6 and P-6 on the diagram). Fixing the cables is simply a matter of attaching the correct cables in the correct sequence. With a little care and attention this is actually fairly simple to achieve and when done correctly you will have the cables for both USB connections attached correctly as in the image on the bottom right.

With the USB cables connected we now only have the system panel headers to attach.

System Panel Header

PC Build: System panel header cables and connections

The final stage of attaching the various case cables is to attach the standard front panel connectors to the motherboard. These are the connectors for the case’s power switch, its reset switch, the case’s power LED and the LED which shows that the hard drive is being accessed. Depending on your case you may also have a system speaker cable (though this was not present on my system). The image top left shows how the cables attach to the pins on the motherboard and the motherboard header itself is shown bottom left. The cables and connectors that need to be attached are shown top right. For my case the white cable on each connector represented the negative lead (marked – on the schematic). Once I’d identified the cables it was a fairly simple process to match the cable to the description of it on the schematic (such as POWER SW matching to PWRBTN#) and then to plug it into the motherboard in the appropriate position so that I attained the image on the bottom right. I tend to work from the left hand edge of the header to the right as it’s then easier to see what’s going on and more difficult to make an error as a result.

The Power Supply Unit (PSU)


In general, a Power Supply Unit (PSU) is defined as: ‘a device or system that supplies electrical or other types of energy to an output load or group of loads.’ Most commonly the term PSU is used in reference to the power supplies of electrical or electronic devices.

Page Map

Power Supplies and Computers Power Supplies and Case ‘Modding’
Computer Power Requirements Power Supply and Cooling



Power Supplies and Computers

Build PC CPU: Back wiring of ESDAC computer

In computer terms the power supply is a transformer that converts the 110V/240v alternating current of the domestic power supply into the 12v direct current supply required to drive all a computer’s components.

In general, the power supply comes as an integral component of the computer case but it is often only attached by five or six screws and if you are upgrading it can be swapped for a more powerful power supply unit.

In common with all electrical devices how powerful a power supply is measured in terms of their wattage. Low-end power supplies usually start at 350W and truly high-end power supplies can reach 600W. Given this range, what is the right power of PSU for you?

Computer Power Requirements

Being electronic devices all the components of the modern PC require electrical power. Everything from the DVD and CD-rom drives, the motherboard itself, the fans, and most especially the CPU and the graphics card.

Build PC PSU: Image of AT and ATX power connectors

Power supplies come in two main flavours: AT and ATX named after the design of the motherboard and the power connectors on those motherboards. Though the AT motherboard is now a legacy system power supplies for this type of motherboard (with the single row connector, far left) are still sold. Modern motherboards, however, all come with an ATX connector (dual row connector immediate left). Please note that the two connectors are not interchangeable and a PSU designed for an ATX system will not work in an AT system (and vice-versa).

Considering that motherboards are now using two power connectors (one for the motherboard and a four-pin connector for the CPU), video cards now need additional power connectors, and not to mention all the additional components, it’s not surprising that today’s computers simply need more than 350W to run everything effectively. As a minimum you need a 400W power supply and you are probably best buying a case with a 450W power supply. Indeed, a 450W power supply is becoming an increasingly-common component in most modern system cases.

It is also the case that if your power supply isn’t strong enough to support all the components in your PC you will soon know about it. The machine may work erratically, you may suffer system errors and crashes, the video display may freeze and some of your components such as DVD drives and hard drives may simply fail to work. If this is the case, and there is no other obvious cause for these errors you should upgrade your power supply. (This is really not as daunting a procedure as it sounds. All you need is a screwdriver and a little confidence).


Power Supplies and Case ‘Modding’

500W ATX power supply with blue LEDs

‘Modding’, or ‘modifying’ is a relatively new phenomenon in PC terms. Here components are bought or upgraded because they will add something to the overall appearance of the PC. Often they will include LEDs (light emitting diodes) that display coloured lights of patterns of lights. A good example being the 500W power supply shown here on the left. This has a number of blue LEDs built in and can be used with a transparent or semi-transparent computer case so that when switched on the entire inside of the case glows a blue colour.

Power Supply and Cooling

The power supply is one of the noisiest components in a modern computer and this is mainly due to the design of the ATX case. Indeed, in the ATX case specification, cool air is drawn into a typical case from vents in the front panel. The incoming air helps cool components as it moves through the case, becoming warm in the process. It is evacuated through the PSU and out the rear by the PSU fan. So the loud, fast fans do help to keep a case cooler. Manually varying a high airflow PSU fan can cause CPU temperature to be affected as much as 5–6°C. The problem is that the ATX specification was designed in the 1980s when computers ran at no more than 30W and a relatively slow turning (and therefore quieter) CPU fan was sufficient to cool the entire machine. These days modern PCs typically run at 130W and to cope with the additional heat generated CPU fans need to run much faster. This causes more vibration and more noise. If noise is a real concern for you you may be able to use a PSU with a slower-spinning fan (which are generally available) as long as your fit enough additional fans in your machine’s case.

Building your PC: The Motherboard


Though used almost synonymously with the mainboard in computer systems, the motherboard is actually a generic term referring to the mainboard of any device where substantial or complex calculations occur. Thus a computer, games console, PDA or handheld game will usually have a motherboard whilst other complex devices such as a TV, stereo or microwave will have a mainboard. A motherboard can also be called a mainboard, logic board, system board or it may even be named using the abbreviation mobo. A typical modern motherboard provides direct connections for the CPU, memory, graphics card, sound card and network cards. The motherboard also provides controllers and connectors for hard disk drives, CD and DVD drives as well as a floppy drive by means of ribbon cables.

Page Map

Motherboard History     FDD Header
    8086 CPU     Graphics Connector
How it Works         AGP Graphics
    CPU Zif Socket         PCI-E Graphics
        Intel Processors     Expansion Ports
        AMD Processors         PCI Bus
    Northbridge and Southbridge Chipsets         PCIe Bus
        Northbridge Chipset     Back Pane Connectors
        Southbridge Chipset     Additional Connectors
    Memory Slots         ATX Power Connector
    Bios         CPU Supplementary Power
    CMOS         Fan Connector
    ATA Headers         Audio Connector
    Serial ATA Headers Summary


The Motherboard:


Build PC Motherboard:  AT form-factor motherboard

Almost by definition the motherboard has been a feature of computers since the very earliest days. However, it was only with the advent of semiconductor devices and automated assembly during the early 1970s that the motherboard as we know it today was born. For the early years of the computer, however, most computers were produced as single units by the machine’s manufacturer. Even here motherboard design was evolving from the PC to the XT form factors. IBM made this an ‘open standard’ allowing a number of other manufacturers to copy their deigns. However, it was not until IBM came up with the AT (advanced technology) design in 1984 that the component-based PC which we know today was born. The AT form factor (see image, left) proved very popular and the various IBM PC clone manufacturers of the time began to use AT-compatible designs. Though not revolutionary in design the AT form factor provided for a remote power switch (now standard) as well as the ability to build a motherboard that would fit into desktop, mini-tower and full tower cases. The power supply for the AT motherboard was also increased from the 84W of previous models to 194W allowing for the addition of far more components and peripherals.

Build PC Motherboard: Image of an ATX motherboard

In 1995 Intel developed the ATX (Advanced Technology Extended) motherboard form factor. This represented the first major design evolution in motherboard and case designs for almost a decade and ATX motherboards were not compatible with previous AT designs. Unlike the AT design an ATX power supply does not directly connect to the system power button, allowing the computer to be powered off via software. However, many ATX power supplies have a switch on the back to ensure no power is flowing to the motherboard (a trickle of energy is normally sent to an ATX-style motherboard even if the computer appears to be “off”). Since the ATX PSU uses the motherboard’s power switch, to turn on the power for use in situations that do not utilize an ATX motherboard it is possible to activate the power supply by shorting the green wire from the ATX connector to any black wire on the connector (or ground). In addition the power supply’s connection to the motherboard was changed. An ATX power supply uses a single connector that can only be plugged into the motherboard in a single orientation (thus alleviating the problem encountered with the AT motherboard that the power supply was reversible and could short-circuit the motherboard). The other major change was to dedicate a single area at the back of the motherboard where all connectors could be placed. (In the AT design only the keyboard connector could go here). ATX motherboards also saw the ubiquitous introduction of the PS/2 keyboard and mouse rather than the large 5-pin DIN connector of the AT design.

In 2005 Intel introduced the BTX (Balanced Technology Extended) form factor for PC motherboards. This motherboard was designed to alleviate some of the power and thermal problems of the ageing ATX design. However, as yet it has not received widespread support and the ATX form factor remains the current industry standard.

The Motherboard: How it Works

Build PC Motherboard: Components on the Motherboard

Admittedly, the image above is quite ‘busy’. Seemingly there’s a lot happening. So let’s focus on individual components and explain what they do.

1. CPU Zif Socket

Build PC Motherboard: CPU ZIF socket

This is the socket in which the CPU (the main chip or ‘central processing unit’) actually sits. In fact the socket (left) is composed of two parts. The white central region where the CPU itself sits and the black outer layer where the CPU’s cooling fan sits. The central CPU housing is called a ZIF (which stands for zero insertion force) as it has a plastic upper layer and a metal lower layer. By means of a crank handle the upper and lower parts slide apart leaving a clear path by which the pins on the underside of the CPU housing can be inserted. Once the CPU is firmly in place the handle is levered down and this pinches the pins of the CPU against the metal base plate, making an electrical connection.

Each zif socket is specific for a certain type of CPU. As such the CPU has to be matched exactly to the ‘socket type’ of the CPU. As of 2006 the current socket types are:

Intel Processors

  • Socket 6 — 80486DX4
  • Socket 7 — Pentium and Pentium MMX, AMD and some Cyrix CPUs)
  • Socket 8 — Intel Pentium Pro
  • Slot 1 — Intel Pentium II, older Pentium III, and Celeron processors (233 MHz–1.13 GHz)
  • Slot 2 — Intel Xeon processors based on Pentium II/III cores
  • Socket 370 — Celeron processors and newer Pentium IIIs (800 MHz–1.4 GHz)
  • Socket 423 — Intel Pentium 4 and Celeron processors (based on the Willamette core)
  • Socket 478 — Intel Pentium 4 and Celeron processors (based on Northwood, Prescott, and Willamette cores)
  • Socket 479 — Intel Pentium M and Celeron M processors (based on the Banias and Dothan cores)
  • Socket 480 — Intel Pentium M processors (based on the Yonah core)
  • Socket 603/604 — Intel Xeon processors based on the Northwood and Willamette Pentium 4 cores
  • Socket T/LGA 775 (Land Grid Array) — Intel Pentium 4 and Celeron processors (based on Northwood and Prescott cores)

AMD Processors

  • Slot A — original AMD Athlon processors
  • Socket 462 (aka Socket A) — newer AMD Athlon, Athlon XP, Sempron, and Duron processors
  • Socket 754 — lower end AMD Athlon 64 and Sempron processors with single-channel memory support
  • Socket 939 — AMD Athlon 64 and AMD Athlon FX processors with dual-channel memory support
  • Socket 940 — AMD Opteron and early AMD Athlon FX processors

In matching processors to motherboard the easiest way is to decide on which particular processor you want, check its socket type and then look at all the matching motherboards. Make a list of these and then check their features against what you want your computer to achieve. That way you can decide on purchasing your CPU and motherboard in tandem to ensure an exact match.

2. Northbridge and Southbridge Chipsets

The Northbridge Chipset Build PC Motherboard: Northbirdge chipset

This rather unprepossessing pair of chips actually play a very important role on the motherboard. Along with the CPU the Northbridge and Southbridge chips form the ‘core logic chipset’ of the motherboard. Though both chipsets have been incorporated into a single die in the past it is far more common to see them as two separate microprocessors.

Build PC Motherboard: motherboard core logic system

The schematic (left) shows the relationship between the CPU and the Northbridge and Southbridge chipsets. This diagram also explains the names Northbridge and Southbridge. If the CPU is considered as lying due North then the next chip in the sequence is the northernmost ‘bridge’ between the CPU and the remainder of the system whilst the final (most southernmost bridge) is the Southbridge.

It is the Northbridge chipset that is often the primary factor in deciding the number, speed and type of CPU that can be utilized as well as the amount, speed and type of RAM that can be accessed. It is the Northbridge’s clock frequency that is used as a baseline for the CPU to determine its own frequency. Because different processors and RAM require different signalling, a northbridge will typically work with only one or two classes of CPUs and generally only one type of RAM. Also, a Northbridge chip will typically only work with one or two different Southbridge integrated circuits. Thus it could be argued that the northbridge is the most important factor in determining which technologies are available on a given motherboard.

However, the Northbridge may be a technology on the verge of obsolescence. In the latest generation of AMD 64 processors the memory controller has already been incorporated into the CPU itself and with the evolution of PCI express (see below) as a video card technology the need for a Northbridge chipset to drive video controllers is also falling by the wayside.

The Southbridge Chipset

In contrast with the Northbridge chipset which mediates all the ‘fast’ capabilities of the computer (memory and graphics) the Southbridge chip effectively implements the ‘slower’ capabilities of the motherboard. On contemporary motherboards the Southbridge chipset will contain interfaces for the PCI bus (where additional cards are added [see below]), the system management bus (which handles things like the interrupt button and a modem’s ‘wake on LAN’ functionality) the DMA controller (which allows certain subsystems on the motherboard to directly access system memory), the drive controller (both IDE and SATA), the LPC (low pin count controller) that allows low bandwidth devices such as audio controllers and the boot ROM to communicate with the CPU, serial and parallel ports, keyboard and mouse connectors), the RTC (real-time clock) which is the motherboard chip that maintains track of the current date and time even when the computer is turned off, the power management subsystem (which allows the BIOS to perform power management tasks such as slowing the speed of the CPU and the BIOS memory. In addition the Southbridge of modern systems may include support for Ethernet, RAID, audio codec and FireWire devices.

3. Memory Slots

Build PC Motherboard: memory slots

Under the control of the Northbridge chipset, these memory slots represent where the memory modules are inserted in a modern PC. For a modern motherboard the memory will undoubtedly be DDR or DDR2 with the type determined by the clock-speed of the CPU (and ultimately on the clock-speed of the Northbridge chip). To learn more about memory modules and which ones to purchase for your system please visit this page.


Build PC Motherboard: the BIOS

The acronym BIOS stands for Basic Input/Output System and represents the software code that’s run by a computer when first turned on. The primary function of this is to prepare the machine so other software programs stored on various media (such as hard drives, floppies, and CDs) can load, execute, and assume control of the computer and it is this process which his generally known as ‘booting-up’.

In effect the BIOS is a program that’s encoded a chip (an example of which is shown above). Originally the BIOS was encoded onto PROM or EPROM programmable chips, but these days is is more commonly written onto flash memory which is included as an integral part of the motherboard. After initialization the BIOS performs an initial check on system integrity and then decompresses itself from the BIOS memory space on the flash RAM and loads itself into main memory where it starts executing. In addition nearly all BIOS implementations can optionally execute a setup program interfacing the nonvolatile BIOS memory (CMOS) which holds user-defined data accessed by BIOS code.

BIOS can sometimes be referred to as firmware as it is an integral part of the motherboard and thus the system hardware. With the advent of flash memory BIOS it is now possible to update the bios of a motherboard by flashing it (replacing the original code with new code) so that the motherboard can keep pace with updates in technology. This obviously can be a dangerous process because the BIOS can become corrupted and if it does the system will not boot. However, modern BIOSes evaluate their own integrity and if there is a problem with the BIOS (if it is ‘corrupt’) they will boot to a floppy drive so that the user can try flashing it again (this remains the most convincing reason for purchasing an integral floppy drive with a new computer system).


Build PC Motherboard: button battery

On almost every modern motherboard you will notice a tiny button battery (like the one on the left). This is a small rechargeable cell that is used to power the Nonvolatile BIOS memory (generally referred to as CMOS) when the main power is off. In effect, the CMOS is an area of memory that contains BIOS settings and sometimes the code used to initialize the computer and load the operating system. Though modern motherboards utilize a flash memory chip or an EPROM for this function the original versions used a low-power CMOS memory chip and this was kept powered by the back-up battery. These days, however, as the memory itself is non-volatile the function of the battery is to power the RTC chip (this provides the real-time clock function that keeps track of time and date). On occasion, such as when a device is added to or removed from your PC the CMOS internal integrity check may report a CMOS mismatch. In this case the POST (power-on self-test) will fail and the computer will refuse to boot. The only way to overcome this is to re-set the CMOS which can be done by means of a jumper located on the motherboard. All settings are set to default and, with any luck, the problem can be rectified.

6. ATA Headers

Build PC Motherboard: ATA cable Build PC Motherboard: ATA headers

ATA stands for Advanced Technology Attachment and represents a standard interface for connecting storage devices such as hard disks and CD/DVD drives inside personal computers. With the introduction of serial ATA technology in 2003 (see below) the standard ATA interface is now known as parallel ATA. Although the standard for this interface as always had the official name “ATA”, marketing dictates dubbed an early version of the standard Integrated Drive Electronics (IDE), and the one following it Enhanced IDE (EIDE). The parallel ATA interface itself comprises of an 40-pin connector on the motherboard (above) which connects to an 80-wire ribbon cable that links the motherboard to the drive being attached. It should be noted that the specified maximum cable length for the ATA standard is just 46cm which can make connecting multiple drives on large cases difficult and though longer cables are available (and though they will generally work) they do lie outside the specified parameters of the standard. Modern motherboards generally have two ATA risers which are usually of the ATA-133 standard (often known as Ultra DMA 133 [UDMA133]) where the maximum data transfer rate over the PCI bus on the motherboard is 133 Mb/sec. This standard also removed the bottleneck of older specifications, allowing drives of 200Gb or greater to be attached.

Most modern motherboards have two ATA risers allowing up to four drives to be attached. Each riser can handle two drives; one device 0 (master) and one device 1 (slave). In the ATA133 standard the master device is defined as the drive at the end of the cable and the slave is the device attached to the middle connector. The jumper settings on the attached drives should correspond to this (see the hard drive page for more information).

7. Serial ATA Headers

Build PC Motherboard: Serial ATA cable Build PC Motherboard: Serial ATA header

Serial-ATA (usually abbreviated to S-ATA or SATA) stands for Serial Advanced Technology Attachment and is a standard for the attachment of external drives introduced in 2003. Primarily intended for the attachment of hard disks, SATA is seen as the successor to Parallel ATA technology (see above). First-generation Serial ATA interfaces, also known as SATA/150, run at 1.5 gigahertz, resulting in an actual data transfer rate of 1.2 gigabits per second (Gbit/s), or 150 megabytes per second. This transfer rate is only slightly higher than that provided by the fastest Parallel ATA mode, Ultra ATA at 133 MB/second (UDMA/133), but the relative simplicity of a serial link allows for the use of longer cables and also provides a much easier path for transitioning to higher speeds. Almost all modern motherboards now come with at least one SATA connector (left). In 2004 the SATA/300 specification was released, allowing a doubling of the clock rate to 3GHz for a maximum throughput of 300 MB/s or 2.4 gigabits per second (Gbit/s). SATA/300 is backwards-compatible with SATA/150 devices, allowing SATA/150 hardware to interface with SATA/300 ports and SATA/300 hardware with SATA/150 ports (albeit the latter at the slower 150 MB/s data rate). Somewhere in 2007 it is expected that there will be a further increase in the maximum throughput of Serial ATA to 600 MB/s, 4.8 gigabits per second (Gbit/s).

The most notable difference between parallel-ATA and S-ATA lies in the cabling. he Serial ATA standard defines a data cable using seven conductors and 8 mm wide wafer connectors on each end and these cables can be up to a metre in length.

SATA also drops the master/slave concept, giving each device its own dedicated connection, once again allowing for better data throughput. As all modern motherboards can boot from a SATA drive, if you have a motherboard that supports this technology buy a hard drive that supports it and use this as your main drive. Relegate parallel-ATA to DVD/CD drives and secondary hard drives.

8. FDD Header

Build PC Motherboard: Floppy drive cable Build PC Motherboard: Floppy drive header

The floppy drive is considered a ‘legacy’ item on the motherboard these days. It is no longer viewed as an essential component of a PC build, though it can still be useful in recovering from a BIOS update problem. However, all motherboards currently include a 34-pin FDD header. This can be connected to a floppy disk drive via a 34-wire ribbon cable which implements an ATA interface specifically for the floppy disk drive. Plugging the floppy drive is a simple matter of adding the drive to the case, plugging-in the power cable and attaching the data cable to the drive and the motherboard. It should be noted that some (especially older) BIOSes require the presence of a floppy drive before they pass the initial POST (power-on self-test) process and begin loading the operating system. For more on the floppy disk drive please view this page.

9. Graphics Connector

Build PC Motherboard: Graphics Card slots

In common with many other components of the motherboard, graphics connectors have undergone considerable change, from dedicated cards to standard PCI cards to AGP and now PIC-E. Most modern motherboards (even the ones that come with on-board graphics) will contain either and AGP or a PCI-E slot for a dedicated graphics card.

AGP Graphics

Of the two current formats for graphics cards this is the oldest (the slot for these types of cards is shown above, top. AGP is an acronym for Advanced Graphics Port which is a high-speed speed point-to-point channel for attaching a graphics card to a computer’s motherboard, primarily to assist in the acceleration of 3D computer graphics. The original specification of AGP (1x) used a 32-bit channel operating at 66 MHz resulting in a maximum data rate of 266 megabytes per second (MB/s), doubled from the 133 MB/s transfer rate of PCI bus 33 MHz / 32-bit; 3.3 V signaling. Every few years the data transfer rate was doubled until 2004 when the AGP 8x standard was released. This delivered an effective 533 MHz resulting in a maximum data rate of 2133 MB/s (2 GB/s); 0.8 V signaling.

AGP grew from the need for more complex image processing that the limited standard PCI bus could deliver. A dedicated graphics bus and slot allowed better data throughput and more realistic rendering of three-dimensional environments.

PCI-E Graphics

PCI-E stands for PCI-Express (sometimes known as PCIe). This is the latest specification for graphics card technologies and the port for these cards is shown in the bottom pane of the above image. It is an implementation of the PCI computer bus that uses existing PCI programming concepts, but bases it on a completely different and much faster serial physical-layer communications protocol which allows for considerable parallelism in the interface and therefore large data throughput. Because of this parallelism the size of the PCIe socket grows between the 1x, 4x, 8x and 16x versions of this interface.

The PCIe link is built around a bidirectional, serial (1-bit), point-to-point connection known as a “lane”. This is in sharp contrast to the PCI connection, which is a bus-based system where all the devices share the same unidirectional, 32-bit, parallel bus. A connection between any two PCIe devices is known as a “link”, and is built up from a collection of 1 or more lanes. All devices must minimally support single-lane (x1) links. Devices may optionally support wider links composed of 2, 4, 8, 12, 16, or 32 lanes. As a result of this PCIe is useful in applications other than graphics cards and currently PCI Express appears to be well on its way to becoming the new backplane standard in personal computers.

Most of the latest graphics cards from all the major manufacturers now use PCI-Express and if you’re buying a new motherboard it is worth buying one employing this technology.

10. Expansion Ports

Build PC Motherboard: PCI expansion slot

The expansion ports as sited at the back of the motherboard and align with the back-plate of the case. It is via these prots that expansion cards such as sound cards, network cards, wireless cards and firewire cards. Originally such devices were attached to an ISA (Industry Standard Architecture) bus which was developed by IBM around 1981. This provided the first standard whereby any manufacturer could provide an add-on card to extend the capabilities of a PC. The original bus was 8-bit but was replaced by a 16-bit architecture in the mid 1980s. The ISA bus survived into the early 1990s when it was finally displaced by the PCI bus.


Like its predecessor the VESA bus, the PCI bus (short for Peripheral Component Interconnect) standard specifies a computer bus for attaching peripheral devices to a computer motherboard. The standard itself was released in 1992 and the full specification of the connector and motherboard slot was published in 1993 and it was not until the release of the second generation pentium processors in late 1994 that the PCI bus began to achieve some measure of market penetration in consumer PCs. By the early 2000s the PCI bus had become common, though EISA (Extended Industry Standard Architecture, a 32-bit extension to VESA) slots continued to be added alongside. It was not until 2003 that the EISA bus disappeared entirely and newer motherboards came with PCI buses only (or maybe a single low-throughput CNR slot for modems.

The PCIe Bus

The PCI-express (PCIe) bus standard was published in 2003 and was initially adopted for video cards (see above). However, the architecture is multi-purpose and obviates the need for a Northbridge chip. Modern motherboards tend to come with a few PCI connectors and a number of PCIe connectors (see above) and, as well as graphics cards, most new Gigabit ethernet cards and even some wireless cards now use PCIe. Undoubtedly the trend will be to replace PCI with PCIe and other peripheral cards (such as sound cards) will soon be based around the PCIe standard. For the moment ensure you have a PCIe card for your graphics card and make sure you have enough PCI slots for your standard add-on cards.

11. Back Pane Connectors

Build PC Motherboard: Back pane connectors

According to the ATX motherboard specification a square region to the far right of the motherboard’s rear edge is reserved for all the connectors and devices directly supported by the motherboard itself. As standard there will be PS/2 D-type connectors for a keyboard and a mouse as well as a parallel prot to support a printer and probably at least one COM port (these are considered the most basic external connector ports on a computer and provide serial connectivity for modems and PDAs). However, all the ports mentioned thus far could be considered legacy systems and all their functionality is now encapsulated within the USB universal serial bus protocol. You should have at least two of these prots on a modern motherboard and many manufacturers have done away with the serial ports entirely so that they can include more USB connectors.

Most modern motherboards will also include an RJ-45 connector for ethernet connection to a local network. Many modern motherboards will also come with audio connectors and a game port connector. It may even have a video port if the motherboard supports on-board graphics.

All these port functions are supported by add-on cards, so the lesson is to pick a motherboard that has all the connections built-in for what you want to do (another option available with some motherboards being FireWire).

12. Additional Connectors

There are a few other connectors on the motherboard that you need to know about and I’ll run through these quickly:

ATX Power Connector Build PC Motherboard: ATX power connector

This is the main power connector on the motherboard and a single connector from the ATX power supply will plug in here. The connector is directional so that it can only fit one way.

CPU supplementary Power Connector Build PC Motherboard: cpu power connector

On many modern ATX motherboards there is an additional power connector that provides power solely to the CPU.

Fan Connector Build PC Motherboard: Fan connector

Next to the CPU Zif socket there is a three-pin connector where the CPU fan plugs in. This both powers the fan and controls the fan speed and power-up cycles. On some motherboards there may be a second one of these to power a case fan.

Audio Connector Build PC Motherboard: Audio Connector

Almost all motherboards these days have a supplementary connector to which the audio cable from a CD or DVD can be connected. This links the audio output from those devices to the motherboard’s audio system so that sound can be output to external speakers.


  • An opening in a computer where a circuit board can be inserted to add new capabilities to the computer.
  • Nearly all personal computers except portables contain expansion slots for adding more memory, graphics capabilities, and support for special devices.
  • The boards inserted into the expansion slots are called expansion boards, expansion cards , cards , add-ins , and add-ons.

Expansion slots for PCs come in two basic sizes: half- and full-size.

  • Half-size slots are also called 8-bit slots because they can transfer 8 bits at a time.
  • Full-size slots are sometimes called 16-bit slots. I
  • In addition, modern PCs include PCI slots for 
  • expansion boards that connect directly to the PCI bus.

Expansion slot standards

  • PCI Express
  • AGP
  • PCI
  • ISA
  • MCA
  • VLB
  • CardBus/PC card/PCMCIA (for notebook computers)
  • ExpressCard
  • Compact flash (for handheld computers)
  • SBus (1990s SPARC-based Sun computers)
  • Zorro (Commodore Amiga)
  • NuBus (Apple Macintosh)

Expansion card types

  • Video cards
  • Sound cards
  • Network cards
  • TV tuner cards
  • Video processing expansion cards
  • Modems
  • Host adapters such as SCSI and RAID controllers.
  • POST cards
  • BIOS Expansion ROM cards
  • Compatibility card (legacy)
  • Physics cards, only recently became commercially available.
  • Disk controller cards (for fixed- or removable-media drives)
  • Interface adapter cards, including parallel port cards, serial port cards, multi-I/O cards, USB port cards, and proprietary interface cards.
  • RAM disks, e.g. i-RAM
  • Memory expansion cards (legacy)
  • Hard disk cards (legacy)
  • Clock/calendar cards (legacy)
  • Security device cards
  • Radio tuner cards

What is the Category Rating System?

Electronic Industries Association (EIA) developed the TIA/EIA-568-A standard, which specifies wiring and performance standards for Unshielded Twisted Pair (UTP) cabling. Category Rating System specifies the definition of performance categories for 100 ohm UTP cabling system.

Category 3 specifies the twisted pair cable and connecting hardware that can support transmission frequency up to 16MHz, and data rates up to 10Mbps. This is primarily used in telephone wiring.

Category 4 specifies cables and connectors that supports up to 20MHz and data rates up to 16Mbps. With introduction of category 5, this is a rarely used category.

Category 5 specifies cables and connectors that supports up to 100MHz and data rates up to 100Mbps. With 100BaseT Ethernet today, Category 5 is a widely used cabling system that matches todays high-speed data requirements.

Category TIA/EIA Standard Description
Cat 1 None POTS, ISDN and doorbell wiring
Cat 2 None 4 Mbps token ring networks
Cat 3 TIA/EIA 568-B 10 Mbps Ethernet – frequency up to 16MHz
Cat 4 None 16 Mbps token ring networks – frequency up to 20MHz
Cat 5 None 100 Mbps Ethernet – frequency up to 100 MHz
Not suitable for GigE (1000BaseT)
Cat 5e TIA/EIA 568-B 100 Mbps & GigE Ethernet – frequency up to 100 MHz
Cat 6 TIA/EIA 568-B 2x Performance of Cat 5 & 5e – frequency up to 250 MHz
Cat 6a None Future specification for 10Gbps application
Cat 7 ISO/IEC 11801 Class F Designed for transmission at frequencies up to 600 MHz

What is RJ stands for?

RJ stands for Registered Jacks. These are used in telephone and data jack wiring registered with FCC. RJ-11 is a 6-position, 4-conductor jack used in telephone wiring, and RJ-45 is a 8-position, 8-conductor jack used in 10BaseT and 100BaseT ethernet wiring.

Crossover Cable Diagram

  1. Using an cat5 cutter and crimping tool, strip about 1/3″of the out jacket of the cat-5 cable.  Be sure not to strip or damage any of the pairs of inner cables.

    Cut Cat-5 Cable

  2. Assemble the pairs of wires in the following order for network cables (EAI standard / TIA-568B).

    CAT-5 Wiring Diagram

  3. Insert the wires into the RJ45 jack as seen below.  Be sure to keep the wires in the correct order.

    CAT-5 to RJ45

  4. Insert the RJ45 connector into the crimping tool (again carefully make sure the wires stay inserted in the correct order).  Crimp down firmly on the crimping tool to permanently attach the RJ45 to the CAT5 cable.

    Crimp RJ45

Crossover Cable Diagram

To create a crossover cable with cat-5 cable follow the same instructions as above for CAT-5 wiring except when you get to step #2, use the below crossover cable diagram:

Crossover Cable Diagram

RJ-45 Crossover Ethernet Cable

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