Computer motherboard and its constituent components
Computer Motherboard and Its constituent components:
There are primarily two types of motherboards, AT motherboard, and ATX
motherboard. AT motherboards are older, and not commonly used now a days. The
AT and ATX motherboards differ in the form factor. Full AT is 12" wide x 13.8"
deep, and Baby AT is 8.57" wide x 13.04" deep. Full-ATX is 12" wide x 9.6" deep
and Mini-ATX is 11.2" wide x 8.2" deep. Other major differences include power
supply connector, and keyboard connector. AT has 5-pin large keyboard connector,
where as ATX has 6-pin mini connector. Similarly, AT has single row two connectors
+/-5V, and +/-12V, whereas ATX motherboard has double row single connector
providing +/-5V, +/-12V, and +3.3V.
A typical ATX PC motherboard with constituent components is given below:
The important constituent components of an ATX Motherboard are given below:
1. Mouse & keyboard
3. Parallel port
4. CPU Chip
5. RAM slots
6. Floppy controller
7. IDE controller
8. PCI slot
9. ISA slot
13.Power supply plug in
1. Mouse & keyboard: Keyboard Connectors are two types basically. All PCs
have a Key board port connected directly to the motherboard. The oldest, but
still quite common type, is a special DIN, and most PCs until recently retained
this style connector. The AT-style keyboard connector is quickly disappearing,
being replaced by the smaller mini DIN PS/2-style keyboard connector.
You can use an AT-style keyboard with a PS/2-style socket (or
the other way around) by using a converter. Although the AT connector is
unique in PCs, the PS/2-style mini-DIN is also used in more modern PCs for
the mouse. Fortunately , most PCs that use the mini-DIN for both the
keyboard and mouse clearly mark each mini-DIN socket as to its correct use.
Some keyboards have a USB connection, but these are fairly rare compared to
the PS/2 connection keyboards.
2. USB (Universal serial bus):
USB is the General-purpose connection for PC. You can find USB versions of
many different devices, such as mice, keyboards, scanners, cameras, and
even printers. a USB connector's distinctive rectangular shape makes it easily
USB has a number of features that makes it particularly popular on PCs. First,
USB devices are hot swappable. You can insert or remove them without
restarting your system.
3. Parallel port: Most printers use a special connector called a parallel port.
Parallel port carry data on more than one wire, as opposed to the serial port,
which uses only one wire. Parallel ports use a 25-pin female DB connector.
Parallel ports are directly supported by the motherboard through a direct
connection or through a dangle.
4. CPU Chip : The central processing unit, also called the microprocessor
performs all the calculations that take place inside a pc. CPUs come in Variety
of shapes and sizes.
Modern CPUs generate a lot of heat and thus require a cooling fan or heat
sink. The cooling device (such as a cooling fan) is removable, although some
CPU manufactures sell the CPU with a fan permanently attached.
5. RAM slots: Random-Access Memory (RAM) stores programs and data
currently being used by the CPU. RAM is measured in units called bytes. RAM
has been packaged in many different ways. The most current package is called
a 168-pin DIMM (Dual Inline Memory module).
6. Floppy controller: The floppy drive connects to the computer via a 34-pin
ribbon cable, which in turn connects to the motherboard. A floppy controller is
one that is used to control the floppy drive.
7. IDE controller: Industry standards define two common types of hard
drives: EIDE and SCSI. Majority of the PCs use EIDE drives. SCSI drives show
up in high end PCs such as network servers or graphical workstations. The
EIDE drive connects to the hard drive via a 2-inch-wide, 40-pin ribbon cable,
which in turn connects to the motherboard. IDE controller is responsible for
controlling the hard drive.
8. PCI slot: Intel introduced the Peripheral component interconnect bus
protocol. The PCI bus is used to connect I/O devices (such as NIC or RAID
controllers) to the main logic of the computer. PCI bus has replaced the ISA
9. ISA slot: (Industry Standard Architecture) It is the standard architecture
of the Expansion bus. Motherboard may contain some slots to connect ISA
10. CMOS Battery: To provide CMOS with the power when the computer is
turned off all motherboards comes with a battery. These batteries mount on
the motherboard in one of three ways: the obsolete external battery, the most
common onboard battery, and built-in battery.
11. AGP slot: If you have a modern motherboard, you will almost certainly
notice a single connector that looks like a PCI slot, but is slightly shorter and
usually brown. You also probably have a video card inserted into this slot. This
is an Advanced Graphics Port (AGP) slot
12. CPU slot: To install the CPU, just slide it straight down into the slot.
Special notches in the slot make it impossible to install them incorrectly. So
remember if it does not go easily, it is probably not correct. Be sure to plug in
the CPU fan's power.
13. Power supply plug in:
The Power supply, as its name implies, provides the necessary electrical power
to make the pc operate. the power supply takes standard 110-V AC power and
converts into +/-12-Volt, +/-5-Volt, and 3.3-Volt DC power.
The power supply connector has 20-pins, and the connector can go in only one
BIOS stands for Basic Input/Output System. It contains basic instructions to interact with various
hardware modules such as Motherboard controllers or that of interface cards. BIOS is the software
that is run by a computer when first powered on.
A computer motherboard inevitably contains a BIOS chip in the form of an onboard PROM,
EPROM or flash memory. When the computer is powered on, it performs diagnostic tests on the
computer hardware devices such as hard drive, FDD, and memory. It searches for other BIOS's on
the plug-in boards, and takes care of them. It then loads the operating system and passes control to
OS. The BIOS accepts requests from the drivers as well as the applications as shown in the figure
BIOS is also known as PC firmware because it is an integral part of the motherboard.
Firmware on adapter cards: A computer can contain several BIOS firmware chips. The motherboard
BIOS is normally used to access basic hardware components such as the keyboard, floppy drives,
and hard disk controllers. Adapter cards such as SCSI, RAID, and video boards may include their
own BIOS software.
Firmware generally available in different forms:
1. EPROM (Erasable Programmable ROM), for updating a BIOS firm using EPROM, you may need
to get a new chip from the manufacturer.
2. EEPROM (Electrically Erasable Programmable ROM), you can update a BIOS firmware using
EEPROM using "boot to floppy", and running the Firmware update program.
3. Flash ROM - faster at rewriting the chip
A typical BIOS chip used on motherboards is shown in the figure below. Though it is a square-type
PLCC package, BIOS chips come in different forms. Figure 2 shows the BIOS after insertion into the
BIOS chip BIOS chip after insertion into a socket.
PC memory stores data and programs currently being executed by the computer. It is important that
the information is fetched by the CPU quickly to further processing. There are several memory types
available. Important among there include the following:
Dynamic RAM (DRAM)
Synchronous RAM (SRAM)
Synchronous DRAM (SDRAM)
Rambus DRAM (RDRAM)
Video RAM (VRAM)
Windows RAM (WRAM)
RAM stands for Ramdom Access Memory.
Dynamic RAM (DRAM): In dynamic RAM, the RAM gets refreshed continually by the controller.
DRAM has been introduced in the earlier stages, and RAM versions available today are much bigger
and faster than the earlier simple DRAMs.
DRAMs store data in the form of capacitive charges. Since any capacitor tends to be leaky, a DRAM
needs to be refreshed on a continual basis.
Synchronous RAM (SRAM): SRAM contains a clock built onto the memory module, enabling the
SRAM to be in synchronization with the motherboard cloak. SDRAM doesn't require frequent
recharge like DRAM. L-2 memory caches are usually made of SRAM and exhibit very fast read and
Synchronous DRAM (SDRAM): SDRAM works in sync with the motherboard, and hence works
quite fast. SDRAMs have speeds of the order of 133MHz, 800MHz, etc.
Rambus DRAM (RDRAM): RDRAM technology was developed originally by Rambus, Inc.
Rambus memory is integrated onto Rambus Inline Memory Modules (RIMMs). RDRAM chips are
synchronized to the processor's memory bus.
CPU synchronized to the motherboard. DRAM is neither synchronized to the motherboard
SRAM CPU and memory module are synchronized to the motherboard.
Synchronizes to the memory bus clock. Memory bus clock is much faster than the
motherboard clock. Hence faster data transfer between the CPU and the memory module
Video RAM (VRAM): VRAM is primarily used on video cards. It is dual ported, in the sense that
while one device write to VRAM, another device can simultaneously do read operation. This is quite
useful in animation and other speed sensitive video applications. VRAMs are more expensive than
DRAMs, but provide better graphic display. Windows RAM (WRAM) is another type of memory
used for graphics, and is similar to VRAM in functionality. However, with the faster memory access
schemes like DDR, VRAM and WRAM are slowly becoming obsolete.
Double Data Rate SDRAM (DDR SDRAM):
DDR SDRAM is similar to SDRAM, but for the difference that DDR reads data on both the rising
and falling edges of the clock. SDRAM reads only on the rising edge of a signal. This technique
allows the DDR module to achieve speeds twice that of SDRAM. For example, instead of a data rate
of 133MHz, DDR memory transfers data at 266MHz.
Memory modules are printed circuit cards made up of memory chips, and a few other passive
components. Normally, memory modules are the those that get installed on the motherboard, and you
don't handle individual memory chips. The following are the prominently used memory modules
(also called memory cards):
Dual In-line Package (DIP)
Single In-line Memory Modules (SIMM)
Dual In-line Memory Modules (DIMM)
Rambus In-line Memory Modules (RIMM)
Dual In-line Package (DIP): Initially, PC XT, and AT systems came with DIP sockets. Individual
memory chips were inserted into the sockets.
DIP chip DIP socket
A typical DIP socket, and chip are shown in the figure above. As can be seen, there is only one chip
per DIP package. This arrangement resulted in several DIP sockets being present on the
motherboard. If you need to enhance the memory, buy additional chips and insert into any existing
DIP sockets. There are several disadvantages because of this method:
1. Due to size, these chips used to take lot of space,
2. The chips used to dislodge from respective sockets, and give raise to errors
3. It is cumbersome to insert individual chips
4. Used to take more power
Because of the above problems, memory chips were integrated into SIMM (Single In-line Memory
Modules) that overcome several of the said problems.
Single In-line Memory Modules (SIMM):
SIMM modules have several memory chips soldered in-line on its own circuit board. A typical
SIMM is shown in the figure below. There are two types of SIMM modules: 30-pin SIMM modules,
and 72-pin SIMM modules. Typically, a 72-pin SIMM has 32-bit wide memory bus, whereas a
DIMM has 64-bit wide memory bus. On a SIMM, the edge connector pins on either side of a SIMM
are shorted, representing only one signal pin.
Typical SIMM package:
Memory Speed: 60nS
Dual In-Line Memory Modules (DIMM):
DIMMs are very similar to SIMMs. The major difference is that a DIMM has two different signal
pins on each side of the module as shown in the figure. One big advantage of DIMM is that only one
module can be inserted into the motherboard, whereas you need two SIMMs (paired) when working
with 64-bit microprocessors like Pentium II and above. Since SIMM provides only 32-bit bus, you
need to use 2-SIMMs paired together with any modern 64-bit processor.
Typical DIMM package
Memory size: 256MB
SIMMs typically have 72 pins, whereas DIMM have 168 pins.
Rambus In-Line Memory Module (RIMM):
Rambus inline memory modules (RIMMs) use Rambus Dyamic RAM (RDRAM) chips.
A RIMM package using RDRAM
Double Data Rate:
DDR modules are also called DIMMs (Dual-In-Line-Memory Module). A typical DDR module
(DIMM) is shown above. The DIMM package using DDR is twice as fast as the one using SDRAM.
Memory Size: 2X512MB
Memory Speed: 400MHzPC3200
Memory Type: Dual Channel DDR
Micro Processors, and Socket Types:
Micro processor, also called CPU (Central Processing Unit), is a major component of a micro
computer. We discuss various CPUs starting from Pentium IV, and onwards.
Socket 478 is a PGA socket used by Intel Pentium 4 microprocessor family (not all P IV family
processors support Socket 478)..
Socket type: Socket 478 (mPGA478B)
Front Bus Frequencies: 400 MHz - 800 MHz (100 MHz - 200 MHz QDR)
Socket size: 1.38" x 1.38" (3.5 x 3.5 cm)
Number of contacts: 478
Compatible package types:
478-pin micro FC-PGA
478-pin micro FC-PGA2
Processors Intel Pentium 4 (1.4 - 3.4 GHz)
Intel Celeron (1.7 - 3.2 GHz)
Celeron D (to 3.2 GHz)
Intel Pentium 4 Extreme Edition (3.2, 3.4 GHz)
This socket has currently been replaced with socket 775.
The socket was used for low-end Pentium IV processors below 2000MHz frequencies. It became
obsolete with the advent of Socket 478.
Number of Contacts: 423
Compatible package type: Organic Land Grid Array (OLGA)
Front Bus Speed: 100 MHz FSB
Compatible Processors: Intel Pentium 4 (1300 MHz - 2000 MHz)
Socket 370 (PGA370) is a PGA socket compatible with Intel Celeron and Pentium III processors in
Pin Grid Array (PGA) package.
Socket 370 Processor package
Compatible package types:
370-pin Plastic Pin Grid Array (PPGA); 370-pin Flip-Chip Pin Grid Array (FC-PGA)
370-pin Flip-Chip Pin Grid Array (FC-PGA2)
Number of contacts: 370
Front Bus Frequencies: 66, 100 and 133 MHz
Supported Processors include the following:
Intel Celeron (PPGA, 300–533 MHz)
Intel Celeron (FC-PGA, 533–1100 MHz)
Intel Celeron (FC-PGA2, 900–1400 MHz)
Intel Pentium III (FC-PGA, 500–1133 MHz)
Intel Pentium III Tualatin (FC-PGA2)
Celeron Tualatin (FC-PGA2)
VIA C3 (FC-PGA)
Socket 8 was used for a very limited number of processor types. The socket is being no more used
by newer processors.
Number of Contacts: 387
Front Bus Speed : 66-75 MHz
Supported Processors include the following:
Pentium Pro 150~200, Pentium II OverDrive 300~333
Socket 7 was introduced by Intel for Pentium 133 - 200 MHz processors. It is also used in Pentium
MMX processor family.
Socket 7 Compatible Processor
Compatible package types:
296-pin staggered Plastic Pin Grid Array (PPGA)
296-pin staggered Ceramic Pin Grid Array (CPGA or SPGA)
296-pin Flip-Chip staggered Ceramic Pin Grid Array
321-pin ceramic Ping Grid Array (CPGA)
Number of contacts: 321
Front Bus Frequencies: 66 - 83 Mhz System Clock
Supported Processors include the following:
AMD K5 (75 MHz - 200 MHz); AMD K6 (166 MHz - 300 MHz)
AMD K6-2 (200 MHz - 570 MHz)
AMD K6-III (333 MHz - 550 MHz)
Cyrix 6x86, 6x86L and 6x86MX (90 MHz - 266 MHz)
Cyrix MII (233 MHz - 433 MHz)
Intel Pentium (non-MMX) (75 MHz - 200 MHz)
Intel Pentium MMX (166 MHz - 233 MHz)
Socket A (Socket 462):
Socket A (also called Socket 462) is a PGA socket compatible with AMD K7 family of processors.
Socket A Processor for Socket A
Bus Frequencies: 100 MHz, 133 MHz, 166 MHz and 200 MHz
Number of contact pins: 462 pin holes
Compatible Processors include the following:
AMD Athlon (650 MHz - 1400 MHz)
AMD Athlon XP (1500+ - 3300+)
AMD Duron (600 MHz - 1800 MHz)
AMD Sempron (2000+ - 3300+)
AMD Athlon MP (1000 MHz - 3000+)
Compatible package types:
462-pin ceramic Pin Grid Array (PGA) package, 462-pin organic PGA.
Slot A is used by AMD's Athlon family of processors. It has 242 contacts, physically similar to that
of Intel's Slot 1. But Slot A is electrically different from that of Slot 1.
Slot 2 is a 330 contact version of Slot 1. Intel's Xeon processor uses Slot 2. The Slot 2 cartridge may
house as many as four processors and an L2 cache.
Slot 1 is a Slot-type connector. This connector is compatible with Pentium II family of processors,
and some of low-end Celeron processors. Pentium III was the last microprocessor family that used
the Slot 1.
Pentium IV family of processors do not use Slot 1.
Number of Contacts: 242
Processors types supported include the following:
Intel Celeron (SECC, 233-466 MHz)
Processors Intel Pentium II (SECC, 233-450 MHz)
Intel Pentium III (SECC2, 450-1133 MHz)
Compatible package types:
Single Edge Processor Package (SEPP)
Single Edge Connector Cartridge (SECC)
Single Edge Connector Cartridge 2 (SECC 2)
Storage Devices, and Interfaces
A computer normally contains several storage options. Commonly used storage devices include the
1. Hard Disk Drive
4. Flash ROM
Computer Hard Disk Drive
Working of a Hard Disk Drive
Hard Disk Drive Interfaces
Working of a HDD:
The hard disk drive (Abbreviated as HDD or HD) holds the main storage media of a computer. A
HDD consists of several platters (or hard disks) along with head actuator, head arm secured in a
chassis. A schematic diagram of a HDD is shown in the figure below:
Schematic of a Hard Disk Drive Photo of a HDD that has two concentric disks
The individual hard disks (platters) are used to store the information. The storage is achieved by
depositing a thin magnetic film on either side of each disk. The disks are mounted on a rotary drive.
Basically, the surface of each disk is divided into concentric Tracks. Then each track is divided into
Sectors. The data is accessed by specifying the disk number, track number, and the sector number.
The disks rotate at a very high speed ( several thousands of revolutions per minute), enabling very
fast read and write operations. The magnetically sensitive head reads/writes information when the
disks rotate. The disks are sealed to prevent any dust or moisture entering the drive.
A typical hard disk drive is shown in the figure.
Hard Disk Drive Interfaces:
There are several standards connected with the Hard Disk Drives. These include the following:
IDE/EIDE Hard Disk Drives:
IDE (Integrated Drive Electronics), also known as ATA is used with IBM compatible hard drives.
IDE and its successor, Enhanced IDE (EIDE), are the commonly used with most Pentium
Figure: A 40-pin IDE cable connector
Enhanced IDE (EIDE) is the enhanced version of IDE technology, and supports faster access to the
Small Computer Systems Interface (SCSI): SCSI is commonly used with server grade machines. IDE
supports only two drives (one master drive and one slave drive) per channel, whereas SCSI can
support 8 or more hard drives. There are different versions of SCSI available today. Different
versions of SCSI include the following:
9. Serial SCSI
The various standards primarily differ in the following:
Maximum throughput (MB/sec)
Maximum cable length, and
Maximum number of devices that could be connected.
For example, SCSI-I has a throughput of 5MB/sec, where as SCSI-
3 can go up to 40MB/sec.
Serial ATA (SATA):
Serial ATA (SATA) is a next generation technology based on
ATA, and for transfer of data to and from a hard disk. Earlier, ATA
was used to mean parallel transfer of bits between the motherboard
and the hard drive. However, with the advent of SATA, traditional ATA was named as PATA
(Parallel ATA). IDE/EIDE is usually associated with PATA.
DVD and DVD RW
DVD stands for Digital Video Disk (Also known as Digital Versatile Disk). Essentially, DVD looks
very similar to CD-R, but contains larger storage space and can hold video, audio, and/or computer
data. A single-layer, single-sided DVD has a capacity of 4.7GB where as a CD-ROM has a capacity
of around 650MB. A double-layer, double-sided DVD-ROM disk can have capacity over 17GB. The
DVD specification supports access rates of 600KBps to 1.3MBps.
DVD-R is a once-recordable form of DVD. DVD-RW or DVD-R/RW can be written multiple times.
Many DVD drives can read data from a CD-R. But, some DVD drives may not be able to read CD-
Rs. You need to ensure compatibility with CD-R when procuring a DVD drive. Normally, CD-R
drives are not compatible with DVD, and you will not be able to read a DVD using a CDR.
The original speed rating for DVDs is different from that of a CD-R. For a DVD, 1x is
1352.54KB/sec. Following the convention, 2x for a DVD drive is 2,705KB/sec, 4x is 5,410KB/sec,
8x is 10,820KB/sec, and 16x would be 21,640KB/sec.
A DVD drive is shown in the figure above. It looks very similar to that of a CD drive.
Video Display Devices
The display devices used commonly can be classified as below:
Cathode Ray Tube (CRT) display
Liquid Crystal Display (LCD) display
Cathode Ray Tube (CRT):
CRT display is the most commonly used form of visual displays, through it is getting gradually
replaced with LCD and Plasma displays.
A computer monitor using CRT display.
In a CRT, an electron beam sweeps the display screen horizontally, one line at a time, gradually
down the screen. A synchronization (sync) signal brings the beam back to the top row of the display.
This type of scanning (line-by-line) is known as raster scan.
Figure: CRT cross sectional diagram showing important components of a CRT.
There are two types of cathode ray displays. One is non-interlaced, and the other is interlaced.
Normally, all the displays are interlaced to reduce flicker.
As shown in the figure, for non-interlaced display, the scanning is done continuously from top to
bottom. For non-interlaced display, alternate rows are scanned.
A black and white monitor contains only one electron gun, whereas a color display monitor will have
three electron guns, each of which represent red, green, and blue.
The horizontal and vertical deflection takes place by applying appropriate voltages to the horizontal,
and vertical deflection plates. Usually, the screen is refreshed between 60-100 times per second.
The grid shown in the figure controls the speed with with the electrons hit the screen. If a positive
voltage is applied to the screen grid, because of which the electrons are accelerated and hit the
screen, making the screen brighter. If a negative voltage is applied to the grip, the electrons are
decelerated and the screen will not glow. The microscopic control of electron beam flow, produces
images on the screen.
One basic unit of measurement is "pixel". A pixel is the smallest area in a graphics display that can
Given below are the commonly used screen resolutions:
Display Type Number of pixels
Video Graphics Array (VGA) 640 * 480 pixels
Super Video Graphics
800 * 600 pixels
eXtended Graphics Array
1024 * 768 pixels
Super eXtended graphics
1280 * 1024 pixels
Screen resolution is always stated as the horizontal number of pixels by the vertical number of
pixels. A screen displaying 800 x 600 pixels has 600 rows, each 800 pixels wide.
Graphics Cards: The graphics card resides in the CPU box, and drives the video display. A typical
graphics card is shown below:
The graphics card shown includes DVI connector, TV/Video connector, and a VGA connector. The
card has an on-board graphics processor with cooling fan. Usually, for graphic intensive applications,
you need a higher end graphic adapter card. For normal desktop usage, a video adapter will be
PCM CIA Interface Cards (PC Cards)
PCMCIA stands for Personal Computer Memory Card International Association. PCMCIA standards
were developed for small, credit card-sized devices, called PC Cards. Though PCMCIA cards (PC
Cards) were originally developed for use with Notebook computers, the same are being extensively
used in other devices also.
Types of PC Cards:
There are 3 types of PC Cards.
1. Type-I PC Cards are typically used for memory devices such as RAM, Flash, and SRAM cards.
2. Type II PC Cards are typically used for I/O devices such as modems, and LAN cards.
3. Type III PC Cards are used for devices that consist of thicker components, such as rotating mass
Important characteristics of PC Cards:
Length 85.6 mm
Width 54.0 mm
Type-I 3.3 mm
Type-II 5.0 mm
Type-III 10.5 mm
Back-end I/O Connector
Depends on the functionality of the
PCMCIA Card (PC Card) slots:
Just like PC Cards, PCMCIA slots also come in three sizes:
Type I slot: Holds up to one Type I card
Type II slot: Holds up to one Type II card or two Type I cards
Type III slot: Holds up to one Type III card or a Type I and Type II card.
Advantages of PC Cards:
Several advantages offered by PC cards are as given below:
1. CardBus: CardBus allows PC Cards and hosts to use 32-bit bus mastering.
2. DMA: The Standard allows cards to utilize Direct Memory Access technology directly in the
hardware when matched with a corresponding host system.
3. eXecute In Place (XIP): XIP allows operating system and application software to run directly from
the PC Card. This in turn improves performance, and eliminates the need for large amounts of
4. Low Voltage Operation: The Standard enables 3.3V as well as 5V operation.
6. Plug and Play: PC Cards can be inserted or removed while the system is powered-on.
7. Power Management: The Standard provides a means to interface to APM (Advanced Power
PC Card Uses:
Some of the frequently used PC Card types include the following:
Wireless LAN card
ATA flash disk card
A wireless LAN serves the same functionality of a traditional LAN card, but operates without being
physically wired to a network device. It enables a mobile user to connect to a local area network
(LAN) through a wireless connection. IEEE 802.11a, 802.11b, and 802.11g specify the technologies
for wireless LANs.
Linksys 802.11b/g Wireless PC Card
A wireless LAN card is shown in the figure above. Typical specifications for the same are given
Connection Type: Wireless
Supported Wireless Standards: 802.11b, 802.11g
Supported Operating Systems: Windows, 98, ME, 2000, XP
IRQs, and I/O Addresses:
IRQ ( Short for Interrupt ReQuest):
An IRQ allows a device to request certain system resources on priority. Normally, the system
allowcates the different non-competing IRQ numbers to different devices. The system allocated
IRQs may some times be manually changed. However, it may lead to serious IRQ conflicts
resulting in a hung operating system.
The table below gives normally assigned IRQ numbers in Pentium compatible systems:
:IRQ Usage Comments
0 System Timer This IRQ is used within the system board for system timing.
1 Keyboard Controller
This IRQ is assigned to the keyboard. Not available to any other add-
2 Tied to IRQs 8-15 This IRQ was assigned to older EGA video cards.
This IRQ is assigned to the serial ports COM 2, and COM 4. Avoid
setting this IRQ to other devices.
This IRQ is assigned to the serial ports COM 1, and COM 3. Avoid
setting this IRQ to other devices.
5 LPT2 or Sound Card
This IRQ is assigned to a secondary printer port LPT2. If LPT2 is not
available, it is used for sound card.
This IRQ is assigned to the Floppy Diskette Controller. Since most
systems use FDC, do not use this IRQ for other requirements.
This IRQ is assigned to the parallel port LPT1. It can also be made
available to other add-in cards, but avoid using it for any other cards.
It may lead to conflicts at a future time, when you least anticipate it.
8 Real Time Clock
This IRQ is reserved for the internal real-time clock. This line is NOT
available to other add-on cards.
9 Substitutes for IRQ 2
This IRQ uses IRQ 2 to talk to the CPU. Normally used for network
10 Not Assigned Can be used for network card, or others
11 Not Assigned Left open for use with any other device
12 PS/2 Mouse Port
This IRQ is used for the PS/2 style mouse port normally available on
many motherboards. If you are using PS/2 Mouse port on system, do
not use this IRQ for anything else.
Used for math co-processor, and not available for other devices.
Primary Hard Disk
This IRQ is assigned to the primary hard drive interface
Secondary Hard Disk
This IRQ is assigned to the secondary hard drive interface.
Memory Mapped Input/Output Addresses:
Input/output addresses ( I/O addresses for short) are resources used by almpst every device in the
computer. The I/O addresses represent the location in memory for a given device to exchange
information between itself, and the rest of the system. These address spaces are permanently
allocated to the system devices. If an address space allocated to one device (say, LPT) is assigned to
another device (say, IDE controller) than I/O address conflict will arise, and the system may hang or
not function normally.
Given below is the list of commonly used I/O address spaces allocated to various system devices:
I/O Address Common Device using the Address Space
Used by keyboard controller
Used by SCSI host adapters
170h Secondary IDE Interface
1F0h Primary IDE Interface
220 Default address for Sound cards
240 Optional address for sound cards and network cards
Optional address for sound cards and network cards.
278h Assigned to LPT2 or LPT3
280 Optional address for sound cards and network cards
300 Default for many network cards
2E8h Assigned to COM 4 and used with IRQ 3
2F8h Assigned to COM 2 and used with IRQ 3
300h nother Network Interface Card choice
Used for a SCSI host adapter or MIDI device. You can use this for a Network card
when not assigned to any SCSI or MIDI device.
330h Used for the SCSI host adapters
340h Could be used for a SCSI host adapter
360h Could be used for a Network card, provided there are no conflicks.
This is assigned to the first parallel printer port (LPT 1) in color systems. Normally
used with IRQ 7.
3BCh This is assigned to the first parallel printer port (LPT1) in monochrome systems.
3E8h Assigned to COM 3 and used with IRQ 4
3F8h Assigned to COM 1 and used with IRQ 4
A rule of thumb for IEQ, and IO address conflicts is that the PC is unable to boot or giving error
codes during boot process. If you have installed any device recently, remove or disable it and see if
the PC boots properly.
Cables and Connectors
The serial port is an Asynchronous port which transmits data one bit of data at a time. Serial port
hardware usually consists of a UART (Universal Asynchronous Receiver/Transmitter).
Most commonly used serial ports are given below:
DB9 adheres to the RS-232c interface standard. It has 9 pins as shown in the figure. The connector
is "D" shaped, and easy to recognize. The function of each pin is described below.
Outline Diagram of DB-9 DB-9 Female Connector
Pin # Pin Description
Pin 1 Data Carrier Detect DCD
Pin 2 Received Data RxData
Pin 3 Transmitted Data TxData
Pin 4 Data Terminal Ready DTR
Pin 5 Signal Ground Gnd
Pin 6 Data Set Ready DSR
Pin 7 Request To Send RTS
Pin 8 Clear To Send CTS
Pin 9 Ring Indicator RI
DB25 adheres to the RS-232C interface standard. It has 25 pins as shown in the figure. The
connector is "D" shaped, and easy to recognize. DB-25 is normally used in older computers, and not
much used in modern day computers.
DB-25 Male Connector
RJ-11 is a 4-wire connector, commonly used with a modem. It should not be confused with bigger
RJ-45 cable and connector. RJ-45 is commonly used for Ethernet network interface card (NIC).
Schematic of RJ-11 Connector An RJ-11 Cable with Connector
Pin # Function
A2 Rx (Data Input)
A3 Tx (Data Output)
A4 Vc (Power)
An RJ-45 connector has 4 pairs of wires as shown in the schematic diagram below. Note that an RJ-
11 is a 4-wire connector, where as RJ-45 is an 8-wire connector.
RJ-45 connector schematic RJ-45 connector crimped to a cable.
RJ-45 connector is commonly used for Ethernet Networking ports. Devices that normally use RJ-45
ports include NICs, Hubs, Switches, and Routers.
There are basically two types of cables. One is Straight-through cable, and the other is Cross-over
cable. Straight-through cables are used for connecting a network device to a work station. Cross-over
cables are used for connecting a hub to a switch or a hub to another hub.
DB-25 connector is most commonly used in conjunction with a parallel printer. It has an 8 bit data
bus as shown in the figure below.
Pin # Function
Pin 1 Strobe
Pin 2 Data Bit 0
Pin 3 Data Bit 1
Pin 4 Data Bit 2
Pin 5 Data Bit 3
Pin 6 Data Bit 4
Pin 7 Data Bit 5
Pin 8 Data Bit 6
Pin 9 Data Bit 7
Pin 10 Acknowledge
Pin 11 Busy
Pin 12 Paper End
Pin 13 Select
Pin 14 Auto Feed
Pin 15 Error
Pin 16 Initialize Printer
Pin 17 Select Input
Pin 18 - Pin 25 Ground (return for pins 0-7)
The length of Parallel Printer cable usually limited to a maximum of 15 feet
Other Type of Parallel Ports:
Enhanced Parallel Port (EPP): The Enhanced Parallel Port (EPP) operates close to ISA bus speed and
can achieve transfer rates up to 1 to 2MB/sec of data.
Enhanced Capabilities Port (ECP): The Enhanced Capabilities Port (ECP), is an additional enhanced
Connectors and Cables -ii
USB stands for Universal Serial Bus. The most important features of USB bus include the
Plug'n'Play: USB is a truly plug-n-play port. The port is automatically detected by the system,
and its ready to use.
Hot plug and unplug: It is possible to insert an USB device and and unplug without affecting
the functioning of a computer. You dont need to power-off the computer to remove or
insert a USB device.
127 physical devices: A USB can support up to 127 devices. This is much more than what is
Low cost cables and connectors
USB connectros can be broadly divided into USB A, and USB B. The difference between the two is
in the physical layout of pins in the connector. Both are shown in the figure below.
Pin # Function
Pin 1 +5V DC
Pin 2 Data-
Pin 3 Data+
Pin 4 Ground
USB"A" plugs are used towards the host system and USB "B" plugs are used towards the USB
FireWire/IEEE1394 port provides data rates up to 400 Mb/sec. The standard is most suitable for
transferring high volumes of information including video, and voice data.
Given below are some of the important features of IEEE 1394 standard:
Based on open licensing system.
Digital transmission with data rates of 100, 200, or 400 mega bits per second (Mb/sec).
Plug and play: Automatically detected by the operating system during start-up or during
Hot pluggable: Cables can be connected and disconnected without turning off power to the
Flexible: You can connect FireWire cables in a daisy-chain or in branching cable
Compatibility, lower speed USB devices can be connected with higher speed devices.
Supports both asynchronous (Based on request-and-acknowledge form of computer
communication, used typically with dial-up modems). and isochronous data transfer.(
Continuous, guaranteed data transmission at a pre-determined speed. Useful for video and
4 PIN IEEE1394 (without Power)
A IEEE 1394 Connector schematic
Pin #1 Twisted-pair B, differential signals, TPB-
Pin #2 Twisted-pair B, differential signals, TPB+
Pin #3 Twisted-pair A, differential signals, TPA-
Pin #4 Twisted-pair A, differential signals, TPA+
6 PIN IEEE1394 (with Power):
IEEE1394 6-pin connector schematic.
Pin # Function
1 Power, 18-28v no load.
3 Twisted-pair B, differential signals, TPB-
4 Twisted-pair B, differential signals, TPB+
5 Twisted-pair A, differential signals, TPA-
6 Twisted-pair A, differential signals, TPA+
PC Power Supplies
PC power supplies deliver required DC power to the computer electronic sub-systems including
Motherboard, Hard drive, CDROM drive, Keyboard, Mouse, and others. Usually, regular CRT
monitors derive power separately from the mains. PC power supplies work by converting the AC
mains power supply to required DC power supplies.
There are mainly two types of power supplies used in computers:
ATX Power Supply
ATX12V Power Supply
ATX12V power supplies are intended to be downward compatible with ATX power supplies.
As the PC components take more and more power, to enable the delivery of more +12 VDC current
to the motherboard, additional 4-pins are provided for taking care of +12 V power supply. Thus,
ATX12V will have a +12 V power connector, and a simple ATX power supply with not have the
+12V power connector.
An ATX power supply, typically found in all Pentium class computers is shown below:
The voltages produced by ATX/ATX12V power supplies are:
1. +3.3 Volts DC
2. +5 Volts DC
3. -5 Volts DC
4. +5 Volts DC Standby
5. +12 Volts DC
6. -12 Volts DC
ATX and ATX12V power supply connectors pin configuration is shown below:
ATX12V Connector (it has 24 pins) ATX Connector (it has 20 pins)
In addition to the Motherboard power connector, a power supply may have a CPU power connector,
and an Auxiliary power connector.
Following are the most commonly used power supply connectors:
4 Pin Berg Connector: Used to connect the PSU to small form factor devices, such as 3.5"
floppy drives. Available in: ATX & ATX12V
4 Pin Molex Connector 4 Pin Molex Connector: This is used to power various components,
including hard drives and optical drives. Available in: ATX and ATX12V
20 Pin Molex ATX Power Connector: This is used to power the motherboard in ATX
systems.available in: ATX (ATX12V have 24 pins)
4 Pin Molex P4 12V Power Connector: Used specifically for Pentium 4 Processor
Motherboards. available in: ATX (integrated into the power connector in ATX12V)
6 Pin AUX Connector 6 Pin AUX Connector: Provides +5V DC, and two connections of
+3.3V.available in: ATX/ATX12V
To power up an ATX or ATX12V PSU for testing, short pin 14 (PS_ON) with any of the ground