96831 motherboard

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System board

The main component of a personal computer. It contains the major structures thatmake up a computer system.

The system board is the center of the PC-compatible microcomputer system. It

contains the circuitry that determines the computing power and speed of the entire system.In particular, it contains the microprocessor and control devices that form the brains of thesystem. System boards are also referred to as motherboards and planar boards. A typicalATX-style system board layout is depicted in Fig 1

The major components of interest on a PC system board are as follows:

1. Microprocessor CPUThe brains of the system. This component performsmathematical and logical computations at incredible speeds.

2. Primary memoryThe systems primary memory elements are as follows:

RAMRandom access memory (RAM), which is quick enough to operate directly

with the microprocessor and can be read from and written to as often as desired. RAM is a

volatileFig 1Parts of ATX Motherboard

type of memory; its contents disappear when power is removed from the memory. ROMRead-only memory (ROM), which contains the computers permanent

startup programs. ROM is nonvolatile; its contents remain with or without powerbeing applied.

Cache memoryAn area of special high-speed RAM reserved for improvingsystem performance by holding information that the microprocessor is likely to use.Blocks of often-used data are copied into the cache area to permit faster accesstimes.


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3. Expansion slot connectorsConnectors mounted on the system board into which theedge connectors of adapter cards can be plugged to achieve system expansion. Theconnectors interface the adapter to the systems I/O channel and system buses.

4. ChipsetMicroprocessor support ICs that coordinate the operation of the system.

Chipsets

North Bridge:

In Northbridge/Southbridge chipset architecture designs, the Northbridge is the chipor chips that connect a CPU to memory, the PCI bus, Level 2 cache and AGP activities. TheNorthbridge chips communicate with the CPU through the FSB.

The Northbridge chip is one of two chips that control the functions of the chipset. Theother is the Southbridge. The Northbridge can consist of more than one discrete chip whilethe Southbridge is typically only one discrete chip.

South Bridge:

In Northbridge/Southbridge chipset architecture designs, the Southbridge is the chipthat controls all of the computers I/O functions, such as USB, audio, serial, the system BIOS,the ISA bus, the interrupt controller and the IDE channels. In other words, all of the functionsof a processor except memory, PCI and AGP.

Fig 1.1 Chipset Layout

The Southbridge chip is one of two chips that control the functions of the chipset. Theother is the Northbridge. The Northbridge can consist of more than one discrete chips whilethe Southbridge is typically only one discrete chip that rests on the Northbridges PCI bus.


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PCs feature a battery-powered RAM area that holds some of the systems advancedconfiguration information. This configuration storage area became known as CMOS RAM.

Expansion Slots

Most PCs use standardized expansion slot connectors that enable various types of

peripheral devices to be attached to the system. Optional input/output devices, or theirinterface adapter boards, are plugged into these slots to connect the devices to the systemsaddress, data, and control buses.

Several different types of expansion slots are in use today. A particular system boardmay contain only one type of slot, or it may have a few of each type of expansion slot. Beaware that adapter cards are compatible with particular types of slots, so it is important toknow which type of slot is being used. The major expansion slot types are as follows:

8-bit PC-bus slots

16-bit AT-bus or Industry Standard Architecture (ISA) bus slots

32-bit Extended ISA (EISA) and Micro Channel Architecture (MCA) slots

32-bit Video Electronics Standards Association (VESA) and

32/64-bit Peripheral Component Interconnect (PCI) local bus slotsThree additional slot technologies have found their way onto Pentium-class system boards. These specialized

slots are as follows:

Accelerated Graphics Port (AGP) slots

Audio Modem Riser (AMR) slots

Communication and Networking Riser (CNR) slots

Fig 1.9 Expansion slot connectors


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System Board

Because most components are physically attached to the system board, this can be

one of the most time-consuming replacements. If you are replacing one system board with

anotherof exactly the same brand and version, you should make notes about the jumper

and BIOS settings in case they need to be changed on the new board. Most likely, however,you will replace a system board with a newer version, and you should follow the new boards

manual rather than the original setup.

All the expansion cards and cables must be removed from the system board. You

might also need to remove hard drive and floppy drive bays to get them out of the way. Next,

remove any screws or fasteners attaching the system board to the chassis, and lift the board

out.

To place the new (or replacement) board in the computer, line it up properly on

the chassis screw holes, and fix it into place. Although the BIOS, CMOS, and CMOS battery

are included with the system board, you will need to install the processor, memory, and

expansion cards separately. Finally, attach the power and drive connectors.

System boards are generally removed for one of two possible reasons. Either the

system board has failed and needs to be replaced, or the user wants to install a new

system board with better features. In either case, it is necessary to remove the current

system board to replace it. The removal procedure can be defined in five steps:

1. Remove all external I/O systems.2. Remove the system units outer cover.3. Remove the option adapter cards.

4. Remove the cables from the system board.5. Remove the system board.

Removing the System Board

Verify the positions of all jumper settings on the old system board. Record thesesettings and verify their meanings before removing the board from the system. This task mayrequire the use of the boards User Manual, if available. Remove the grounding screws thatsecure the system board to the chassis. Store the screws properly. In a desktop unit thatemploys plastic standoffs, slide the system board toward the left (as you face the front of theunit) to free its plastic feet from the slots in the floor of the system unit. Tilt up the left edge ofthe board, and then lift it straight up and out of the system unit, as illustrated in Fig 1.20. If

the system unit uses threaded brass standoffs, simply remove all the retaining screws and liftthe board out of the case. In a tower unit that has plastic standoffs, slide the system boardtoward the bottom of the system unit to free its plastic feet from the slots in the side panel.Tilt the bottom edge of the board away from the unit and pull it straight out of the chassis, asshown Fig 1.20 a.


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Fig 1.20 Removing the system board from a Desktop case

Installing the System Board

The system board installation process is basically the removal process performed in reverse:1. Verify the hole pattern alignment for the new system board and insert it into the systemunit chassis.2. Install the grounding screws that hold the system board to the brass standoffs in thechassis floor.3. Reconnect all the cables to the system board (that is, power connections, front-panelconnections, and disk-drive connections).4. Install the option adapter cards in the appropriate expansion slot connectors (remove slotcovers from the back panel of the chassis to accommodate any new cards).


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Types of Motherboards

Although motherboards can vary from computer to computer, there are two common

types: the AT and ATX. Their sizes, typical components, and prevalence are discussed

here.

Full and Baby AT

Advanced Technology (AT) motherboards were introduced in 1984 (around the time

of the Intel 80286 processor). They measure approximately 12 by 13 inches andtypically support 80286 or older processors, 5.25 inches floppy drives, and 84-key

keyboards. A smaller version of the AT motherboard, typically measuring around 8.5

by 13 inches was later released. This type was called the Baby AT, and the originalbecame known as the Full AT. The two AT motherboards are similar in layout. The

Full AT motherboard is now practically obsolete, but the Baby AT motherboard is still

being used by some manufacturers. Depending on when it was

Differences between AT and ATX Motherboards

AT Motherboard ATX Motherboard

Introduced by IBM Introduced by Intel

Two 6-pin power connectors 20-pin power connector

Doesn't have on-board cards(like Sound, Video, etc.)

Have on-board cards (likeSound, Video, etc.)

Doesn't have built-in portsHas built-in ports like parallel,serial

Has DIN-5 Ports Has Mini DIN-6 Ports

No Soft SwitchHas Soft Switch (Enabled byACPI)

Differences between AT and ATX Form Factors

manufactured, Baby AT motherboards might contain SIMM and/or DIMM memoryslots and 80386, 80486, or Pentium processor slots. Baby AT motherboards also use

the 3.5-inch floppy drive rather than the older 5.25-inch drive Fig 5-9 shows a Baby

AT motherboard layout.

AT motherboards can be identified by the fact that the parallel and serial ports are not

integrated with the keyboard; rather, they are installed in an empty chassis slot and

attached to motherboard ports via small ribbon connectors. Other identifying


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characteristics of the AT motherboard are the placement of the processor socket near

the end of the expansion card slots, the use of a DIN-5 keyboard connector, two

power connector ports (for P8 and P9 connectors), and support for 12 and 5vDConly. Fig 5-10 shows two six pin power connectors of AT motherboard.

Fig 5-9 Baby AT Style

Fig 5-10 Power connectors of AT Motherboard

ATX


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The ATX motherboard was released by Intel in 1996 and is the most commonly used

form in new PCs. ATX is not an acronym but is the actual trademarked name of the

motherboard form. The ATX is the same size as a Baby AT motherboard but has adifferent orientation and layout (Fig 5-11). Note that the processor is located further

from the expansion slots, and the hard drive and floppy drive connectors are located

closer to the bays on the chassis.The ATX motherboard also includes integrated parallel and serial ports (I/O ports)and a mini-DIN-6, rather than a DIN-5, keyboard connector. The ATX motherboards

power supply uses a single motherboard connector and supplies voltages of 12, 5,

and +3.3vDC. Again, depending on when it was manufactured, an ATX motherboardcan contain SIMM and DIMM memory slots; support for BIOS-controlled power

management; 80386, 80486, or Pentium-class processor sockets; and support for

USB.

Fig 5-11 ATX Motherboard Layout

96831 motherboard

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