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THE SYSTEM BOARD
Mass transit in a city has interconnected streets,
avenues, and highways that are used for transportation. Similarly, with a
system board, data and information is transported via the bus, which can have
numerous components attached to its “roadways.”
In a city, we have stop lights,
stop signs, and other ways of controlling traffic, as well as people who use
the transportation system to get from place to place. If we didn’t have ways to
manage the traffic, we might get lost, end up in a traffic jam, or be involved
in an accident. Similarly, the system board has a system clock to synchronize the operation of the bus and other
components. The system clock keeps data and information flowing smoothly to its
correct destination, and hopefully without error.
The system board is the most important module and the main
PCB in the computer. A system board is the physical component in a computer
that contains the computer’s basic circuitry and parts. On a typical system
board, the circuitry is imprinted or affixed to the surface of a firm planar
surface (usually fiberglass) and typically is manufactured in a single step. A planar surface is flat and
two-dimensional. Fiberglass is typically used because it’s light, durable, and
slightly flexible, yet firm enough to drill holes, mount chips, or imprint
traces. While the system board connects directly or indirectly to every part of
the computer, it’s the hardware that’s mounted on the system board that does
all the work.
The disk drives and the
power supply sit above or to one side of the system board. System board modularity means that the dimensions of slots,
connectors, sockets, and the board itself (form factor) are standardized,
making it easier to repair or upgrade cards, memory, CPUs, and peripheral
devices.
Many system boards manufactured today are integrated units with discrete components that can’t be replaced
without returning the system board to the manufacturer. These boards include
many of the components that would previously have been found on adapter cards.
Today, drive controllers (hard drive, floppy, optical, and zip), video
adapters, I/O ports (serial [COM], universal serial bus [USB], and parallel
[LPT]), integrated audio, network adapters, and even modems are built into the
system board. This type of system board is referred to as an integrated system board because all the
major functions can be found on the system board itself.
Some components located on
the system board can be replaced without sending the entire board back to the
manufacturer. These components include the CPU, ROM BIOS chip, CMOS battery, and random-access memory
(RAM). Each replaceable component is called a field replaceable unit (FRU); that is, it can be easily repaired or
upgraded.
The type and speed of the CPU. Different
CPU's require different socket sizes and shapes, Therefore you’ll need a system board that’s right for
the processor you choose. Each system board description will tell you what
processors are supported. At Intel and AMD Web sites, you can see what system
boards are recommended.
The chipset installed. The
chipset is the group of chips affixed to a system board that are responsible
for control and functionality of the system board and associated peripheral
devices. There are two primary chip architectures, the Northbridge/Southbridge
and the Hub. In the Northbridge/Southbridge scheme, the “North bridge” controls
operations of the CPU to memory, the secondary L2 cache controller, the
real-time clock, the PCI bus and AGP (accelerated graphics port) functionality.
The “South bridge” controls all I/O (input/output) functions, such as BIOS,
the hard-drive controller, and USB and serial ports. Hub architecture is a
newer technology and renames the “North bridge” the memory controller hub (MCH), while the “South bridge” becomes the I/O controller hub (ICH). The main
difference is that the hub interface is twice as fast as the PCI interface, and
PCI bus communication is faster because it doesn’t have to share bandwidth with
the “South bridge” chip.
Chipset manufacturers include Intel,
Via Technologies, Acer Labs, Silicon Integrated Systems, AMD, and
OPTi. Chipset design is determined by the other components in the system. Some
of the criteria affecting chipset construction are CPU type/speed, memory (RAM)
type, memory bus speed, PCI/PCI-E support/bus width, AGP support, USB
functionality, and mouse type.
The
number and style of external connectors or ports. These ports can have
metal pins (male) or metal sockets (female) integrated into the system board.
•
For the keyboard and mouse, most PCs you’ll encounter
will use USB ports, although older PCs may have a DIN (an acronym for the German Institute for Standardization)
connector.
•
The video connection will either be a 15-pin female
D-shell connector or a 29-pin DVI connector.
•
The printer (not on a network) will use either a USB
port or a 25-pin female D-shell connector.
•
Serial ports come in 9-pin male and 25-pin male D-shell
configurations to connect a variety of input devices (e.g., mice, trackballs,
and digitizers).
System boards are available
with integrated video, sound, and modem as well as other components. Embedded
video, sound, and network connectivity have steadily improved in quality and
performance and are generally less expensive than add-on boards, or adapter cards. But for flexibility,
adaptability, performance, and overall higher quality, adapter cards are
preferred.
Even when you’re dealing with embedded functions, there are
slots on the system board for external adapter cards. Most expansion slots
today come in one of three types, PCI/PCI-E, ISA, or AGP.
The oldest currently used architecture is the PCI (peripheral
component interconnect) bus. This is 32 or 64 bits wide and supports Plug
and Play, which allows easy configuration for new devices that support it. PCI
cards don’t have jumpers or switches, and configuration is done automatically
via software. The AGP (accelerated graphics port) bus allows
direct communication with main memory for handling
Identifying System Board
Components
As we mentioned earlier,
the system board itself is a PCB made of fiberglass. System board form-factor
dimensions are standardized and designed to accommodate the various PC case
sizes and configurations. The system board is multilayered (as many as six) and
etched with fine copper traces, some of which are designated data paths, or
buses, for BIOS, memory, the CPU, and other board components; others are
voltage- and ground-return paths. The traces are sandwiched between the layers,
and you’ll find that most board components aren’t field replaceable units, or FRUs.
If an embedded function fails, you may be able to turn it off in the BIOS and restore functionality with an adapter card.
However, because of the complexity of a system board, a failing or “bad” board
generally isn’t analyzed and repaired, but simply replaced.
Manufacturers make hundreds of tiny holes in the system
board on the bus. Chips, sockets, and other electronic components are then
inserted into these holes. They’re held in place by solder joints applied on
the underside of the PCB. Several other holes are drilled into the system board
for spacers and screws that fasten it to the system unit case. If you encounter
a system board that’s outside of its system unit case, spend a few minutes examining
it. Look closely at both sides and try to get a good feel for the “geography”
of the top and the bottom. Most importantly, note the difference between
socketed chips and soldered chips.
Whenever you encounter a new system board, it’s a good
idea to review the manual that came with the unit so that you can become
familiar with the parts. Sometimes additional information and support can be
found on manufacturer sites such as www.asus.com,
vendor sites, online forums, or tech sites such as www.geek.com. You can also find cutting-edge information and
reviews in hardcopy PC magazines and shopper guides.
The Central Processing Unit
The central processing unit (CPU) is the brain of the computer. That’s
why computers are referred to by the type of CPU they contain, such as 8088,
8086, 286, 386, 486, Pentium, Pentium II, etc. The CPU is located underneath
its own cooling fan and/or heat sink.
The Basic Input/Output System
The basic input/output
system (BIOS) is a chip that contains the startup programs, such as power-on self-test (POST), and drivers
of the computer. This chip gets the computer system running and interfaces with
the system hardware and the operating system. BIOS is referred to as firmware.
The term firmware suggests that the
chips are neither hardware nor software but contain elements of both. Almost
all computers include flash ROM to
store the BIOS. (We’ll discuss ROM later in this study unit.) Flash ROM allows
you to erase and upgrade the BIOS software.
Expansion Slots
The expansion slots enable
the CPU of the computer to communicate with peripheral devices that expand the
capabilities of the computer. Different cards can be plugged into these slots
to enhance the computer, such as adding a video card to make graphics redraw
faster on the monitor.
The Battery
The complementary metal oxide semiconductor, or CMOS, is an onboard semiconductor chip used to store system
configuration settings, such as hard-drive parameters, memory configurations,
and the system’s date and time. It’s used by the BIOS and located with the real
clock in the system board chipset or in a separate clock chip. The small amount
of power CMOS requires is used to retain its contents. For this purpose, the
system employs a battery when it’s turned off. Batteries vary in shape, voltage, and
composition—alkaline, nickel/cadmium, or lithium. Lithium batteries can last up
to ten years, depending on how much they’re used and environmental conditions.
If a system battery goes dead, crucial system information and functionality can
be lost. Thus, it’s always a good idea to write down your CMOS
settings.
Chips
Newer system boards have fewer chips than older ones. In the
past, individual chips were needed to perform specific functions. Today, these
functions are being performed by a single chip with more built-in circuitry or
by the chipset. The best example of this trend is the change that has taken
place in RAM chips. Early PCs had as many as eight banks of nine chips each,
for 256 KB (or K) of RAM. Today, a one GB dual inline memory module (DIMM) can
take the place of these 72 chips and more. We’ll discuss RAM and DIMMs and
related items later in this study unit.
The major chips on the system board are supported by a
number of other integrated circuit (IC) chips.
These chips include the system controller, the page mode memory controller, the
data controller, the address controller, and the peripheral controller. The direct memory access (DMA)
circuitry is often included in the peripheral
controller chip. The user’s manual sometimes contains the chip numbers for
these ICs. If you buy a system board from a supplier, it will almost always
contain enough documentation for you to identify the chips that perform
specific functions.
A byte is eight bits.
A megabyte (MB)
is equal to 1,048,576 bytes.
A megabit (Mb or Mbit) is equal to 1,048,576 bits.
A gigabyte (GB) is equal to one billion bytes.
A gibibit (GiB)
is equal to 1,073,741,824 bits.
The Integrated Drive Electronics
The integrated drive electronics
(IDE) controller is responsible for controlling all IDE and serial ATA
(SATA) interface disk drives.
The Power Supply and Other Connectors
A variety of connectors may exist on a system board. Such
connectors are used for plugging into equipment such as the power supply and
keyboard. In older AT PCs, two power cables (often labeled P8 and P9) are
attached to the system board by a multi-connector socket and prongs. On newer
ATX PCs a single power connector (P1) connects the power supply to the system
board using 20 or 24 pins.
Pentium IV (P4) system boards have an extra 4-pin connector
that’s used to connect to the power supply. This ATX 12V or SFX 12V power supply gives
an extra 12 volts to the P4 board. A system board that doesn’t have this
connector is probably a non-P4 ATX board
and shouldn’t be used with that processor. An Intel-based system won’t boot
until the 12V 2X2 connection is made to the system board. Failure to use the
right power supply or connect the 4-pin power cable may result in damage to the
system board and/or the power supply.
SIMMS and DIMMS
The single
inline memory module (SIMM) replaced dual
in-line package (DIP) chips. Instead of having multiple memory DIP chips,
SIMMs and dual inline memory modules
(DIMMs) replaced these individual memory chips. SIMMs, which were replaced
in turn by DIMMs, are considered to be one large memory chip, which is actually
composed of many small memory chips. If a problem develops, the entire SIMM or
DIMM is replaced instead of having to determine which individual chip is
responsible for the problem, as was done with DIPs.
Ports
Computer ports allow external devices to be connected
to the system board. There are a number of different ports that may be found on
a system board, including parallel, serial, universal
serial bus (USB), and small computer
system interface (SCSI) bus. Parallel ports act as printer adapters. Serial
ports support a number of low-speed peripherals, including modems, some
scanners, “serial” mice, and digital cameras. USBs are able to support up to
127 devices, far more than a traditional serial port; they’re also much faster
and can support a number of devices, including cameras, printers, external
drives, and Web cams. A SCSI bus can support a number of peripherals, including
hard disks, tape drives, CD-ROM drives, scanners, and other devices.
Resistors, Capacitors, and Diodes
At various places on the system board, you’ll see resistors
and capacitors. You might even see a set of resistors in something known as a resistor pack. The resistors and
capacitors help maintain the appropriate voltage and current for system board
components. If you look carefully, you’ll see very small devices that look like
resistors, except they don’t have the identifying colored bands. These devices
are diodes.
The Evolution of the System
Board
Since you’ll encounter many different types of systems,
we’ll give you some of the history of the evolution of system boards and then
show you some illustrations so you can see just how much they’ve changed. But
first, let’s discuss the common forms, or form
factors, used for system boards. By common forms, we mean the size and
shape of the boards, because the size of the board dictates what kind of case
the system board will fit into.
An AT System Board for a 486 Computer
Full-Size AT
The full-size AT system boards are large in order to
accommodate a lot of circuitry. Full-size AT system boards fit into
full-size AT desktop and tower cases. They won’t fit into mini-towers or
slimline-type cases. While a baby-AT can replace a full-size AT system board, a
full size board can’t replace a baby-AT board due to size constraints. For the
most part, full-size AT system boards are no longer being produced. With
advances in microprocessor technology, components are getting smaller, allowing
for smaller system boards.
Baby-AT
From 1983 to 1996, the baby-AT form size was the most
popular system board because it would fit into a number of different case
designs. In fact, today you can still get system boards of this design with any
processor installed, from the original 8088 to a Pentium. The baby-AT design
was replaced by the ATX design in 1996.
LPX
The LPX-type design is a
semi proprietary system board design used mostly in the slimline-type case
designs and also in some tower case designs. Because of their proprietary
nature, LPX and other proprietary designs, such as backplane systems, are difficult to repair and upgrade. If
something breaks on one of these designs, you have to go back to the original
manufacturer for replacement parts. Such replacement parts or upgrade parts are
very expensive. In these cases, you’re often better off purchasing a brand new
system that has a non proprietary design.
ATX
ATX designs aren’t
compatible with AT designs, because the system board is turned sideways in the
case and has a different power supply connector and location. Thus, ATX system boards use an entirely
different case design. This case design allows for longer boards and increases
airflow to reduce heat inside the case.
Initially, the airflow design used a positive-pressure
cooling system, blowing air into the case across the system board. However,
newer processors run at higher speeds and produce more heat; the
positive-pressure cooling system usually requires secondary heat sinks or
system board fans to dissipate that heat. As a result, manufacturers are migrating
back to the more effective negative-pressure
(air blowing out) method that’s used in the AT.
Another change from the AT was a new 20-pin keyed power connector. This
replaced the P8 and P9 connectors, which, if reversed, could burn out your
system board. The keyed design also helps in that the connector can only go in
one way.
Unlike the original AT, which required only 5 or 12 volts,
the ATX supports the lower voltages of newer processors (3.3 V and below). Less
voltage equates with less heat and less power consumption. In addition, the ATX
power supply can be turned on or off with software for scheduled startups or
shutdowns.
Just as the AT system board
has its baby-AT form factor, the ATX also has other form factors. The first is
the mini-ATX, which uses the same power supply
and case as the full-size ATX board, with the exception of being about an inch
shorter on each side (11.2 in. 8.2 in.). The micro ATX (9.6 in. 9.6
in.) and Flex ATX (9 in. 7.5 in.) are smaller-board versions.
An NLX System Board with Riser Card
These form factors are for low-end PCs, have their
own power supplies (which supply a constant 90 W), and support only socket-type
CPUs. ATX is the most popular design of newer systems and is easily upgradable
and repairable.
NLX/Micro-ATX
Introduced by Intel in 1997, the NLX format is smaller than
some of the other form factors, making it well suited for low-profile desktop
cases.
The NLX is designed with an
emphasis on easy maintenance. All expansion slots, power cables, and peripheral
connectors are located on an edge-mounted riser card to allow easy removal of
the system board (which is mounted on rails in the chassis). The NLX uses a
full-width input/output (I/O) shield to allow for different combinations of
rear-panel I/O. The NLX also has the ability to support the latest processor
technologies. The processor can be easily accessed and receives greater cooling
than those with a closed-in layout.
The NLX will support a
Pentium II system single-edge contact cartridge and is designed for greater
flexibility in the face of rapidly changing processor technologies. Its backplane-like flexibility is built into the form by allowing a new
system board to be easily and quickly installed without having to disassemble
the entire system.
The last type of system board to be discussed here is the micro ATX . Released around the same time as the NLX factor, the micro-ATX has
experienced more success than many of its fellow factors, as it is
backward-compatible with the ATX, which remains the most popular form factor as
of 2011. The micro-ATX uses less power than a full ATX board, mainly due to the
integration of many of the functions of a PC, such as a video adapter chip and
NIC, in addition to reducing the number of available expansion slots. Most
current micro-ATX boards use the latest version of the ATX standard, including
the 24-pin P1 power connector that carries wider voltage ranges and support for
PCI-E slots.
System board design is
constantly changing. These previous illustrations should give you a good start
in understanding the major differences between form factors. You’ll often come upon system boards and features you haven’t seen before. Take
it in stride and refer to the user’s manual. Look carefully at the system board
before you begin working, and you’ll soon figure it out. Remember that anytime
you have an opportunity to inspect or read about a system board, take advantage
of it, especially if an expert is present to lend a hand.
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