In
personal computers, a
motherboard is the central
printed circuit board (PCB) in many modern
computers
and holds many of the crucial components of the system, providing
connectors for other peripherals. The motherboard is sometimes
alternatively known as the
mainboard,
system board, or, on
Apple computers, the
logic board.
[1] It is also sometimes casually shortened to
mobo.
Motherboard for an Acer desktop personal computer, showing the typical
components and interfaces that are found on a motherboard. This model
was made by Foxconn in 2008, and follows the
ATX layout (known as the "
form factor") usually employed for desktop computers. It is designed to work with AMD's
Athlon 64 processor.
A motherboard of a Vaio E series laptop (right)
[edit] History
Prior to the advent of the
microprocessor, a computer was usually built in a card-cage case or
mainframe with components connected by a
backplane
consisting of a set of slots themselves connected with wires; in very
old designs the wires were discrete connections between card connector
pins, but printed circuit boards soon became the standard practice. The
Central Processing Unit, memory and
peripherals
were housed on individual printed circuit boards which plugged into the
backplate. During the late 1980s and 1990s, it became economical to
move an increasing number of peripheral functions onto the motherboard (
see below). In the late 1980s, motherboards began to include single ICs (called
Super I/O chips) capable of supporting a set of low-speed peripherals:
keyboard,
mouse,
floppy disk drive,
serial ports,
and parallel ports. As of the late 1990s, many personal computer
motherboards supported a full range of audio, video, storage, and
networking functions without the need for any
expansion cards at all; higher-end systems for
3D gaming and
computer graphics typically retained only the graphics card as a separate component.
The early pioneers of motherboard manufacturing were
Micronics,
Mylex,
AMI, DTK,
Hauppauge,
Orchid Technology,
Elitegroup,
DFI, and a number of Taiwan-based manufacturers.
The most popular computers such as the
Apple II and
IBM PC had published schematic diagrams and other documentation which permitted rapid
reverse-engineering
and third-party replacement motherboards. Usually intended for building
new computers compatible with the exemplars, many motherboards offered
additional performance or other features and were used to upgrade the
manufacturer's original equipment
The term mainboard is applied to devices with a single board and no
additional expansions or capability. In modern terms this would include
embedded systems
and controlling boards in televisions, washing machines, etc. A
motherboard specifically refers to a printed circuit board with
expansion capability.
[edit] Overview
A motherboard, like a
backplane,
provides the electrical connections by which the other components of
the system communicate, but unlike a backplane, it also connects the
central processing unit and hosts other subsystems and devices.
A typical
desktop computer has its
microprocessor,
main memory, and other essential components connected to the motherboard. Other components such as
external storage, controllers for
video display and
sound, and
peripheral
devices may be attached to the motherboard as plug-in cards or via
cables, although in modern computers it is increasingly common to
integrate some of these peripherals into the motherboard itself.
An important component of a motherboard is the microprocessor's supporting
chipset,
which provides the supporting interfaces between the CPU and the
various buses and external components. This chipset determines, to an
extent, the features and capabilities of the motherboard.
Modern motherboards include, at a minimum:
- sockets (or slots) in which one or more microprocessors may be installed[2]
- slots into which the system's main memory is to be installed (typically in the form of DIMM modules containing DRAM chips)
- a chipset which forms an interface between the CPU's front-side bus, main memory, and peripheral buses
- non-volatile memory chips (usually Flash ROM in modern motherboards) containing the system's firmware or BIOS
- a clock generator which produces the system clock signal to synchronize the various components
- slots for expansion cards (these interface to the system via the buses supported by the chipset)
- power connectors, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.[3]
The Octek Jaguar V motherboard from 1993.
[4] This board has few onboard peripherals, as evidenced by the 6 slots provided for
ISA cards and the lack of other built-in external interface connectors.
Additionally, nearly all motherboards include logic and connectors to support commonly used input devices, such as
PS/2 connectors for a
mouse and keyboard. Early
personal computers such as the
Apple II or
IBM PC
included only this minimal peripheral support on the motherboard.
Occasionally video interface hardware was also integrated into the
motherboard; for example, on the Apple II and rarely on IBM-compatible
computers such as the
IBM PC Jr. Additional peripherals such as
disk controllers and
serial ports were provided as expansion cards.
Given the high
thermal design power of high-speed computer CPUs and components, modern motherboards nearly always include
heat sinks and mounting points for
fans to dissipate excess heat.
[edit] CPU sockets
A CPU socket or slot is an electrical component that attaches to a
printed circuit board (PCB) and is designed to house a CPU (also called a
microprocessor). It is a special type of integrated circuit socket
designed for very high pin counts. A CPU socket provides many functions,
including a physical structure to support the CPU, support for a heat
sink, facilitating replacement (as well as reducing cost), and most
importantly, forming an electrical interface both with the CPU and the
PCB. CPU sockets can most often be found in most desktop and
server computers (laptops typically use surface mount CPUs), particularly those based on the Intel
x86 architecture on the motherboard. A CPU socket type and motherboard chipset must support the CPU series and speed.
[edit] Integrated peripherals
Block diagram of a modern motherboard, which supports many on-board peripheral functions as well as several expansion slots.
With the steadily declining costs and size of
integrated circuits, it is now possible to include support for many
peripherals on the motherboard. By combining many functions on one
PCB, the physical size and total cost of the system may be reduced; highly integrated motherboards are thus especially popular in
small form factor and budget computers.
For example, the
ECS RS485M-M,
[5] a typical modern budget motherboard for computers based on
AMD processors, has on-board support for a very large range of peripherals:
Expansion cards to support all of these functions would have cost
hundreds of dollars even a decade ago; however, as of April 2007 such highly integrated motherboards are available for as little as $30 in the US.
[edit] Peripheral card slots
A typical motherboard of 2009 will have a different number of connections depending on its standard.
A standard ATX motherboard will typically have one PCI-E 16x
connection for a graphics card, two conventional PCI slots for various
expansion cards, and one PCI-E 1x (which will eventually supersede
PCI). A standard
EATX
motherboard will have one PCI-E 16x connection for a graphics card, and
a varying number of PCI and PCI-E 1x slots. It can sometimes also have a
PCI-E 4x slot. (This varies between brands and models.)
Some motherboards have two PCI-E 16x slots, to allow more than 2
monitors without special hardware, or use a special graphics technology
called
SLI (for
Nvidia) and
Crossfire (for
ATI).
These allow 2 graphics cards to be linked together, to allow better
performance in intensive graphical computing tasks, such as gaming and
video editing.
As of 2007, virtually all motherboards come with at least four
USB
ports on the rear, with at least 2 connections on the board internally
for wiring additional front ports that may be built into the computer's
case.
Ethernet is also included. This is a standard networking cable for connecting the computer to a
network or a
modem. A sound chip is always included on the motherboard, to allow sound output without the need for any extra
components. This allows computers to be far more
multimedia-based
than before. Some motherboards contain video outputs on the back panel
for integrated graphics solutions (either embedded in the motherboard,
or combined with the microprocessor, such as the Intel HD Graphics). A
separate card may still be used.
[edit] Temperature and reliability
Motherboards are generally
air cooled with
heat sinks often mounted on larger chips, such as the
Northbridge,
in modern motherboards. Insufficient or improper cooling can cause
damage to the internal components of the computer and cause it to
crash.
Passive cooling, or a single fan mounted on the
power supply, was sufficient for many desktop computer CPUs until the late 1990s; since then, most have required
CPU fans mounted on their
heat sinks, due to rising clock speeds and power consumption. Most motherboards have connectors for additional
case fans
as well. Newer motherboards have integrated temperature sensors to
detect motherboard and CPU temperatures, and controllable fan connectors
which the
BIOS or
operating system can use to regulate fan speed. Some computers (which typically have high-performance microprocessors, large amounts of
RAM, and high-performance
video cards) use a
water-cooling system instead of many fans.
Some
small form factor computers and
home theater PCs
designed for quiet and energy-efficient operation boast fan-less
designs. This typically requires the use of a low-power CPU, as well as
careful layout of the motherboard and other
components to allow for heat sink placement.
A 2003 study
[6] found that some spurious computer crashes and general reliability issues, ranging from screen image distortions to
I/O read/write errors, can be attributed not to
software or peripheral
hardware but to aging
capacitors on PC motherboards. Ultimately this was shown to be the result of a faulty electrolyte formulation.
[7]
A microATX motherboard with some faulty capacitors.
- For more information on premature capacitor failure on PC motherboards, see capacitor plague.
Motherboards use
electrolytic capacitors to filter the
DC power distributed around the board. These capacitors age at a temperature-dependent rate, as their water based
electrolytes slowly evaporate. This can lead to loss of capacitance and subsequent motherboard malfunctions due to
voltage instabilities. While most capacitors are rated for 2000 hours of operation at 105 °C,
[8]
their expected design life roughly doubles for every 10 °C below this.
At 45 °C a lifetime of 15 years can be expected. This appears reasonable
for a computer motherboard. However, many manufacturers have delivered
substandard capacitors,
[9]
which significantly reduce life expectancy. Inadequate case cooling and
elevated temperatures easily exacerbate this problem. It is possible,
but tedious and time-consuming, to find and replace failed capacitors on
PC motherboards.
[edit] Form factor
Motherboards are produced in a variety of sizes and shapes called
computer form factor, some of which are specific to individual
computer manufacturers. However, the motherboards used in IBM-compatible systems are designed to fit various
case sizes. As of 2007, most
desktop computer motherboards use one of these
[which?] standard form factors—even those found in
Macintosh and
Sun
computers, which have not been built from commodity components. A
case's motherboard and PSU form factor must all match, though some
smaller form factor motherboards of the same family will fit larger
cases. For example, an ATX case will usually accommodate a
microATX motherboard.
Laptop
computers generally use highly integrated, miniaturized and customized
motherboards. This is one of the reasons that laptop computers are
difficult to upgrade and expensive to repair. Often the failure of one
laptop component requires the replacement of the entire motherboard,
which is usually more expensive than a desktop motherboard due to the
large number of integrated components.
[edit] Bootstrapping using the BIOS
Motherboards contain some
non-volatile memory to initialize the system and load an
operating system from some external peripheral device. Microcomputers such as the Apple II and IBM PC used
ROM
chips, mounted in sockets on the motherboard. At power-up, the central
processor would load its program counter with the address of the boot
ROM and start executing ROM instructions, displaying system information
on the screen and running memory checks, which would in turn start
loading memory from an external or peripheral device (disk drive). If
none is available, then the computer can perform tasks from other memory
stores or display an error message, depending on the model and design
of the computer and version of the BIOS.
Most modern motherboard designs use a
BIOS, stored in an
EEPROM chip soldered or socketed to the motherboard, to
bootstrap an
operating system.
When power is first applied to the motherboard, the BIOS firmware tests
and configures memory, circuitry, and peripherals. This
Power-On Self Test (POST) may include testing some of the following things:
On recent motherboards, the BIOS may also patch the central processor
microcode if the BIOS detects that the installed CPU is one in for
which
errata has been published. Many of the above devices can be stored with
machine code instructions to load an
operating system or
program.
[edit] See also
[edit] References
[edit] External links
