From Wikipedia, the free encyclopedia
A motherboard (sometimes alternatively known as the mainboard, system board, planar board or logic board, or colloquially, a mobo) is a printed circuit board (PCB) found in computers and other expandable systems. It holds many of the crucial components of the system, such as the central processing unit (CPU) and memory, and provides connectors for other peripherals. Unlike a backplane, a motherboard contains significant sub-systems such as the processor.
Motherboard specifically refers to a PCB with expansion
capability - the board is the "mother" of all components attached to it,
which often include sound cards, video cards, network cards, hard drives or other forms of persistent storage, TV tuner cards, cards providing extra USB or Firewire slots, and a variety of other custom components. (The term mainboard
is applied to devices with a single board and no additional expansions
or capability - in modern terms this would include controlling boards in
televisions, washing machines and other embedded systems, which are not true motherboards.)
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. 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.
By 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 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.
Design
A motherboard provides the electrical connections by which the other
components of the system communicate. 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, 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. In the case of CPUs in BGA packages, such as the VIA C3, the CPU is directly soldered to the motherboard.
- 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. As of 2007, some graphics cards (e.g. GeForce 8 and Radeon R600)
require more power than the motherboard can provide, and thus dedicated
connectors have been introduced to attach them directly to the power
supply. Most disk drives also connect to the power supply via dedicated 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.
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 on the motherboard 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. A CPU socket type and motherboard chipset must support the CPU series and speed.
Integrated peripherals
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, a typical modern budget motherboard for computers based on AMD processors, has on-board support for a very large range of peripherals:
- Disk controllers for a floppy disk drive, up to 2 PATA drives, and up to 6 SATA drives (including RAID 0/1 support)
- integrated graphics controller supporting 2D and 3D graphics, with VGA and TV output
- integrated sound card supporting 8-channel (7.1) audio and S/PDIF output
- Fast Ethernet network controller for 10/100 Mbit networking
- USB 2.0 controller supporting up to 12 USB ports
- IrDA controller for infrared data communication (e.g. with an IrDA-enabled cellular phone or printer)
- Temperature, voltage, and fan-speed sensors that allow software to monitor the health of computer components
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.
Peripheral card slots
A typical motherboard of 2012 will have a different number of connections depending on its standard. A standard ATX motherboard will typically have two or three PCI-E 16x
connection for a graphics card, one or two legacy PCI slots for various
expansion cards, and one or two PCI-E 1x (which has superseded PCI). A standard EATX motherboard will have two to four PCI-Express
16x connection for graphics cards, 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 or more 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 to 4 graphics cards to be linked together, to allow
better performance in intensive graphical computing tasks, such as
gaming, video editing, et cetera.
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 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, an issue termed 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 (221 °F), their expected design life roughly doubles for every 10 °C (50 °F) below this. At 45 °C (113 °F)
a lifetime of 15 years can be expected. This appears reasonable for a
computer motherboard. However, many manufacturers have delivered
substandard capacitors,
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
Personal Computer (PC) motherboards.
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 the ATX standard form factor — 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.
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.
These instructions displayed system information on the screen, ran
memory checks, and then loaded an operating system from an external or
peripheral device (disk drive). If none was available, then the computer
would 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:
- Video adapter
- Cards inserted into slots, such as conventional PCI
- Floppy drive
- Thermistors, voltages, and fan speeds for hardware monitoring
- CMOS used to store BIOS setup configuration
- keyboard and mouse
- network controller
- Optical drives: CD-ROM or DVD-ROM
- SCSI hard drive
- IDE, EIDE, or SATA hard disk
- Security devices, such as a fingerprint reader or the state of a latch switch to detect intrusion
- USB devices, such as a memory storage device
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.