Motherboard

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See also: PC motherboard
Motherboard

The ASUS CUSL2-C motherboard
Connects to:
Form factors:
Common Manufacturers:

A motherboard is the central or primary circuit board (PCB) making up a complex electronic system, such as a modern computer. It is also known as a mainboard, baseboard, system board, planar board, or, on Apple computers, a logic board, and is sometimes abbreviated casually as mobo.[1]

Most motherboards produced today are designed for so-called IBM-compatible computers, which held over 96% of the global personal computer market in 2005.[2] Motherboards for IBM-compatible computers are specifically covered in the PC motherboard article.

A motherboard, like a backplane, provides the electrical connections by which the other components of the system communicate, but unlike a backplane also contains the central processing unit and other subsystems such as real time clock, and some peripheral interfaces.

A typical desktop computer is built with the microprocessor, main memory, and other essential components on the motherboard. Other components such as external storage, controllers for video display and sound, and peripheral devices are typically attached to the motherboard via edge connectors and cables, although in modern computers it is increasingly common to integrate these "peripherals" into the motherboard.

Contents

[edit] Components and functions

The 2004 K7VT4A Pro motherboard by ASRock. The chipset on this board consists of northbridge and southbridge chips.
The 2004 K7VT4A Pro[3] motherboard by ASRock. The chipset on this board consists of northbridge and southbridge chips.

The motherboard of a typical desktop consists of a large printed circuit board. It holds electronic components and interconnects, as well as physical connectors (sockets, slots, and headers) into which other computer components may be inserted or attached.

Most motherboards include, at a minimum:

  • sockets (or slots) in which one or more microprocessors (CPUs) are installed[4]
  • slots into which the system's main memory is 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 and circuits, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.[5]
The Octek Jaguar V motherboard from 1993. This board has 6 ISA slots but few onboard peripherals, as evidenced by the lack of external connectors.
The Octek Jaguar V motherboard from 1993.[6] This board has 6 ISA slots but few onboard peripherals, as evidenced by the lack of external 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-comatible 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 heatsinks and mounting points for fans to dissipate excess heat.

[edit] Integrated peripherals

Diagram of a modern motherboard, which supports many on-board peripheral functions as well as several expansion slots.
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,[7] 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 USA.

[edit] Temperature and reliability

Motherboards are generally air cooled with heat sinks often mounted on larger chips, such as the northbridge, in modern motherboards. 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 heatsinks, 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 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[8] 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.[9]

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,[10] 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, which significantly reduce this 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; it is less expensive to buy a new motherboard than to pay for such a repair.

[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 backplane.

During the late 1980s and 1990s, it became economical to move an increasing number of peripheral functions onto the motherboard (see above). 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 support 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 retain 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.

Popular personal 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.

[edit] Bootstrapping using the BIOS

Main article: booting

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 read-only memory 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, which would in turn start loading memory from an external peripheral device (disk drive).


Most modern motherboard designs use a BIOS, stored in a EEPROM chip soldered to the motherboard, to bootstrap the motherboard. (Socketed BIOS chips are widely used, also.) By booting the motherboard, the memory, circuitry, and peripherals are tested and configured. This process is known as a Power On Self Test or POST. Errors during POST result in POST error codes, ranging from simple audible beeps from the speaker to complex diagnostic messages displayed on the video monitor.

The BIOS often requires configuration settings to be stored on the motherboard. Since configuration settings must be easily edited, these settings are often stored in non-volatile RAM (NVRAM) rather than in some sort of read-only memory (ROM). When a user makes configuration changes or alters the date and time of the computer, this small NVRAM circuit stores the data. Typically, a small, long-lasting battery (e.g. a lithium coin cell CR2032) is used to keep the NVRAM "refreshed" for many years. Therefore, a failing battery on a motherboard will produce the symptoms of a computer that cannot determine the correct date and time, nor remember what hardware configuration the user has selected. The BIOS itself is unaffected by the status of the battery.

When IBM first introduced the PC in the 1980s, imitations were quite common. (The physical parts which made up the motherboard were trivial to acquire.) However, the imitations were never successful until the IBM ROM BIOS was legally copied.[11] To understand why copying the BIOS was an important step, consider that the BIOS contained vital instructions which interacted with peripherals. Without these software instructions in the BIOS, a PC would not function properly. (In most modern computer operating systems, the BIOS is bypassed for most hardware functions, but in the 1980s, the BIOS served many vital low-level functions.)

So when Compaq Computer Corp. spent US$1 million to clone the IBM BIOS using reverse engineering, they became an elite computer manufacturer of IBM PC Clones. Phoenix Technology soon matched their feat and began reselling BIOSes to other clone makers.[12] It has been noted that Microsoft was more than happy to license the operating system (DOS), and IBM was more than happy to sue companies[13] that violated the copyright of their BIOS. But by documenting and publicizing the reverse engineering of the BIOS, Compaq and Phoenix were legally competing with IBM using their own copyrighted BIOS.

Once the bootstrapping of the computer's peripherals are complete, the BIOS will normally pass control to another set of instructions stored on a bootable device.

Devices which are normally used to boot a computer:

Any of the above devices can be stored with machine code instructions to load an operating system or a program.

[edit] Form factors

Main article: Comparison of computer form factors

Motherboards are produced in a variety of sizes and shapes ("form factors"), some of which are specific to individual computer manufacturers. However, the motherboards used in IBM-compatible commodity computers have been standardized to fit various case sizes. As of 2007, most desktop computer motherboards use one of these standard form factors—even those found in Macintosh and Sun computers which have not traditionally been built from commodity components.

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] See also

Motherboards for sale at retail.
Motherboards for sale at retail.

[edit] Notes

  1. ^ The abbreviated name "mobo" is generally credited to the magazine Maximum PC.
  2. ^ See the chart at the bottom of this page: http://arstechnica.com/articles/culture/total-share.ars/10
  3. ^ K7VT4A Pro. Retrieved on 2007-06-27.
  4. ^ In the case of CPUs in BGA packages, such as the VIA C3, the CPU is directly soldered to the motherboard.
  5. ^ 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. (Note that most disk drives also connect to the power supply via dedicated connectors.)
  6. ^ Golden Oldies: 1993 mainboards. Retrieved on 2007-06-27.
  7. ^ RS485M-M (V1.0). Retrieved on 2007-06-27.
  8. ^ c't Magazine, vol. 21, pp. 216-221. 2003.
  9. ^ Yu-Tzu Chiu, Samuel K. Moore "Faults & Failures: Leaking Capacitors Muck up Motherboards" (2003-02-19) IEEE Spectrum accessed 2008-03-10
  10. ^ See the capacitor lifetime formula at [1].
  11. ^ I, Cringely . The Pulpit . Real Trouble | PBS
  12. ^ Reverse-Engineering
  13. ^ like Eagle Computer and Corona Data Products, both sued by IBM.

[edit] External links

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