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Motherboard

The ASUS CUSL2-C motherboard

Connects to: Microprocessors via sockets Main memory via Slots Peripherals via one of External ports Internal cables

Form factors: ATX microATX Others

Common Manufacturers: ASUS Foxconn Intel Others

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

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

The basic purpose of the motherboard, like a backplane, is to provide the electrical and logical connections by which the other components of the system communicate.

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.

Components and functions

The motherboard of a typical desktop consists of a large PCB. 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 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]

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. 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.

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,^[7] 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 4 SATA drives (including RAID 0/1 support)
• integrated ATI Radeon graphics controller supporting 2D and 3D graphics, with VGA and TV output
• integrated sound card supporting 6-channel audio and S/PDIF output
• fast Ethernet network controller for 10/100 Mbit networking
• USB 2.0 controller supporting up to 8 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 USA.

History

Prior to the advent of the Apple II in 1977, a computer was usually built in a case or mainframe with components connected by a backplane consisting of a set of slots themselves connected with wires. The CPU, memory and I/O peripherals were housed on individual PCBs or cards which plugged into the backplane. With the arrival of the microprocessor, it became more cost-effective to place the backplane connectors, processor and glue logic onto a single "mother" board, with video, memory and I/O functions on "child" cards — hence the terms "motherboard" and daughterboard. The Apple II computer featured a motherboard with 8 expansion slots.

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 early 2000s, many 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.

It can be argued that the motherboard industry was born by IBM in 1981 with the release their entry level 5150 Personal Computer (IBM PC) which was based on a motherboard. The motherboard provided an Intel 4.77MHz 8088 with 16K bytes of on-board memory, expandable to 640K through the use of plug-in memory boards, eight 8-bit ISA expansion connectors, cassette tape port and keyboard port. All other I/O such as the interface for 160K 5-1/4" floppy drives, serial and parallel ports were provided by plug-in boards. IBM approached Digital Research about using DR/DOS as an operating system but was rebuffed. IBM approached Microsoft and licensed PC-DOS. Microsoft released PC-DOS 1.1 in 1982 by retaining rights to the operating system allowing them to sell it to other manufacturers.

IBM published the schematics and I/O map allowing the birth of the clone motherboard industry.

Software Meets Hardware: the BIOS

A computer motherboard is a piece of hardware: it is the physical circuits and interconnecting wires that forms the backbone of a computer. It has logic circuits which can be manipulated and controlled by the operator, the software program, and input peripherals. But in order to begin operating from a power-off state, a motherboard must be bootstrapped (or simply, booted) by an initial set of software instructions. Without this vital software, the motherboard is rendered useless.

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 CR3032) 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.^[8] 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.^[9] 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^[10] 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:

• floppy drive
• network controller
• CD-ROM drive
• DVD-ROM drive
• SCSI hard drive
• IDE, EIDE, or SATA hard drive
• External USB memory storage device

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

Form factors

Computer case form factors

Types: AT ATX microATX Mini-ITX Nano-ITX Pico-ITX Baby-AT BTX DTX ETX FlexATX LPX Mini-DTX NLX WTX Comparisons

Main article: PC motherboard

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.

These are some of the more popular motherboard form factors:

• PC/XT - created by IBM for the IBM PC, its first home computer. As the specifications were open, many clone motherboards were produced and it became a de facto standard.
• AT form factor (Advanced Technology) - created by IBM for its PC/XT successor, the AT. Also known as Full AT, it was popular during the era of the Intel 80386 microprocessor. Superseded by ATX.
• Baby AT - IBM's 1985 successor to the AT motherboard. Functionally equivalent to the AT, it became popular due to its significantly smaller size.
• ATX - created by Intel in 1995. As of 2007, it is the most popular form factor for commodity motherboards. Typical size is 9.6x12" although some companies extend that to 10x12".
• EATX - Refers to Extended ATX with a size of 13x12". Typically used for Server Class type motherboards with dual processors and too much circuitry for a standard ATX motherboard. The mounting hole pattern for the upper portion of the board matches ATX.
• ETX - used in embedded systems and single board computers.
• microATX - a smaller variant of the ATX form factor (about 25% shorter). It is compatible with most ATX cases, but supports fewer expansion slots due to its smaller size. Very popular for desktop and small form factor computers as of 2007.
• FlexATX - a subset of microATX developed by Intel in 1999. Allows more flexible motherboard design, component positioning and shape.
• LPX - based on a design by Western Digital, it allowed smaller cases than the AT standard, by putting the expansion card slots on a riser (image). LPX was never standardized and generally only used by large OEMs.
• NLX - a low-profile design released in 1997. It also incorporated a riser for expansion cards, and never became popular.
• BTX (Balanced Technology Extended) - a standard proposed by Intel as a successor to ATX in the early 2000s.
• Mini-ITX - a small, highly-integrated form factor created by VIA in 2001. Mini-ITX was designed for small devices such as thin clients and set-top boxes.
• WTX - created by Intel in 1998. A large design for servers and high-end workstations featuring multiple CPUs and hard drives.

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.

Visual comparison

This image compares the sizes of common form factors to ISO 216 paper sizes (e.g. A4):

See also

• PC motherboard, for more information about motherboards for "IBM-compatible" personal computers
• Computer case
• BIOS
• Chipset
• Front side bus
• List of manufacturers
• Offboard
• PCI
• PCI Express
• Accelerated Graphics Port (AGP)

Notes

External links

• Jumper settings for Motherboard
• What is a motherboard?
• The Making of a Motherboard: ECS Factory Tour
• The Making of a Motherboard: Gigabyte Factory Tour
• Motherboard reviews
• Open Directory motherboard links