PC motherboard

A PC motherboard is a printed circuit board used in a personal computer. It is also known as the mainboard or planar board and occasionally abbreviated to mobo or MB. The term mainboard is also used for the main circuit board in this and other electronic devices.

A typical motherboard provides attachment points for one or more of the following: CPU, graphics card, sound card, hard disk controller, memory (RAM), and external peripheral devices. The connectors for external peripherals are nearly always color coded according to the PC 99 specification.

All of the basic circuitry and components required for a computer to function sit either directly on the motherboard or in an expansion slot of the motherboard or are connected with a cable. The most important component on a motherboard is the chipset. It often consists of two components or chips known as the Northbridge and Southbridge, though they may also be integrated into a single component. These chips determine, to an extent, the features and capabilities of the motherboard.

The remainder of this article discusses the state of the so-called "IBM compatible PC" motherboard in the early 2000s. It contains the chipset, which controls the operation of the CPU, the PCI, ISA, AGP, and PCI Express expansion slots, and (usually) the IDE/ATA controller as well. Most of the devices that can be attached to a motherboard are attached via one or more slots or sockets, although some modern motherboards support wireless devices using the IrDA, Bluetooth, or 802.11 (Wi-Fi) protocols.

CPU sockets

There are different slots and sockets for CPUs, and it is necessary for a motherboard to have the appropriate slot or socket for the CPU. Newer sockets, those with a three digit number, are named after the number of pins they contain. Older ones are simply named in the order of their invention, usually with a single digit.

Sockets supporting Intel processors

• Socket 6 - 80486DX4
• Socket 7 - Intel Pentium and Pentium MMX, AMD K6 and some Cyrix CPUs)
• Socket 8 - Intel Pentium Pro
• Slot 1 - Intel Pentium II, older Pentium III, and Celeron processors (233 MHz - 1.13 GHz)
• Slot 2 - Intel Xeon processors based on Pentium II/III cores
• Socket 370 - Celeron processors and newer Pentium IIIs (700 MHz - 1.4 GHz)
• Socket 423 - Intel Pentium 4 and Celeron processors (based on the Willamette core)
• Socket 478 - Intel Pentium 4 and Celeron processors (based on Northwood, Prescott, and Willamette cores)
• Socket 479 - Intel Pentium M and Celeron M processors (based on the Banias and Dothan cores)
• Socket M - Intel Core processors (based on the Yonah core)
• Socket 603 / 604 - Intel Xeon processors based on the Northwood and Willamette Pentium 4 cores
• Socket T / LGA 775 (Land Grid Array) - Intel Pentium 4, Core 2, and Celeron processors (based on Northwood, Prescott, Conroe, Kentsfield, and Cedar Mill cores)
• LGA 771 (Land Grid Array) - Intel Xeon Core 2, and based on Woodcrest cores

Sockets supporting AMD CPUs

• Slot A - or Socket 462 (aka Socket A) - newer AMD Athlon, Athlon XP, Sempron, and Duron processors
• Socket 754 - lower end AMD Athlon 64 and Sempron processors with single-channel memory support
• Socket 939 - AMD Athlon 64, AMD Athlon 64 FX, AMD Athlon 64 X2, and AMD Opteron processors with dual-channel memory support
• Socket 940 - AMD Opteron and early AMD Athlon FX processors
• Socket AM2 - Sempron, AMD Athlon 64, AMD Athlon 64 X2, AMD Athlon 64 FX and AMD Opteron (AMD Phenom?)
• Socket F - AMD Opteron and high-end AMD Athlon 64 FX

Peripheral card slots

There are usually a number of expansion card slots to allow peripheral devices and cards to be inserted. Each slot is compatible with one or more industry bus standards. Commonly available buses include: PCI (Peripheral Component Interconnect), PCI-X, AGP (Accelerated Graphics Port), and PCI Express.

ISA was the original bus for connecting cards to a PC. Despite significant performance limitations, it was not superseded by the more advanced but incompatible MCA (Micro Channel Architecture) (IBM's proprietary solution which appeared in their PS/2 series of computers and a handful of other models) or the equally advanced and backward-compatible EISA (Extended Industry Standard Architecture) bus. It endured as a standard feature in PCs till the end of the 20th century, aided first by the brief dominance of the VESA (Video Electronic Standards Association) extension during the reign of the 486 and later by the need to accommodate the large number of existing ISA peripheral cards. The more recent PCI bus is the current industry standard, which initially was a high-speed supplement to ISA for high-bandwidth peripherals (notably graphics cards, network cards, and SCSI host adaptors), and gradually replaced ISA as a general-purpose bus. An AGP slot is a high speed, single-purpose port designed solely for connecting high performance graphics cards (which produce video output) to the monitor. Both AGP and PCI buses are marked for replacement by PCI Express, although this is unlikely to happen prior to 2006 because of the large established base of AGP/PCI motherboards and add-in cards.

A typical motherboard of 1999 might have had one AGP slot, four PCI slots, and one (or two) ISA slots; since about 2002 the last ISA slots in new boards have been replaced with extra PCI slots. Sometimes an Advanced Communications Riser slot is used instead on less expensive motherboards.

As of 2001, most PCs also support Universal Serial Bus (USB) connections, and the controller and ports required for this are usually integrated onto the motherboard. An ethernet interface and a basic audio processor are now almost universally integrated into current motherboards as well.

Temperature and reliability

Generally, motherboards are air cooled with heat sinks on the larger chips such as the northbridge and CPU, and they have monitored sockets for case fans. Newer motherboards have integrated temperature sensors to detect motherboard and CPU temperatures, which can be used by the BIOS or Operating system to regulate fan speed. The removal of waste thermal energy became a major concern for workstation PCs around 2000, with the problem becoming more severe over time as computer systems continued to consume more and more power.

A study of the German c't computer magazine c't 2003, vol. 21 pg. 216-221 found that some spurious computer crashes and general reliability issues ranging from screen image distortions to I/O read/write errors can surprisingly be attributed not to software or peripheral hardware but to aging PC motherboards.

Motherboard voltage regulation uses electrolytic capacitors. These capacitors exhibit aging effects which depend on the temperature of the parts, since their water based electrolytes slowly evaporate leading to capacity loss and motherboard malfunctions due to voltage instabilities. While most capacitors are rated for 2000 hours at 105 °C (life formula ), their expected design life roughly doubles for every 10 °C below this. At 45 °C a lifetime of 15 years can be expected, which appears reasonable for a computer mainboard. In the past, many manufacturers delivered substandard capacitors, which would reduce this life expectancy figure. With inadequate case cooling this can become a serious problem. It is, however, possible to find and replace broken capacitors on PC mainboards. For more information on certain types of premature capacitor failure on PC motherboards, see capacitor plague.

Form factor

The motherboard fits into the computer case with screws or clips. There are many form factors, or sizes of motherboard. In general, it is necessary for the case, power supply, and motherboard to conform to the same standard in order for them to operate properly.

As new generations of components have been developed, the standards for motherboard design have changed too - for example with AGP being introduced, and more recently PCI Express. However the basic standardized size and layout of motherboard have changed much more slowly, and are controlled by their own standards. This is helped by the fact that in many ways, the list of components a motherboard must include changes far slower than the components themselves. For example, north bridge controllers have changed many times since their original introduction, with many manufacturers bringing out their own versions, but in terms of form factor standards, the requirement to allow for a north bridge has remained fairly static for many years.

Differences between form factors are most apparent in terms of their intended market sector, and involve variations in size, design compromises and typical features. Most modern computers have very similar requirements, so form factor differences tend to be based upon subsets and supersets of these. For example, a desktop computer may require more sockets for maximal flexibility and many optional connectors and other features on-board, whereas a computer to be used in a multimedia system may need to be optimized for heat and size, with additional plug-in cards being less common. The smallest motherboards may sacrifice CPU flexibility in favor of a fixed manufacturer's choice.