The motherboard has been an integral part of most personal computers for more than 20 years. Think of a motherboard as a scale model of a futuristic city with many modular plug-in buildings, each using power from a common electrical system. Multiple-lane highways of various widths transport data between the buildings. The motherboard is the data and power infrastructure for the entire computer.

Motherboards (also called mainboards) are actually a carryover from architecture used for years in mainframe computers. Various circuit cards performing various functions all plug into many similar sockets on a common circuit board. Each circuit card performs a unique function in the computer and gets its power from the socket as well.

Due to improvements in circuitry and packaging, motherboards have essentially stayed the same size or shrunk (in square inches), while their functionality has skyrocketed in the past 20 years. In this edition of, you will learn more about how the motherboard works, and about a motherboard's many sockets and connectors.

A Brief History
The original IBM PC contained the original PC motherboard. In this design, which premiered in 1982, the motherboard itself was a large printed circuit card that contained the 8088
microprocessor, the BIOS, sockets for the CPU's RAM and a collection of slots that auxiliary cards could plug into. If you wanted to add a floppy disk drive or a parallel port or a joy stick, you bought a separate card and plugged it into one of the slots. This approach was originally pioneered in the mass market by the Apple II machine. By making it easy to add cards, Apple and IBM accomplished two things:

  • They made it easy to add new features to the machine over time
  • They opened the computer to creative opportunities by third-party vendors
Motherboard Sizes
Different motherboards of different vintages typically have different form factors. Form factor essentially means the size and shape of the actual motherboard. There are more than a half-dozen form factors for motherboards, with the most recent ones having the designation of NLX. Right now, the designation ATX is the most prevalent. By buying a computer with a true ATX motherboard, you are assured that you will have the ability to upgrade by being able to re-use the personal computer case with a more recent replacement ATX board design.

Motherboards have helped to keep the "personal" in personal computing since pluggable components allow the user to personalize the system depending on their applications and needs. For example:

  • A prolific collector of digital camera images or video will want to add a SCSI hard disk drive to an open bay and use an empty socket on the motherboard for the SCSI controller card.
  • A serious game enthusiast will want the fastest video card possible with as much memory on the card as possible.

Common Motherboard Items
A motherboard is a multi-layered printed circuit board. Copper circuit paths called traces that resemble a complicated roadmap carry signals and voltages across the motherboard. Layered fabrication techniques are used so that some layers of a board can carry data for the input/output, processor and memory buses while other layers can carry voltage and ground returns without circuit paths short-circuiting at intersections. The insulated layers are manufactured into one complete, complex "sandwich."

Chips and sockets are soldered onto the motherboard. For example, you will typically find:

  • one or more microprocessors
  • a basic input/output system chip (BIOS)
  • memory slots
  • a chip set that adds lots of features like I/O ports and controllers
  • peripheral component interconnect (PCI) adapter card slots
  • industry standard architecture (ISA) adapter slots
  • accelerated graphics port (AGP) video card slots
  • universal serial bus (USB) ports
  • cooling fan(s) on heat sinks of processor and some video cards

The MSI 694D Pro AR supports dual Pentium central processing units (CPUs), has five PCI slots and a communications network riser (CNR) slot. The board supports 133 MHz bus speeds and ultra-direct memory access-100 (UDMA). There are four USB ports and onboard audio in the ATX form factor board.

MSI 694D Pro AR Dual Flip Chip Socket 370 Motherboard

The Abit KT-7A supports Advanced Micro Devices (AMD) processors and has the KT-133A chipset. The card slots on the Abit KT-7A, from bottom to top, shows below that ISA has one slot, PCI has six slots and AGP has one slot. A special fan cools the chipset.

Abit KT-7A AMD Processor Motherboard

A partial view of the TechRam S3ProM motherboard shows slots, from bottom to top, that ISA has one slot, PCI has two slots, audio modem riser (AMR) has one slot, and AGP has one slot.

TechRam S3ProM Motherboard

The BIOS chip is common to many motherboards.


Data Bus Width
Modern Pentium class motherboards have a data bus with 64 bits. That is the width of the data highway that goes in and out of the processor. The Pentium processors, however, do use 32-bit registers to handle 32-bit instructions.

Bus speeds and widths have increased due to faster processors and the needs of multimedia applications. Typical bus names and widths (in bits) are:

How Have Motherboards Changed?
Speeds, temperatures, density, faster chipset designs and component count have driven the need for circuit cooling via miniature electric fans. These fans mount inside the actual computer case. Heat sinks act like an automobile radiator and provide additional surface area to help cool a component. Replaceable fan-heat sink assemblies are often used to help dissipate the considerable amount of heat on modern processor chips. The fan-heat sink assembly conducts heat away from the chip by convection, using a layer of thermal grease between the two mating metal surfaces. Fans often have a third wire used for monitoring the speed of the fan.

Modern motherboard designs include provisions for monitoring:

  • fan speed in RPM for the personal computer case, processor and power supply fans
  • temperatures of motherboard and processor
  • personal computer case intrusion
PCI slots are replacing the older ISA slots, and both types of slots are being replaced by USB ports. USB ports can also be used to replace the usual keyboard, mouse and printer ports. Sound card function is also typically incorporated into modern motherboards. Multifunction chips are on the horizon that will do even more multiple tasks.

The additional function on the motherboard saves the motherboard manufacturer costs because:

  • there are less warranty claims due to problems associated with all the many electrical contacts (fasteners) in the usual card slot
  • there are lower power supply wattage requirements
  • there are savings from elimination of a slot's socket and its space on the motherboard
The consumer can still upgrade function integrated on the motherboard (such as audio and game controls) so long as the motherboard manufacturer provides a means of disabling the function in order to prevent subsequent system resource conflicts.

A motherboard still may have voltages present on it even if the computer is switched off due to recent advances in power management and power controls. Always make sure that the power cord is unplugged!

Chipsets provide the support for the
processor chip on the motherboard. The Intel 440BX is the dominant chipset in the non-Apple personal computers. The chipset is the heart of the computer since it controls and determines how fast and which type of processor, memory, and slots are used. Another chip on the motherboard is called the Super I/O controller. Its main function is to control the floppy disk drive, keyboard, mouse, serial and printer ports.

Recent motherboard designs include additional chips to support USB, sound card, video adapter, computer host and network adapter. These chips save the cost of an adapter slot.

Advice on Motherboards
When buying a motherboard, follow these tips:

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