When you’re responsible for maintaining and upgrading the personal computers in your organization, you need to recognize the functions performed by the various components of the computer. The central component in all systems is the motherboard (sometimes referred to as the system board or the main board). Regardless of the name, these boards contain the essential elements of the computer.

Motherboards are designed around three key pieces:

  • The processor, or, more generally, the processor family (e.g., Pentium II)
  • The main logic chipset
  • The system BIOS

In other words, once you have selected a motherboard, you will not be able to change any of these elements. They are integral to the design of the board. Let’s take a brief look at the functions performed by each of these.

The processor family
The processor family defines the type of processor the motherboard will support. Modern boards will have one of three possible sockets for the various processors that are available:

  • Socket 7 supports classic Intel Pentium processors at speeds up to 233 MHz, with the additional MMX instructions supported on 166 and faster versions of the chip. An enhanced version of Socket 7, Super Socket 7, also supports AMD and Cyrix processors at speeds up to 550 MHz in the same socket. In Socket 7 architecture, the processor is mounted flat on the motherboard.
  • Slot 1 supports the newer Intel Pentium II processors and the first generation of Celerons (up to 300 MHz), as well as the earlier Pentium III chips (up to 667 MHz). This slot mounts the processor on edge, so that it stands up vertically on the board. Intel now calls this connector the SC242 slot connector and refers to this form factor as S.E.C.C., or Single Edge Contact Cartridge.
  • Socket 370 is the newest socket for the latest Celeron and Pentium III processors. This socket has packaging similar to the classic Pentiums, with a design similar to the Socket 7 architecture.

In buying a new or replacement motherboard, a Socket 370 design will accommodate any of the latest Celeron or Pentium III chips, but not the Pentium II or earlier Celerons or Pentium IIIs. Intel provides a list of form factors for Celeron chips. The situation for Pentium IIIs is a bit confusing. Intel is phasing out the Slot 1 for Pentium IIIs, but some models come in both versions. There are Pentium III models from 500 MHz to 1 GHz in the Socket 370 version, and models from 450 MHz to 1.13 GHz in the Slot 1, or SC242 version. For more information, check out Intel’s Pentium III Product Overview. If you require the flexibility to accept any of the Pentium II generation or later chips, this can be accomplished by choosing a Slot 1 design. For less than $10 U.S. you can purchase a Socket 370 to Slot 1 adapter, which will allow use of a Socket 370 processor in a Slot 1 motherboard.

Figure A shows an ASUS motherboard (model CUBX) with a Socket 370 connector. Figure B is a diagram of the same motherboard, showing where all the components are placed.

Figure A
This photo of an advanced ASUS CUBX motherboard shows a typical layout. The Socket 370 appears, circled, at the upper left. Courtesy of ASUS.

Figure B
A diagram of how all the components are placed on the ASUS CUBX motherboard. Courtesy of ASUS.

The main logic chipset
The second design element, the main logic chipset, describes the chips that govern communications across the motherboard by all the components through the system bus. This will usually be accomplished by one or two chips on the board, which can often be identified by markings on the chips. The chipsets you are most likely to encounter come from Intel, Via, SiS, or ETEQ. In some cases the main component of the chipset is covered by a heat sink, so you may not be able to read the markings on the chip.

In sending signals across the system bus, the chipset handles various communications at different speeds. For example, the fastest communications across the bus involve memory access; the slowest involve keyboard input. The chipset defines speeds for different parts of the system. The Intel chipset architecture has two parts, the North Bridge and the South Bridge. The North Bridge is responsible for the fastest devices, and the South Bridge, which is further divided into multiple speed levels, handles slower devices. Figure C views the Intel main logic chipset from the ASUS motherboard shown above.

Figure C
Note that the Intel chip of this ASUS CUBX motherboard is marked AGP. This Intel main logic chipset is designed to optimize the Pentium III and can boost the external bus speed from 66 to 100 MHz. Courtesy of ASUS.

The system BIOS
The third design element on the motherboard is the BIOS, or Basic Input/Output System. This chip is responsible for conducting all input/output operations, and also contains the logic that runs the Power On Self Test (POST) when the system is powered up. The BIOS gives the user the ability to specify various system options, including the input/output devices supported and attached, memory configuration, and features for the convenience and/or security of the user.

The BIOS will usually be a single integrated circuit on the motherboard, sometimes in a socket but frequently soldered permanently in place. The most popular BIOS manufacturers on modern motherboards are Award and American Megatrends, Inc. (AMI). Some computer manufacturers have their own custom BIOS chips. Figure D shows an Award BIOS positioned next to a motherboard’s CMOS battery (discussed below).

Figure D
This motherboard uses an Award BIOS. Note the CMOS battery to the left.

Other components on the motherboard
Expansion slots
A basic consideration in the design of a motherboard is the provision for inserting expansion cards to handle optional input/output devices that are not included on the motherboard itself. The traditional expansion slots are the older ISA slots, which handle 16 bits of data at a time at a speed of 8 MHz, and the newer, 32-bit PCI slots, which communicate with the system bus at 33 MHz.

The newest type of expansion slot is the Accelerated Graphics Port, or AGP, which is found only on Pentium II–class or Super Socket 7 or greater motherboards. This slot is used only for video and offers several advantages over the earlier ISA and PCI video cards, including communications across the system bus at up to four times the standard bus speed.

Newer motherboards will include some combination of these expansion slots, although the newest have few, if any, ISA slots. On the other hand, there will rarely be more than one AGP slot, and none will appear on older Socket 7 boards. Most boards will include at least one pair of adjacent ISA and PCI slots, which will share a space on the back of the computer case for the bracket of the inserted card. In a shared ISA/PCI environment, one slot or the other may contain an expansion card, but there is only one hole in the back of the case to accommodate either card. The ASUS motherboard shown in Figure A has one ISA slot (at the extreme left), and six PCI slots.

Memory sockets
One critical component of any motherboard is a socket or several sockets for Random Access Memory (RAM). This is the main system memory, which on Pentium-class motherboards will be packaged in Single Inline Memory Modules (SIMMs) or Dual Inline Memory Modules (DIMMs).

Early Socket 7 Pentium motherboards typically use 72-pin SIMMs for main system memory. If a motherboard uses 72-pin SIMM sockets, this memory must be installed in pairs. Most Pentium motherboards with 72-pin SIMMs will have four, six, or eight sockets. These pairs of memory sockets will be designated bank 0, bank 1, etc.

The latest Slot 1 and Socket 370 Pentium motherboards use only DIMMs for main system memory. A single DIMM contains a full bank of memory for a Pentium-class processor, and a motherboard may contain one to three DIMM sockets. Using today’s memory technology, a single DIMM socket may contain up to 256 MB of RAM, and 512 MB will be available in the near future. Consequently, a single DIMM socket on the motherboard may be adequate for most users.

Some Pentium motherboards contain both SIMM and DIMM sockets, giving you the flexibility to use old 72-pin SIMMs that you may already have in inventory, or the newer DIMMs if you need to buy new memory anyway. In most cases you probably will not be able to use both on the same motherboard at the same time. The ASUS motherboard in Figure A has four DIMM sockets. They appear below the Socket 370.

The CMOS battery
The Complementary Metal Oxide Semiconductor (CMOS) battery (shown in Figure C) is used to maintain the settings in the BIOS. This type of memory is volatile, which means it does not retain its information when power is no longer applied to the CMOS.

When a PC is powered up, the normal flow of electricity through the motherboard keeps the CMOS settings alive. When the system is powered down or suffers a power interruption, the CMOS battery takes over to maintain those settings. The CMOS battery will normally last from three to five years; your mileage may vary considerably, depending upon various factors. The most important variable in this equation is probably the amount of time the computer spends powered off—the more time it spends in this environment, the shorter your battery life will be.

An important point to note here is that the CMOS battery is never rechargeable. There are only two possible states for the battery—it is either being discharged, or it is not. In other words, every motherboard will require a battery replacement eventually, if it stays in service long enough.

When it is time to replace the CMOS battery, the system will give you adequate warning. You will start to notice the system clock slowing down at an accelerating rate. So, if your system clock has been fairly accurate—then it loses 10 minutes in one day, and 30 minutes the next day—you can be fairly certain it’s telling you that it’s time to replace the battery.

If you ignore the warning signs and wait until the battery is completely exhausted, the symptom will be a PC that doesn’t know how to boot. The CMOS contains information about what devices (hard drives, floppies, CD-ROMs) are attached to your system and the desired sequence of boot devices. When that information is missing, you will see an error message. The exact error message will vary depending on your specific BIOS and motherboard, but it will say something like “CMOS information not found” or “Must run Setup.” If you can’t replace the battery when you see such a message, you can still go into the Setup routine and enter the necessary information. Those settings will remain in effect until you power off the system.

Connectors for I/O devices
Every motherboard contains connectors that are used to attach input/output devices, such as your hard drives, CD-ROMs, and floppy drives. These connectors will have a varying number of pins, depending on the type of device that is to be attached. The Primary and Secondary IDE Controllers will each have 40 pins, which correspond to the 40 pins on IDE devices. The Floppy Disk Controller uses 34 pins, and you may also have additional connectors for other devices. In Figure A, IDE connectors appear in the bottom center of the ASUS motherboard. The floppy drive connector is to the left.

In attaching these devices, you must connect Pin 1 of the motherboard connector to Pin 1 on the device, using a flat ribbon cable. (Use the red stripe on the ribbon cable as a guide.)

You may find that some of these motherboard connectors have plastic frames around them, which is a great help in properly aligning the cable connector with the pins on the motherboard. Without these frames, it’s easy to miss a pin or two on the vertical axis, or an entire row of pins on the horizontal axis.

You might experience a false sense of security when you see these frames, because most of them contain what appears to be a “key” notch on one side. Many technicians assume that this key will prevent them from attaching a cable “backwards,” with Pin 1 facing the wrong end. In most cases, you will find that the cable itself is not keyed, meaning that it can be attached properly or 180 degrees out of proper alignment. So, always assume it is possible to connect a cable the wrong way!

Jumpers and/or DIP switches
Most motherboards have jumpers and/or DIP switches with which you can change the settings that are in effect, or the features and capabilities of some of the components on the board.

Jumpers are used on most motherboards today to configure settings. A jumper is a small piece of plastic that contains a flat piece of metal. When the jumper is applied, it connects two adjacent pins on a jumper block. Some jumper blocks contain two-pin jumpers. Others contain three-pin jumpers, on which the jumper may connect pins 1 and 2, 2 and 3, or make no connection.

DIP switches are less frequently found on newer motherboards. These are small banks of two-position (on and off) switches. These are usually slide-type switches, but occasionally rocker switches are used, which pivot in the middle to select the “on” or “off” setting.

The most common use of jumpers and DIP switches is to change the speed of the processor installed on the motherboard.

Connectors for front panel lights and switches
Every motherboard needs a way to connect to the lights and switches on the front cover of the case in which it is housed. Depending on the design and age of the case, there will be lights for Power On, Hard Drive Activity, and possibly Turbo and Suspend Mode.

Switches will usually include Reset, and sometimes Turbo and Suspend. In addition to these lights and switches, the bundle of wires coming from the front of the case will usually include a connector for the small internal speaker as well.

The motherboard may have a single block of pins where all these wires connect, or the connections may be found on separate parts of the board. Note that most cases will have at least some wires for lights and switches that are not used or supported by some motherboards. These extra wires can be ignored. In any case, the lights and switches won’t make a difference in whether the system works or not; they just give you a better indication of the status of the system at any given time.

In rare cases, connecting these wires improperly can prevent the system from coming up at all. If you’re not sure where any of these wires should go, the safest approach is to leave them disconnected initially. Once the system is performing properly, you can experiment with different locations for the wires in question. Move only one at a time, until you’ve found the proper position.

In connecting these wires, you’ll want to be aware that the Light Emitting Diodes (LEDs) are polarity sensitive. If you have the wires connected to the right pins but the light isn’t working, reverse the position of the pins and the LED should operate properly. The switches and speaker are not polarity sensitive, so they will work every time if they are connected to the correct pins on the motherboard.

In this Daily Drill Down, I’ve presented an overview of the architecture of that vital piece of hardware, the motherboard. Next time, I’ll cover motherboard form factors, jumper/DIP switch settings for processor upgrades, identifying Pin 1 connectors, and some of the other components you’re likely to find on motherboards.

Ken Dwight has been in the computer industry since 1966 and has worked on personal computers since 1982. He is founder and president of The TeleProcessors, Inc., a 28-year-old software and consulting firm based in Houston, TX. Ten days per month, he teaches PC Troubleshooting and Advanced PC Troubleshooting for a major international training company.

The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.