Replacement Motherboards
Knowing What to Look for (Selection Criteria)
Rom BIOS Compatibility
Using Correct Speed-Rated Parts
Motherboard Form Factors
Backplane Systems
Full-Size AT
Motherboard Interface Connectors
Motherboard CMOS RAM Addresses
Easily the most important component in a PC system is the main board or motherboard. Some companies, such as IBM, refer to the motherboard as a system board or planar. The terms motherboard, main board, system board, and planar are interchangeable. In this chapter, we will examine the different types of motherboards available, as well as those components usually contained on the motherboard and motherboard interface connectors.

Replacement Motherboards

Some manufacturers go out of their way to make their systems as physically incompatible as possible with any other system. Then replacement parts, repairs, and upgrades are virtually impossible to find--except, of course, from the original system manufacturer, at a significantly higher price than the equivalent part would cost to fit a standard PC-compatible system.

For example, if the motherboard in my current AT-chassis system (or any system using a Baby-AT motherboard and case) dies, I can find any number of replacement boards that will bolt directly in, with my choice of processors and clock speeds, at very good prices. If the motherboard dies in a newer IBM, Compaq, Hewlett-Packard, Packard Bell, Gateway, AST, or other proprietary shaped system, you'll pay for a replacement available only from the original manufacturer, and you have little or no opportunity to select a board with a faster or better processor than the one that failed. In other words, upgrading or repairing one of these systems via a motherboard replacement is difficult and usually not cost-effective.

Knowing What to Look For (Selection Criteria)

As a consultant, I am often asked to make a recommendation for purchases. Making these types of recommendations is one of the most frequent tasks a consultant performs. Many consultants charge a large fee for this advice. Without guidance, many individuals don't have any rhyme or reason to their selections and instead base their choices solely on magazine reviews or, even worse, on some personal bias. To help eliminate this haphazard selection process, I have developed a simple checklist that will help you select a system. This list takes into consideration several important system aspects overlooked by most checklists. The goal is to ensure that the selected system truly is compatible and has a long life of service and upgrades ahead.

It helps to think like an engineer when you make your selection. Consider every aspect and detail of the motherboards in question. For instance, you should consider any future uses and upgrades. Technical support at a professional (as opposed to a user) level is extremely important. What support will be provided? Is there documentation, and does it cover everything else?

In short, a checklist is a good idea. You can use the following check list to evaluate any PC-compatible system. You might not have to meet every one of these criteria to consider a particular system, but if you miss more than a few of these checks, consider staying away from that system. The items at the top of the list are the most important, and the items at the bottom are perhaps of lesser importance (although I think each item is important). The rest of this chapter discusses in detail the criteria in this checklist:

Processor. A Pentium motherboard should use as a minimum the second-generation 3.3v Pentium processor, which has a 296-pin Socket 5 or Socket 7 configuration that differs physically from the 273-pin Socket 4 first-generation design. Pentium motherboards with the Socket 7 configuration also support newer processors with MMX technology, including AMD's K6. All second-generation Pentiums (75MHz and up) are fully SL Enhanced. Newer Pentium Pro and Pentium II processors have their own unique motherboard configurations, and are not compatible with other Pentium-based motherboards.

Processor Sockets. A Pentium motherboard should have at least one ZIF socket that follows the Intel Socket 7 (321-pin) specification. The Socket 7 with an adjacent VRM (Voltage Regulator Module) socket will allow the best selection of future Pentium processors that will be available at higher speeds. Although Socket 5 is similar to Socket 7, many of the newer and faster Pentiums--including the MMX equipped processors--require Socket 7. Pentium Pro (P6) motherboards use Socket 8, and many are set up for multiple processors. Before going to the expense of buying a multi-processor board, ensure that your operating system is able to handle it. For instance, while Windows 95 cannot really benefit from more than one CPU, Windows NT, OS/2, and some others may run considerably faster.

Motherboard Speed. A Pentium or Pentium Pro motherboard should run at 60 or 66MHz and be speed-switchable between these speeds. Notice that all the Pentium and Pentium Pro processors sold today run at a multiple of the motherboard speed. For example, Pentium 75 runs at a motherboard speed of 50MHz; Pentium 60, 90, 120, 150, and 180MHz chips run at a 60MHz base motherboard speed; and the Pentium 66, 100, 133, 166, and 200 run at a 66MHz motherboard speed setting. The Pentium Pro 150, 180, and 200 run at 50, 60, and 66MHz speeds, respectively. All components on the motherboard (especially cache memory) should be rated to run at the maximum allowable motherboard speed.

Cache Memory. All Pentium motherboards should have 256K to 512K of Level 2 cache on-board. Most Pentium Pro processors have a built-in 256K or 512K Level 2 cache, but may also have more Level 2 cache on the motherboard for even better performance. The Level 2 cache should be of a Write-Back design, and must be populated with chips that are fast enough to support the maximum motherboard speed, which should be 15ns or faster for 66MHz maximum motherboard speeds. For Pentium boards, the cache should be a Synchronous SRAM (Static RAM) type, which is also called Pipelined Burst SRAM.

SIMM Memory. All Pentium and Pentium Pro motherboards should use either 72-pin SIMMs or 168-pin DIMMs (Dual In-line Memory Modules). Due to the 64-bit design of these boards, the 72-pin SIMMs must be installed in matched pairs, while DIMMs are installed one at a time (one per 64-bit bank). Carefully consider the total amount of memory that the board supports. While 16M is regarded as a bare minimum for today's memory-hungry applications, you may actually require much more. Pentium motherboards should support at least 128M, and many current Pentium II boards support more than 1G! A motherboard should contain at least four memory sockets (72-pin, 168-pin, or a combination) and the more the better. For maximum performance, look for systems that support SDRAM (Synchronous DRAM) or EDO (Extended Data Out) type SIMMs/DIMMs. The SIMMs should be rated at 70ns or faster.
Mission-critical systems ideally should use Parity SIMMs and ensure that the motherboard fully supports parity checking or even ECC (Error Correcting Code) as well. Note that the popular Intel Triton Pentium chipset (82430FX) does not support parity checked memory at all, but their other Pentium chipsets such as the older Neptune (82430NX) and newer Triton II (82430HX) do indeed offer parity support. Triton II even offers ECC capability using standard parity SIMMs. All the current Pentium Pro chipsets also support Parity memory and are ideal for file servers and other mission critical use when equipped with parity SIMMs or DIMMs.

Bus Type. Pentium, Pentium Pro, and Pentium II motherboards should have three or four ISA bus slots and three or four PCI local bus slots. Take a look at the layout of the slots to ensure that cards inserted in them will not block access to memory sockets, or be blocked by other components in the case.

BIOS. The motherboard should use an industry-standard BIOS such as those from AMI, Phoenix, Microid Research, or Award. The BIOS should be of a Flash ROM or EEPROM (Electrically Erasable Programmable Read Only Memory) design for easy updating. The BIOS should support the Plug and Play (PnP) specification, Enhanced IDE or Fast ATA, as well as 2.88M floppy drives. APM (Advanced Power Management) support should be built into the BIOS as well.

Form Factor. For maximum flexibility, the Baby-AT form factor is still a safe bet. It can be installed in the widest variety of case designs, and is retrofittable in most systems. For the greatest performance and future flexibility, many newer motherboards and systems incorporate the new ATX form factor, which has distinct performance and functional advantages over Baby-AT. Additionally, the new NLX form factor has been developed by Intel as an improvement on the ATX. Although it is new, the NLX specification is supported by a number of manufacturers, so it could prove to be a popular board in the coming years.

Built-in interfaces. Ideally, a motherboard should contain as many built-in standard controllers and interfaces as possible (except video). A motherboard should have a built-in floppy controller that supports 2.88M drives, built-in primary and secondary local bus (PCI or VL-Bus) Enhanced IDE (also called Fast ATA) connectors, two built-in high-speed serial ports (must use 16550A type buffered UARTs), and a built-in high-speed parallel port (must be EPP/ECP-compliant). A built-in PS/2 type mouse port should be included, although one of the serial ports can be used for a mouse as well.
Some newer systems, particularly those with ATX and NLX form factors, should include a built-in USB (Universal Serial Bus) port. USB ports will become a "must-have" item on multimedia systems in the near future. A built-in SCSI port is a bonus as long as it conforms to ASPI (Advanced SCSI Programming Interface) standards. Built-in network adapters are acceptable, but usually an ISA slot card network adapter is more easily supported via standard drivers and is more easily upgraded as well. Built-in video adapters are also a bonus in some situations, but because there are many different video chipset and adapter designs to choose from, generally there are better choices in external local bus video adapters. The same goes for built-in sound cards; they usually offer basic Sound Blaster compatibility and function, but often do not include other desirable features found on most plug-in sound cards, such as wavetable support. Plug and Play (PnP). The motherboard should fully support the Intel PnP specification. This will allow automatic configuration of PCI adapters as well as PnP ISA adapters.

TIP: Even if a motherboard doesn't list that it's PnP-compatible, it may be. PCI motherboards are required to be PnP-compatible, as it is a part of the PCI standard.
Power Management. The motherboard should fully support SL Enhanced processors with APM (Advanced Power Management) and SMM (System Management Mode) protocols that allow for powering down various system components to different levels of readiness and power consumption.

Motherboard Chipset. Pentium and Pentium MMX motherboards should use a high-performance chipset--preferably one that allows parity checking, such as the Intel Triton II (430HX). The popular original Intel Triton (430FX) chipset, along with the newer 430TX and 430VX chipsets, does not support parity-checked memory. For critical applications using Pentium motherboards where accuracy and data integrity is important, I recommend you use a board based on the Triton II (430HX) chipset or any others like it that support ECC memory using true parity memory modules. As a bonus, the 430HX chipset supports USB and dual CPUs, making it truly versatile.
Pentium Pro and Pentium II motherboards currently have the high-end Orion (450KX and 450GX) chipsets, as well as the less expensive Natoma (440FX) chipset. All three chipsets support parity memory, USB, and multiple CPUs, and are suitable for critical application use.

Documentation. Good technical documentation is a requirement. Documents should include information on any and all jumpers and switches found on the board, connector pinouts for all connectors, specifications for cache RAM chips, SIMMs, and other plug-in components, and any other applicable technical information. I would also acquire separate documentation from the BIOS manufacturer covering the specific BIOS used in the system, as well as the data books covering the specific chipset used in the motherboard. Additional data books for any other controller or I/O chips on-board are a bonus, and may be acquired from the respective chip manufacturers.
Another nice thing to have is available online support and documentation updates, although this should not be accepted in place of good hardcopy manuals.

You may notice that these selection criteria seem fairly strict and may disqualify many motherboards on the market, including what you already have in your system! These criteria will, however, guarantee you the highest quality motherboard offering the latest in PC technology that will be upgradable, expandable, and provide good service for many years. Most of the time I recommend purchasing boards from better-known motherboard manufacturers such as Intel, SuperMicro, Micronics, AMI, Biostar, Tyan, Asus, and so on. These boards might cost a little more than others that you have never heard of, but there is some safety in the more well-known brands; that is, the more boards that they sell, the more likely that any problems will have been discovered by others and solved long before you get yours. Also, if service or support are needed, the larger vendors are more likely to be around in the long run.


As mentioned, extensive documentation is an important factor to consider when you're planning to purchase a motherboard. Most motherboard manufacturers design their boards around a particular chipset, which actually counts as the bulk of the motherboard circuitry. There are a number of manufacturers offering chipsets, such as Intel, Opti, VIA, SiS, and others. I recommend obtaining the data book or other technical documentation on the chipset directly from the chipset manufacturer.

One of the more common questions I hear about a system relates to the BIOS Setup program. People want to know what the "Advanced Chipset Setup" features mean and what the effects of changing them will be. Often they go to the BIOS manufacturer thinking that the BIOS documentation will offer help. Usually, however, people find that there is no real coverage of what the chipset setup features are in the BIOS documentation. You will find this information in the data book provided by the chipset manufacturer. Although these books are meant to be read by the engineers who design the boards, they contain all the detailed information about the chipset's features, especially those that might be adjustable. With the chipset data book, you will have an explanation of all the controls in the Advanced Chipset Setup section of the BIOS Setup program.

Besides the main chipset data books, I also recommend collecting any data books on the other major chips in the system. This would include any floppy or IDE controller chips, Super I/O chips, and of course the main processor. You will find an incredible amount of information on these components in the data books.

CAUTION: Most chipset manufacturers only make a particular chip for a short time, rapidly superseding it with an improved or changed version. The data books are only available during the time the chip is being manufactured, so if you wait too long, you will find that such documents may no longer be available. The time to collect documentation on your motherboard is now!
ROM BIOS Compatibility

The issue of ROM BIOS compatibility is important. If the BIOS is not compatible, any number of problems can result. Several reputable companies that produce compatibles have developed their own proprietary ROM BIOS that works just like IBM's. Also, many of the compatibles' OEMs have designed ROMs that work specifically with additional features in their systems while effectively masking the effects of these improvements from any software that would "balk" at the differences.


Many OEMs (Original Equipment Manufacturers) have developed their own compatible ROMs independently. Companies such as Compaq and AT&T have developed their own BIOS products which are comparable to those offered by AMI, Phoenix, and others. These companies also offer upgrades to newer versions that often can offer more features and improvements or fix problems with the older versions. If you use a system with a proprietary ROM, make sure that it is from a larger company with a track record and one that will provide updates and fixes as necessary. Ideally, upgrades should be available for download from the Internet.

Several companies have specialized in the development of a compatible ROM BIOS product. The three major companies that come to mind in discussing ROM BIOS software are American Megatrends, Inc. (AMI), Award Software, and Phoenix Software. Each company licenses its ROM BIOS to a motherboard manufacturer so that the manufacturer can worry about the hardware rather than the software. To obtain one of these ROMs for a motherboard, the OEM must answer many questions about the design of the system so that the proper BIOS can be either developed or selected from those already designed. Combining a ROM BIOS and a motherboard is not a haphazard task. No single, generic, compatible ROM exists, either. AMI, Award, Microid Research, and Phoenix ship to different manufacturers many variations of their BIOS code, each one custom-tailored to that specific system, much like DOS can be.

A good source of information on currently available BIOS products is available from the System Optimization Web site at:


Although AMI customizes the ROM code for a particular system, it does not sell the ROM source code to the OEM. An OEM must obtain each new release as it becomes available. Because many OEMs don't need or want every new version developed, they might skip several version changes before licensing a new one.

The AMI BIOS is currently the most popular BIOS in PC systems today. Newer versions of the AMI BIOS are called Hi-Flex due to the high flexibility found in the BIOS configuration program. The AMI Hi-Flex BIOS is used in Intel, AMI, and many other manufacturers' motherboards. One special AMI feature is that it is the only third-party BIOS manufacturer to make its own motherboard.

During powerup, the BIOS ID string is displayed on the lower-left of the screen. This string tells you valuable information about which BIOS version you have, as well as certain settings which are determined by the built-in setup program.

TIP: A good trick to help you view the BIOS ID string is to shut down and either unplug your keyboard, or hold down a key as you power back on. This will cause a keyboard error, and the string will remained displayed.
The primary BIOS Identification string (ID String 1) is displayed by any AMI BIOS during the POST (Power On Self-Test) at the bottom-left corner of the screen, below the copyright message. Two additional BIOS ID strings (ID Strings 2 and 3) can be displayed by the AMI Hi-Flex BIOS by pressing the Insert key during POST. These additional ID strings display the options that are installed in the BIOS.

The general BIOS ID String 1 format for older AMI BIOS versions is shown in Table 4.1.

Table 4.1 ABBB-NNNN-mmddyy-KK

Position Description
A BIOS Options:
D = Diagnostics built-in.
S = Setup built-in.
E = Extended Setup built-in.
BBB Chipset or Motherboard Identifier:
C&T = Chips & Technologies chipset.
NET = C&T NEAT 286 chipset.
286 = Standard 286 motherboard.
SUN = Suntac chipset.
PAQ = Compaq motherboard.
INT = Intel motherboard.
AMI = AMI motherboard.
G23 = G2 chipset 386 motherboard.
NNNN The manufacturer license code reference number.
mmddyy The BIOS release date, mm/dd/yy.
KK The AMI keyboard BIOS version number.
The BIOS ID String 1 format for AMI Hi-Flex BIOS versions is shown in Table 4.2.


Position Description
A Processor Type:
0 = 8086 or 8088.
2 = 286.
3 = 386.
4 = 486.
5 = Pentium.
6 = Pentium Pro.
B Size of BIOS:
0 = 64K BIOS.
1 = 128K BIOS.
CCcc Major and Minor BIOS version number.
DDDDDD Manufacturer license code reference number.
0036xx = AMI 386 motherboard, xx = Series #.
0046xx = AMI 486 motherboard, xx = Series #.
0056xx = AMI Pentium motherboard, xx = Series #.
0066xx = AMI Pentium Pro motherboard, xx = Series #.
E 1 = Halt on Post Error.
F 1 = Initialize CMOS every boot.
G 1 = Block pins 22 and 23 of the keyboard controller.
H 1 = Mouse support in BIOS/keyboard controller.
I 1 = Wait for key on POST errors.
J 1 = Display floppy error during POST.
K 1 = Display video error during POST.
L 1 = Display keyboard error during POST.
mmddyy BIOS Date, mm/dd/yy.
MMMMMMMM Chipset identifier or BIOS name.
N Keyboard controller version number.
AMI Hi-Flex BIOS ID String 2 is shown in Table 4.3.


Position Description
AA Keyboard controller pin number for clock switching.
B Keyboard controller clock switching pin function:
H = High signal switches clock to high speed.
L = High signal switches clock to low speed.
C Clock switching through chip set registers:
0 = Disable.
1 = Enable.
DDDD Port address to switch clock high.
EE Data value to switch clock high.
FF Mask value to switch clock high.
GGGG Port Address to switch clock low.
HH Data value to switch clock low.
II Mask value to switch clock low.
JJJ Pin number for Turbo Switch Input.
AMI Hi-Flex BIOS ID String 3 is shown in Table 4.4.


Position Description
AA Keyboard controller pin number for cache control.
B Keyboard controller cache control pin function:
H = High signal enables the cache.
L = High signal disables the cache.
C 1 = High signal is used on the keyboard controller pin.
DDD Cache control through Chipset registers:
0 = Cache control off.
1 = Cache control on.
EE Port address to enable cache.
FF Data value to enable cache.
GGGG Mask value to enable cache.
HH Port address to disable cache.
II Data value to disable cache.
JJ Mask value to disable cache.
K Pin number for resetting the 82335 memory controller.
L BIOS Modification Flag:
0 = The BIOS has not been modified.
1-9, A-Z = Number of times the BIOS has been modified.
The AMI BIOS has many features, including a built-in setup program activated by pressing the Delete or Esc key in the first few seconds of booting up your computer. The BIOS will prompt you briefly as to which key to press and when to press it. The AMI BIOS offers user-definable hard disk types, essential for optimal use of many IDE or ESDI drives. The newer BIOS versions also support Enhanced IDE drives and will auto- configure the drive parameters.

A unique AMI BIOS feature is that, in addition to the setup, it has a built-in, menu-driven, diagnostics package--essentially a very limited version of the stand-alone AMIDIAG product. The internal diagnostics are not a replacement for more comprehensive disk-based programs, but they can help in a pinch. The menu-driven diagnostics does not do extensive memory testing, for example, and the hard disk low-level formatter works only at the BIOS level rather than at the controller register level. These limitations often have prevented it from being capable of formatting severely damaged disks.

The AMI BIOS is sold through distributors, a list of which is available at You may also contact Washburn and Co., listed in the vendor list in Appendix A. However, keep in mind that you cannot buy upgrades and replacements direct from AMI.


Award is unique among BIOS manufacturers because it sells its BIOS code to the OEM and allows the OEM to customize the BIOS. Of course, then the BIOS no longer is Award BIOS, but rather a highly customized version. AST uses this approach on its systems, as do other manufacturers, for total control over the BIOS code, without having to write it from scratch. Although AMI and Phoenix customize the ROM code for a particular system, they do not sell the ROM's source code to the OEM. Some OEMs that seem to have developed their own ROM code started with a base of source code licensed to them by Award or some other company.

The Award BIOS has all the normal features you expect, including a built-in setup program activated by pressing Ctrl+Alt+Esc. This setup offers user-definable drive types, required in order to fully use IDE or ESDI hard disks. The POST is good, and Award runs technical support on its Web site at They also run a BBS whose number is listed in the vendor list in Appendix A.

In all, the Award BIOS is high quality, has minimal compatibility problems, and offers a high level of support.


The Phoenix BIOS for many years has been a standard of compatibility by which others are judged. It was one of the first third-party companies to legally reverse-engineer the IBM BIOS using a "clean room" approach. In this approach, a group of engineers studied the IBM BIOS and wrote a specification for how that BIOS should work and what features should be incorporated. This information then was passed to a second group of engineers who had never seen the IBM BIOS. They could then legally write a new BIOS to the specifications set forth by the first group. This work would then be unique and not a copy of IBM's BIOS; however, it would function the same way. This code has been refined over the years and has very few compatibility problems compared to some of the other BIOS vendors.

The Phoenix BIOS excels in two areas that put it high on my list of recommendations. One is that the POST is excellent. The BIOS outputs an extensive set of beep codes that can be used to diagnose severe motherboard problems which would prevent normal operation of the system. In fact, this POST can isolate memory failures in Bank 0 right down to the individual chip with beep codes alone. The Phoenix BIOS also has an excellent setup program free from unnecessary frills, but that offers all the features one would expect, such as user-definable drive types, and so on. The built-in setup is activated by pressing either Ctrl+Alt+S or Ctrl+Alt+Esc, depending on the version of BIOS you have.

The second area in which Phoenix excels is the documentation. Not only are the manuals that you get with the system detailed, but also Phoenix has written a set of BIOS technical-reference manuals that are a standard in the industry. The set consists of three books, titled System BIOS for IBM PC/XT/AT Computers and Compatibles, CBIOS for IBM PS/2 Computers and Compatibles, and ABIOS for IBM PS/2 Computers and Compatibles. Phoenix is one of few vendors who has done extensive research on the PS/2 BIOS and produced virtually all the ROMs in the PS/2 Micro Channel clones on the market. In addition to being an excellent reference for the Phoenix BIOS, these books serve as an outstanding overall reference to any company's IBM-compatible BIOS. Even if you never have a system with a Phoenix BIOS, I highly recommend these books.

Micronics motherboards have always used the Phoenix BIOS, and these motherboards are used in many of the popular name-brand compatible systems. Phoenix has been one of the largest OEMs of Microsoft MS-DOS. If you have MS-DOS, you also have the Phoenix OEM version. Phoenix licenses its DOS to other computer manufacturers so long as they use the Phoenix BIOS. Because of its close relationship with Microsoft, it has had access to the DOS source code, which helps in eliminating compatibility problems.

Although Phoenix does not operate a technical support service by itself, their largest nationwide distributor does, which is Micro Firmware Inc. Online information is available at, or check the phone numbers listed in the vendor list in Appendix B. Micro Firmware offers upgrades to many systems with a Phoenix BIOS, including many Packard Bell, Gateway 2000 (with Micronics motherboards), Micron Technologies, and other systems.

Unless the ROM BIOS is a truly compatible, custom OEM version such as Compaq's, you might want to install in the system the ROM BIOS from one of the known quantities, such as AMI, Award, or Phoenix. These companies' products are established as ROM BIOS standards in the industry, and frequent updates and improvements ensure that a system containing these ROMs will have a long life of upgrades and service.

A good source of online information about BIOS basics can be found at:

Using Correct Speed-Rated Parts

Some compatible vendors use substandard parts in their systems to save money. Because the CPU is one of the most expensive components on the motherboard, and many motherboards are sold to system assemblers without the CPU installed, it is tempting for the assembler to install a CPU rated for less than the actual operating speed. A system could be sold as a 100MHz system, for example, but when you look "under the hood," you may find a CPU rated for only 90MHz. The system does appear to work correctly, but for how long? If the company that manufactures the CPU chip installed in this system had tested the chip to run reliably at 100MHz, it would have labeled the part accordingly. After all, the company could sell the chip for more money if it worked at the higher clock speed.

When a chip is run at a speed higher than it is rated for, it will run hotter than it would normally. This may cause the chip to overheat occasionally, which would appear as random lockups, glitches, and frustration. I highly recommend that you avoid systems whose operation speed exceeds the design of the respective parts.

This practice is easy to fall into because the faster rated chips cost more money, and Intel and other chip manufacturers usually rate their chips very conservatively. I have taken several 25MHz 486 processors and run them at 33MHz, and they seemed to work fine. The Pentium 90 chips I have tested seem to run fine at 100MHz. Although I might purchase a Pentium 90 system and make a decision to run it at 100MHz, if I were to experience lockups or glitches in operation, I would immediately return it to 90MHz and retest. If I purchase a 100MHz system from a vendor, I fully expect it to have 100MHz parts, not 90MHz parts running past their rated speed! These days, many chips will have some form of heat sink on them, which helps to prevent overheating, but which can also sometimes cover up for a "pushed" chip. If the price is too good to be true, ask before you buy: "Are the parts really manufacturer-rated for the system speed?"

To determine the rated speed of a CPU chip, look at the writing on the chip. Most of the time, the part number will end in a suffix of -xxx where the xxx is a number indicating the maximum speed. For example, -100 indicates that the chip is rated for 100MHz operation.

CAUTION: Be careful when running software to detect processor speed. Such programs can only tell you what speed the chip is currently running at, not what the true rating is. Also ignore the speed indicator lights on the front of some cases. These digital displays can literally be set via jumpers to read any speed you desire! They have no true relation to actual system speed.
Motherboard Form Factors

There are several compatible form factors used for motherboards. The form factor refers to the physical dimensions and size of the board, and dictates what type of case the board will fit into. The types of motherboard form factors generally available are the following:

Backplane Systems LPX

Full-size AT ATX




Backplane Systems

Not all systems have a motherboard in the true sense of the word. In some systems, the components normally found on a motherboard are located instead on an expansion adapter card plugged into a slot. In these systems, the board with the slots is called a backplane, rather than a motherboard. Systems using this type of construction are called backplane systems.

Backplane systems come in two main types: passive and active. A passive backplane means the main backplane board does not contain any circuitry at all except for the bus connectors and maybe some buffer and driver circuits. All the circuitry found on a conventional motherboard is contained on one or more expansion cards installed in slots on the backplane. Some backplane systems use a passive design that incorporates the entire system circuitry into a single mothercard. The mothercard is essentially a complete motherboard that is designed to plug into a slot in the passive backplane. The passive backplane/mothercard concept allows the entire system to be easily upgraded by changing one or more cards. Because of the expense of the high function mothercard, this type of system design is rarely found in PC systems. The passive backplane design does enjoy popularity in industrial systems, which are often rack-mounted. Some high-end file servers also feature this design.

An active backplane means the main backplane board contains bus control and usually other circuitry as well. Most active backplane systems contain all the circuitry found on a typical motherboard except for the processor complex. The processor complex is the name of the circuit board that contains the main system processor and any other circuitry directly related to it, such as clock control, cache, and so forth. The processor complex design allows the user to easily upgrade the system later to a new processor type by changing one card. In effect, it amounts to a modular motherboard with a replaceable processor section. Most modern PC systems that use a backplane design use an active backplane/processor complex. Both IBM and Compaq have used this type of design in some of their high-end (server class) systems, for example. This allows an easier and generally more affordable upgrade than the passive backplane/mothercard design since the processor complex board is usually much cheaper than a mothercard. Unfortunately, because there are no standards for the processor complex interface to the system, these boards are proprietary and can only be purchased from the system manufacturer. This limited market and availability causes the prices of these boards to be higher than most complete motherboards from other manufacturers.

The motherboard system design and the backplane system design have both advantages and disadvantages. Most original personal computers were designed as backplanes in the late 1970s. Apple and IBM shifted the market to the now traditional motherboard with a slot-type design because this type of system generally is cheaper to mass-produce than one with the backplane design. The theoretical advantage of a backplane system, however, is that you can upgrade it easily to a new processor and new level of performance by changing a single card. For example, you can upgrade a system's processor just by changing the card. In a motherboard-design system, you often must change the motherboard itself, a seemingly more formidable task. Unfortunately, the reality of the situation is that a backplane design is often much more expensive to upgrade, and because the bus remains fixed on the backplane, the backplane design precludes more comprehensive upgrades that involve adding local bus slots, for example.

Another nail in the coffin of backplane designs is the upgradable processor. Intel has designed all 486, Pentium, Pentium MMX, and Pentium Pro processors to be upgradable to faster (sometimes called OverDrive) processors in the future by simply swapping (or adding) the new processor chip. Changing only the processor chip for a faster one is the easiest and generally most cost-effective way to upgrade without changing the entire motherboard.

Because of the limited availability of the processor complex boards or mothercards, they usually end up being more expensive than a complete new motherboard that uses an industry standard form factor. Intel recently announced the new NLX form factor for the Pentium II, and it shares some traits with traditional backplane systems. The NLX has been promised considerable industry support, so we may well see affordable backplane systems in the near future.

Full-Size AT

The full-size AT motherboard is so named because it matches the original IBM AT motherboard design. This allows for a very large board of up to 12 inches wide by 13.8 inches deep. The keyboard connector and slot connectors must conform to specific placement requirements to fit the holes in the case. This type of board will fit into full-size AT or Tower cases only. Because these motherboards will not fit into the popular Baby-AT or Mini-Tower cases, and because of advances in component miniaturization, they are no longer being produced by most motherboard manufacturers.


The Baby-AT form factor is essentially the same as the original IBM XT motherboard, with modifications in screw hole positions to fit into an AT-style case (see Figure 4.1). These motherboards also have specific placement of the keyboard connector and slot connectors to match the holes in the case. Note that virtually all full-size AT and Baby-AT motherboards use the standard 5-pin DIN type connector for the keyboard. Baby-AT motherboards will fit into every type of case except the Low Profile or Slimline cases. Because of their flexibility, this is now the most popular motherboard form factor. Figure 4.1 shows the dimensions and layout of a Baby-AT motherboard.

FIG. 4.1 Baby-AT motherboard form factor.


Another popular form factor used in motherboards today is the LPX and Mini-LPX form factors. This form factor was first developed by Western Digital for some of their motherboards. Although they no longer produce PC motherboards, the form factor lives on and has been duplicated by many other motherboard manufacturers. These are used in the Low Profile or Slimline case systems sold widely today. These are often lower-cost systems like those sold at retail electronics superstores. It should be noted that systems using LPX boards may have other differences which can cause compatibility problems similar to those of proprietary systems.

The LPX boards are characterized by several distinctive features. The most noticeable is that the expansion slots are mounted on a bus riser card that plugs into the mother- board. Expansion cards must plug sideways into the riser card. This sideways placement allows for the low profile case design. Slots are located on one or both sides of the riser card depending on the system and case design.

Another distinguishing feature of the LPX design is the standard placement of connectors on the back of the board. An LPX board has a row of connectors for video (VGA 15-pin), parallel (25-pin), two serial ports (9-pin each), and mini-DIN PS/2 style Mouse and Keyboard connectors. All of these connectors are mounted across the rear of the motherboard and protrude through a slot in the case. Some LPX motherboards may have additional connectors for other internal ports such as Network or SCSI adapters. Figure 4.2 shows the standard form factors for the LPX and Mini-LPX motherboards used in many systems today.

FIG. 4.2 LPX and Mini-LPX motherboard form factors.


The ATX form factor is a recent evolution in motherboard form factors. ATX is a combination of the best features of the Baby-AT and LPX motherboard designs, with many new enhancements and features thrown in. The ATX form factor is essentially a Baby-AT motherboard turned sideways in the chassis, along with a modified power supply location and connector. The most important thing to know initially about the ATX form factor is that it is physically incompatible with either the previous Baby-AT or LPX designs. In other words, a different case and power supply are required to match the ATX motherboard. These new case and power supply designs have become common, and can be found in many new systems.

The official ATX specification was released by Intel in July 1995, and has been written as an open specification for the industry. The latest revision of the specification is Version 2.01, published in February 1997. Intel has published detailed specifications so other manufacturers can use the ATX design in their systems.

ATX improves on the Baby-AT and LPX motherboard designs in several major areas:

Built-in double high external I/O connector panel. The rear portion of the motherboard includes a stacked I/O connector area, which is 6.25 inches wide by 1.75 inches tall. This allows external connectors to be located directly on the board and negates the need for cables running from internal connectors to the back of the case as with Baby-AT designs.

Single keyed internal power supply connector. This is a boon for the average end user, who always had to worry about interchanging the Baby-AT power supply connectors and subsequently blowing the motherboard! The ATX specification includes a single keyed and shrouded power connector that is easy to plug in, and which cannot be installed incorrectly. This connector also features pins for supplying 3.3v to the motherboard, which means that ATX motherboards will not require built-in voltage regulators that are susceptible to failure.

Relocated CPU and memory. The CPU and memory modules are relocated so they cannot interfere with any bus expansion cards, and they can easily be accessed for upgrade without removing any of the installed bus adapters. The CPU and memory are relocated next to the power supply, which has a single fan blowing air across them, thus eliminating the need for inefficient and failure-prone CPU cooling fans. There is room for a large passive heat sink above the CPU as well.

Relocated internal I/O connectors. The internal I/O connectors for the floppy and hard disk drives are relocated to be near the drive bays and out from under the expansion board slot and drive bay areas. This means that internal cables to the drives can be much shorter, and accessing the connectors will not require card or drive removal.

Improved cooling. The CPU and main memory are cooled directly by the power supply fan, eliminating the need for separate case or CPU cooling fans. Also, the ATX power supply fan blows into the system chassis, thus pressurizing it which greatly minimizes dust and dirt intrusion into the system. If desired, an air filter can be easily added to the air intake vents on the power supply, creating a system that is even more immune to dirt or dust in the environment.

Lower cost to manufacture. The ATX specifications eliminate the need for the rats nest of cables to external port connectors found on Baby-AT motherboards, eliminates the need for additional CPU or chassis cooling fans, eliminates the need for on-board 3.3v voltage regulators, uses a single power supply connector, and allows for shorter internal drive cables. These all conspire to greatly reduce not only the cost of the motherboard, but also significantly reduces the cost of a complete system including the case and power supply.
Figure 4.3 shows the new ATX system layout and chassis features. Notice how the entire motherboard is virtually clear of the drive bays, and how the devices like CPU, memory, and internal drive connectors are easy to access and do not interfere with the bus slots. Also notice the power supply orientation and the single power supply fan that blows into the case directly over the high heat, generating items like the CPU and memory.

FIG. 4.3 ATX system chassis layout and features.

The ATX motherboard is basically a Baby-AT design rotated sideways. The expansion slots are now parallel to the shorter side dimension and do not interfere with the CPU, memory, or I/O connector sockets. In addition to a full-sized ATX layout, Intel also has specified a mini-ATX design as well, which will fit into the same case. Although the case holes are similar to the Baby-AT case, cases for the two formats are generally not compatible. The power supplies would require a connector adapter to be interchangeable, but the basic ATX power supply design is similar to the standard Slimline power supply. The ATX and mini-ATX motherboard dimensions are shown in Figure 4.4.

FIG. 4.4 ATX and Mini-ATX motherboard form factors.

Clearly, the advantages of the ATX form factor make it a good choice for high-end systems. For backwards compatibility, Baby-AT is still hard to beat, and there are still more Baby-AT motherboards, cases, and power supplies on the market than the ATX versions. With the coming of NLX motherboards and the support that form factor is receiving from the industry, it seems unlikely that ATX will be the all encompassing wave of the future.

For complete specifications, check out the ATX Motherboard Specification page at


NLX is the latest development in desktop motherboard technology, and may prove to be the form factor of choice in the near future. It is a low-profile form factor similar in appearance to LPX, but with a number of improvements designed to allow full integration of the latest technologies. Whereas the primary limitation of LPX boards includes an inability to handle the physical size of newer processors, as well as their higher thermal characteristics, the NLX form factor has been designed specifically to address these problems.

Specific advantages of the NLX form factor include:

Support for current processor technologies. This is especially important in Pentium II systems because the size of the Single Edge Contact cartridge this processor uses can limit its use on existing Baby-AT and LPX motherboards. Although a few motherboard manufacturers currently offer ATX-based Pentium II systems, they generally only have room for two 72-pin SIMM sockets!

Flexibility in the face of rapidly changing processor technologies. Backplane-like flexibility has been built into the form by allowing a new motherboard to be easily and quickly installed without tearing your entire system to pieces. But unlike traditional backplane systems, many industry leaders are putting their support behind NLX, including AST, Digital, Gateway, Hewlett-Packard, IBM, Micron, NEC, and others.

Support for other emerging technologies. This includes Accelerated Graphics Port (AGP) high-performance graphic solutions, Universal Serial Bus (USB), and tall memory modules and DIMM technology. Furthermore, with the emerging importance of multimedia applications, connectivity support for such things as video playback, enhanced graphics, and extended audio have been built into the motherboard. This should represent a good cost savings over expensive daughterboard arrangements, which have been necessary for many advanced multimedia uses in the past.
Figure 4.5 shows the basic NLX system layout. Notice that, like ATX, the system is clear of the drive bays and other chassis-mounted components. Also, the motherboard and I/O cards (which, like the LPX form factor, are mounted parallel to the motherboard) can easily be slid in and out of the side of the chassis, leaving the riser card and other cards in place. The processor itself can be easily accessed and enjoys greater cooling than in a more closed in layout.

FIG. 4.5 LX system chassis layout and features.

As you can see, the NLX form factor has been designed for maximum flexibility and space efficiency. Even extremely long I/O cards will fit easily, without fouling on other system components as has been such a problem with Baby-AT form factor systems.

Complete design specifications and information on NLX boards can be found online at the official NLX Motherboard Specification page, located at:

ATX and NLX form factors will probably be used in most future systems. I usually do not recommend LPX style systems if upgradability is a factor because it is not only difficult to locate a new motherboard that will fit, but LPX systems are also limited in expansion slots and drive bays as well. Baby-AT systems still offer a great deal of flexibility at present, but for future systems, ATX and NLX configurations are the way to go.

Motherboard Interface Connectors

There are a variety of different connectors on a modern motherboard. Tables 4.5 through 4.14 contain the pinouts of most of the different interface and I/O connectors you will find.

Table 4.5 ATX Motherboard Power Connector

Pin Signal Name Pin Signal Name
1 +3.3 V 11 +3.3 V
2 +3.3 V 12 -12 V
3 Ground 13 Ground
4 +5 V 14 PS-ON# (Power Supply Remote On/Off Control)
5 Ground 15 Ground
6 +5 V 16 Ground
7 Ground 17 Ground
8 PWRGD (Power Good) 18 -5 V
9 +5 VSB (Standby) 19 +5 V
10 +12 V 20 +5 V
Table 4.6 Baby-AT Motherboard Power Connectors

Pin Name Pin Name
1 PWRGD (Power Good) 7 Ground
2 +5 V 8 Ground
3 +12 V 9 -5 V
4 -12 V 10 +5 V
5 Ground 11 +5 V
6 Ground 12 +5 V
Table 4.7 Serial Port Pin-Header Connectors

Pin Signal Name Pin Signal Name
3 Serial In - (SIN) 8 RI
5 Serial Out - (SOUT) 10 Not Connected
Table 4.8 Parallel Port Pin-Header Connector

Signal Name Pin Pin Signal Name
Data Bit 0 3 4 ERROR-
Data Bit 1 5 6 INIT-
Data Bit 2 7 8 SLCT IN-
Data Bit 3 9 10 Ground
Data Bit 4 11 12 Ground
Data Bit 5 13 14 Ground
Data Bit 6 15 16 Ground
Data Bit 7 17 18 Ground
ACJ- 19 20 Ground
BUSY 21 22 Ground
PE (Paper End) 23 24 Ground
SLCT 25 26 N/C
Table 4.9 Motherboard Mouse Pin-Header Connector

Pin Signal Pin Signal
1 Gnd 5 CLK
2 Data 6 KEY
3 N/C 7 KEY
4 Vcc 8 N/C
Table 4.10 Infrared Data (IrDA) Pin-Header Connector

Pin Signal Name Pin Signal Name
1 +5 V 4 Ground
2 Key 5 IrTX
3 IrRX 6 CONIR (Consumer IR)
Table 4.11 Battery Connector

Pin Signal Pin Signal
1 Gnd 3 KEY
2 Unused 4 +6v
Table 4.12 LED and Keylock Connector

Pin Signal Pin Signal
1 LED Power (+5v) 4 Keyboard Inhibit
2 KEY 5 Gnd
3 Gnd
Table 4.13 Speaker Connector

Pin Signal Pin Signal
1 Ground 3 Board-Mounted Speaker
2 KEY 4 Speaker Output
Table 4.14 Microprocessor Fan Power Connector

Pin Signal Name
1 Ground
2 +12V
3 Sense tachometer
CAUTION: Do not place a jumper on this connector; serious board damage will result if the 12v is shorted to ground.
Note that some boards have a board mounted piezo speaker. It is enabled by placing a jumper over pins 3 and 4, which routes the speaker output to the board mounted speaker. Removing the jumper allows a conventional speaker to be plugged in.

Motherboard CMOS RAM Addresses

Table 4.15 shows the information maintained in the 64-byte standard CMOS RAM module. This information controls the configuration of the system and is read and written by the system Setup program.

In the original AT system, a Motorola 146818 chip was used. Modern systems incorporate the CMOS into the chipset, Super I/O chip, or use a special battery and NVRAM (Non-Volatile RAM) module from companies like Dallas or Benchmarq. The standard format of the information stored in the CMOS RAM is shown in Table 4.15.

Table 4.15 AT CMOS RAM Addresses

Offset Hex Offset Dec Field Size Function
00h 0 1 byte Current second in binary coded decimal (BCD)
01h 1 1 byte Alarm second in BCD
02h 2 1 byte Current minute in BCD
03h 3 1 byte Alarm minute in BCD
04h 4 1 byte Current hour in BCD
05h 5 1 byte Alarm hour in BCD
06h 6 1 byte Current day of week in BCD
07h 7 1 byte Current day in BCD
08h 8 1 byte Current month in BCD
09h 9 1 byte Current year in BCD
0Ah 10 1 byte Status register A
Bit 7 = Update in progress
0 = Date and time can be read
1 = Time update in progress
Bits 6-4 = Time frequency divider
010 = 32.768KHz
Bits 3-0 = Rate selection frequency
0110 = 1.024KHz square wave frequency
0Bh 11 1 byte Status register B
Bit 7 = Clock update cycle
0 = Update normally
1 = Abort update in progress
Bit 6 = Periodic interrupt
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 5 = Alarm interrupt
0 = Disable interrupt (default)
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 4 = Update-ended interrupt
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 3 = Status register A square wave frequency
0 = Disable square wave (default)
1 = Enable square wave
Bit 2 = Date format
0 = Calendar in BCD format (default)
1 = Calendar in binary format
Bit 1 = 24-hour clock
0 = 24-hour mode (default)
1 = 12-hour mode
Bit 0 = Daylight Savings Time
0 = Disable Daylight Savings (default)
1 = Enable Daylight Savings
0Ch 12 1 byte Status register C
Bit 7 = IRQF flag
Bit 6 = PF flag
Bit 5 = AF flag
Bit 4 = UF flag
Bits 3-0 = Reserved
0Dh 13 1 byte Status register D
Bit 7 = Valid CMOS RAM bit
0 = CMOS battery dead
1 = CMOS battery power good
Bits 6-0 = Reserved
0Eh 14 1 byte Diagnostic status
Bit 7 = Real-time clock power status
0 = CMOS has not lost power
1 = CMOS has lost power
Bit 6 = CMOS checksum status
0 = Checksum is good
1 = Checksum is bad
Bit 5 = POST configuration information status
0 = Configuration information is valid
1 = Configuration information is invalid
Bit 4 = Memory size compare during POST
0 = POST memory equals
1 = POST memory not equal to
Bit 3 = Fixed disk/adapter initialization
0 = Initialization good
1 = Initialization failed
Bit 2 = CMOS time status indicator
0 = Time is valid
1 = Time is Invalid
Bits 1-0 = Reserved
0Fh 15 1 byte Shutdown code
00h = Power on or soft reset
01h = Memory size pass
02h = Memory test pass
03h = Memory test fail
04h = POST end; boot system
05h = JMP double word pointer with EOI
06h = Protected mode tests pass
07h = Protected mode tests fail
07h = Protected mode tests fail
08h = Memory size fail
09h = Int 15h block move
0Ah = JMP double word pointer without
0Bh = used by 80386
10h 16 1 byte Floppy disk drive types
Bits 7-4 = Drive 0 type
Bits 3-0 = Drive 1 type
0000 = None
0001 = 360K
0010 = 1.2M
0011 = 720K
0100 = 1.44M
11h 17 1 byte Reserved
12h 18 1 byte Hard disk types
Bits 7-4 = Hard disk 0 type (0-15)
Bits 3-0 = Hard disk 1 type (0-15)
13h 19 1 byte Reserved
14h 20 1 byte Installed equipment
Bits 7-6 = Number of floppy disk drives
00 = 1 floppy disk drive
01 = 2 floppy disk drives
Bits 5-4 = Primary display
00 = Use display adapter BIOS
01 = CGA 40-column
10 = CGA 80-column
11 = Monochrome Display Adapter
Bits 3-2 = Reserved
Bit 1 = Math coprocessor present
Bit 0 = Floppy disk drive present
15h 21 1 byte Base memory low-order byte
16h 22 1 byte Base memory high-order byte
17h 23 1 byte Extended memory low-order byte
18h 24 1 byte Extended memory high-order byte
19h 25 1 byte Hard Disk 0 Extended Type (0-255)
1Ah 26 1 byte Hard Disk 1 Extended Type (0-255)
1Bh 27 9 bytes Reserved
2Eh 46 1 byte CMOS checksum high-order byte
2Fh 47 1 byte CMOS checksum low-order byte
30h 48 1 byte Actual extended memory low-order byte
31h 49 1 byte Actual extended memory high-order byte
32h 50 1 byte Date century in BCD
33h 51 1 byte POST information flag
Bit 7 = Top 128K base memory status
0 = Top 128K base memory not installed
1 = Top 128K base memory installed
Bit 6 = Setup program flag
0 = Normal (default)
1 = Put out first user message
Bits 5-0 = Reserved
34h 52 2 bytes Reserved
Table 4.16 shows the values that may be stored by your system BIOS in a special CMOS byte called the diagnostics status byte. By examining this location with a diagnostics program, you can determine whether your system has set trouble codes, which indicate that a problem has occurred previously.

Table 4.16 CMOS RAM (AT and PS/2) Diagnostic Status Byte Codes

Bit Number
7 6 5 4 3 2 1 0 Hex Function
1 . . . . . . . 80 Real-time clock (RTC) chip lost power
. 1 . . . . . . 40 CMOS RAM checksum is bad
. . 1 . . . . . 20 Invalid configuration information found at POST
. . . 1 . . . . 10 Memory size compare error at POST
. . . . 1 . . . 08 Fixed disk or adapter failed initialization
. . . . . 1 . . 04 Real-time clock (RTC) time found invalid
. . . . . . 1 . 02 Adapters do not match configuration
. . . . . . . 1 01 Time-out reading an adapter ID
. . . . . . . . 00 No errors found (Normal)


Author: suraj26 Jul 2009 Member Level: Silver   Points : 1

Mother board is one of the important part in computer. All the parts computer gets power and communication mechanism through motherboards only.

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