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.
Documentation
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.
OEMs.
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:
http://www.sysopt.com/bios.html
AMI.
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.
Table
4.2 AB-CCcc-DDDDDD-EFGHIJKL-mmddyy-MMMMMMMM-N
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 <F1> 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.
Table
4.3 AAB-C-DDDD-EE-FF-GGGG-HH-II-JJJ
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.
Table
4.4 AAB-C-DDD-EE-FF-GGGG-HH-II-JJ-K-L
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 http://www.ami.com/distributor.html.
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.
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 http://www.award.com.
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.
Phoenix.
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 o 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 http://www.firmware.com,
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:
http://www.lemig.umontreal.ca/bios/bios_sg.htm
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
- Baby-AT NLX
- LPX
- ATX
- NLX
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.
Baby-AT
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.
LPX
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.
ATX
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
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