Download What is a Motherboard

What is a Motherboard ?
A motherboard is the physical arrangement in a computer that contains the
computer's basic circuitry and components. On the typical motherboard, the circuitry
is imprinted or affixed to the surface of a firm planar surface and usually
manufactured in a single step. The most common motherboard design in desktop
computers today is the AT, based on the IBM AT motherboard. A more recent
motherboard specification, ATX, improves on the AT design. In both the AT and ATX
designs, the computer components included in the motherboard are:

The microprocessor

(Optionally) coprocessors

Memory

Basic input/output system (BIOS)

Expansion slot

Interconnecting circuitry
Additional components can be added to a motherboard through its expansion slot.
The electronic interface between the motherboard and the smaller boards or cards in
the expansion slots is called the bus.
The best way to describe the motherboard goes along well with my human body
analogy that I used for the CPU. The CPU is the brain, and the motherboard is the
nervous system. Therefore, just as a person would want to have fast communication
to the body parts, you want fast communication between the parts of your computer.
Fast communication isn't as important as reliable communication though. If your
brain wanted to move your arm, you want to be sure the nervous system can
accurately and consistently carry the signals to do that! Thus, in my opinion, the
motherboard is the second most important part of the computer.
The motherboard is the circuit board to which all the other components of the
computer connect in some way. The video card, sound card, IDE hard drive, etc. all
plug into the motherboard's various slots and connectors. The CPU also plugs into
the motherboard via a Socket or a Slot.
The motherboard has been an integral part of most personal computers for more
than 20 years. Think of a motherboard as a scale model of a futuristic city with many
modular plug-in buildings, each using power from a common electrical system.
Multiple-lane highways of various widths transport data between the buildings. The
motherboard is the data and power infrastructure for the entire computer.
Motherboards (also called main boards) are actually a carryover from architecture
used for years in mainframe computers. Various circuit cards performing various
functions all plug into many similar sockets on a common circuit board. Each circuit
card performs a unique function in the computer and gets its power from the socket
as well.
Due to improvements in circuitry and packaging, motherboards have essentially
stayed the same size or shrunk (in square inches), while their functionality has
skyrocketed in the past 20 years. In this edition, you will learn more about how the
motherboard works, and about a motherboard's many sockets and connectors.
As can be seen from the plan of the computer
system, the mother, or main board is at the
center of the PC computer system. Effectively it
is a printed circuit board containing the central
processing unit (CPU) and the memory modules
(Simm's). It allows the CPU to interfaces with
other parts of the computer via a 'BUS' system,
into which sockets are fitted for connection of
various 'expansion' boards. Also on the
motherboard is the RAM memory, normally in
the form of SIMM, or DIMM modules in the
modern PC computer, and cache memory in the
form of integrated circuits (chips). All these various components will be examined in more
detail in their appropriate sections.
Modern boards utilize a Pentium CPU with four PCI and three, or four, ISA expansion
slots. Most video boards now use the PCI connection. So, if you are upgrading from
a 386, or 486, mother board that have a video board with VESA connectors, you may
well have to purchase a new video panel.
Most Pentium boards are also fitted with controllers for Floppy Drives, 4 EIDE
devices and Enhanced Parallel and Fast Serial Ports. This 'fee’s up' a couple of
expansion slots for other devices
Motherboard Sizes :
Different motherboards of different vintages typically have different form factors.
Form factor essentially means the size and shape of the actual motherboard. There
are more than a half-dozen form factors for motherboards, with the most recent ones
having the designation of NLX. Right now, the designation ATX is the most
prevalent. By buying a computer with a true ATX motherboard, you are assured that
you will have the ability to upgrade by being able to re-use the personal computer
case with a more recent replacement ATX board design.
Motherboards have helped to keep the "personal" in personal computing since plug
able components allow the user to personalize the system depending on their
applications and needs. For example:

A prolific collector of digital camera images or video will want to add a SCSI
hard disk drive to an open bay and use an empty socket on the motherboard
for the SCSI controller card.

A serious game enthusiast will want the fastest video card possible with as
much memory on the card as possible.
Common Motherboard Items :
A motherboard is a multi-layered printed circuit board. Copper circuit paths called
traces that resemble a complicated roadmap carry signals and voltages across the
motherboard. Layered fabrication techniques are used so that some layers of a board
can carry data for the input/output, processor and memory buses while other layers
can carry voltage and ground returns without circuit paths short-circuiting at
intersections. The insulated layers are manufactured into one complete, complex
"sandwich."
Chips and sockets are soldered onto the motherboard. For example, you will typically
find:

One or more microprocessors

A basic input/output system chip (BIOS)

Memory slots

A chip set that adds lots of features like I/O ports and controllers

Peripheral component interconnect (PCI) adapter card slots

Industry standard architecture (ISA) adapter slots

Accelerated graphics port (AGP) video card slots

Universal serial bus (USB) ports
Cooling fan(s) on heat sinks of processor and some video cards
Examples :
This motherboard supports one processing unit (CPU), has six PCI slots and an
ISA Slot as well as an AGP Slot. The board supports 133 MHz bus speeds and
ultra-direct memory access-100 (UDMA). There are four USB ports and onboard
audio in the ATX form factor board.
The Abit KT-7A supports Advanced Micro Devices (AMD) processors and has
the KT-133A chipset. The card slots on the Abit KT-7A, from bottom to top,
shows below that ISA has one slot, PCI has six slots and AGP has one slot. A
special fan cools the chipset.
The BIOS chip is common to many motherboards.
How Have Motherboards Changed ?
Speeds, temperatures, density, faster chipset designs and component count have
driven the need for circuit cooling via miniature electric fans. These fans mount inside
the actual computer case. Heat sinks act like an automobile radiator and provide
additional surface area to help cool a component. Replaceable fan-heat sink
assemblies are often used to help dissipate the considerable amount of heat on
modern processor chips. The fan-heat sink assembly conducts heat away from the
chip by convection, using a layer of thermal grease between the two mating metal
surfaces. Fans often have a third wire used for monitoring the speed of the fan.
Modern motherboard designs include provisions for monitoring:

Fan speed in RPM for the personal computer case, processor and power
supply fans

Temperatures of motherboard and processor

Personal computer case intrusion
PCI slots are replacing the older ISA slots, and USB ports are replacing both types of
slots. USB ports can also be used to replace the usual keyboard, mouse and printer
ports. Sound card function is also typically incorporated into modern motherboards.
Multifunction chips are on the horizon that will do even more multiple tasks.
The additional function on the motherboard saves the motherboard manufacturer
costs because:

There are less warranty claims due to problems associated with all the many
electrical contacts (fasteners) in the usual card slot

There are lower power supply wattage requirements

There are savings from elimination of a slot's socket and its space on the
motherboard
The consumer can still upgrade function integrated on the motherboard (such as
audio and game controls) so long as the motherboard manufacturer provides a
means of disabling the function in order to prevent subsequent system resource
conflicts.
A motherboard still may have voltages present on it even if the computer is switched
off due to recent advances in power management and power controls. Always make
sure that the power cord is unplugged!
Advice on Motherboards :
When buying a motherboard, follow these tips:

Deal only with a reputable manufacturer.

Ensure that it has the same form factor as your current case.

Read the booklet that comes with your motherboard. It should fully cover the
motherboard's settings and specifications.

Check the power supply requirements for AMD processors. Some
motherboards have unique requirements.

Verify the form factor of your computer case matches the form factor of any
motherboard you plan to buy.
Avoid tweaking voltages and timings to get more speed out of a computer ("over
clocking")."
Motherboard Requirements :
For maximum compatibility with Dig design hardware your computer's motherboard
should be based on the Intel chipset and meet the ATX form factor specifications.
The ATX form factor is the standard in PC motherboard architecture. If you are
unsure whether you have an ATX motherboard look at the back of your computer. If
the ports match up with the photo below your computer most likely has an ATX
motherboard.
One exception to this is with higher end workstation motherboards that include
daughter boards. These are circuit board extensions off of the main motherboard that
are not expansion cards. Motherboards with daughter boards do not meet the ATX
specification.
The Connectivity Test of the Mother Board :
1. Necessary information for the test
Before starting the test, we determined the function of each connector on the Mother
Board. Fig1 shows the arrangement of connectors.
Fig 1. Connector arrangement on the Mother Board
There are 33 connectors on the outside of the Mother Board arranged by function. In
the case of Timing & Control, LVDS output, and Serial & Control connectors, the
group1 connector is the top one and the group7 connector is the bottom one in Fig1
(a). On the inside, group arranges connectors. Six 160-pin connectors (groups 1-6)
plug into power/comm. boards and two 80-pin connectors (group7) plug into a
daughter board.
We made various fan out boards, a 160 pin, an 80 pin, a 50 pin, a 40 pin, a 36 pin, a
26 pin, and a 20-pin fan out board.

160 pin fan-out board ------> for inside connectors J36-J41 (groups 1-6)

80 pin fan-out board -------> for inside connectors J34-J35 (group7).
J35; signals 1-80, J34; signals 81-160

50 pin fan-out board -------> for Silicon Bias1 or Analog Sum output, J24, J26.

40 pin fan-out board -------> for Silicon Bias2 or Monitoring signal, J23, J25.

36 pin fan-out board -------> for LVDS output, J16-J22.

26 pin fan-out board -------> for Serial/control signal, J27-J33.

20 pin fan-out board -------> for Timing/control signal, J9-J15.
We can find a pair of connected pins, a pin on the inside connector and a pin on the
outside connector, by using fan out boards and Fig 1. Because of the fine pitch,
direct contact to a pin with a probe or lead is difficult, but fan out boards make it easy
to touch a pin.
2. Simple setup for the connectivity test
Fig 2. The resistance measurement using fan-out boards
We measured resistances for all pass-through signals. Starting with group 1, we
identified a target signal then plugged the appropriate fan out board into the Mother
Board. A pair of pins (or several pins in case of ground connection) with the same
name were located, one pin on the inside and one pin on the outside of the board.
Finally, a resistance value was measured with a FLUKE 8060A multimeter. This
value included resistances of the two fans out boards, which we didn't want.
Therefore we measured the resistance of each fan out board and subtracted it from
the total measured resistance. The resistances of the fan out boards are shown
below, in table 1. The resistance values appearing in table 2 of section 3 below
exclude the portion from the two fans out boards.
Table 1. The resistances of the fan out boards
Fan out board
Resistance (ohm)
160 pin
0.38
80 pin
0.38
50 pin
0.39
40 pin
0.35
36 pin
0.37
26 pin
0.35
20 pin
0.35
3. Results
The resistance values, classified by the group # and signal function, are shown in
table 2. Generally the measured values are distributed between 0.1 and 2.2 ohm,
which we believe as acceptable. Comparing the values between groups, the
measured resistances show the tendency to decrease when the group # increases
from 1 to 6, which is due to the decrease of the trace length on the motherboard.
Because two 80-pin connectors for group 7 are far away from the 160 pin
connectors, resistances for group 7 don't follow that trend.
Because the analog discriminator sum is not used for groups 4-6,which relate to the
outer layer strip detector, the table shows blank values for groups 4-6 analog sum
signal connections. The below table doesn't contain the resistance values for ground
connections. In fact, we had trouble with some DGND and AGND lines which are
summarized at the end of this section. Except for these bad grounds, the remaining
ground connections have relatively small resistance (< 0.2 ohm) regardless of the
group #.
References :
http://www.basichardware.com/motherboard.html
http://www.digidesign.com/compato/mobo.html
http://whatis.techtarget.com/definition/0,sid9_gci212594, 00.html
http://www.phenix.bnl.gov/phenix/WWW/publish/mvd/ygkim/note_357/con_thru.html
Prepared by
ID NUMBER
Mohammed A. Al-najem
422001524