Transcript ic100eHD ch08

OBJECTIVES
1. Demonstrate proper cleaning procedures for various system components.
2. Describe electrostatic discharge hazards and methods of preventing ESD.
3. List the steps for proper IC handling.
4. Define the term ground.
5. Describe the two types of uninterruptible power supplies (UPS) and state
their qualities.
6. State typical precautions that should be observed when working on computer
equipment.
7. Perform generic preventive maintenance routines as required (for example,
remove excess toner, defragment hard drives, and create back-up copies).
8. Detail routine preventive maintenance procedures as they apply to hard and
floppy disks.
9. Perform basic disk management functions on a hard drive, including using
ScanDisk, CHKDSK, and Defrag utilities.
10. Use backup software to create backups of important data.
11. Use software utilities to identify and remove viruses from computer
systems.
12. List precautionary steps that should be taken when handling floppy disks.
(continued)
OBJECTIVES (CONTINUED)
13. List steps to clean a dot-matrix, ink-jet, or laser printer.
14. Establish and maintain preventive maintenance schedules for users.
15. Differentiate between various UPS specifications and state how they apply
to a given situation.
16. State potential hazards that are present when working with laser printers,
monitors, and other equipment.
Identify the purpose of various types of preventive
maintenance measures, products and procedures,
and when and how to use them. Content may
include the following:
Liquid cleaning compounds
Types of materials to clean contacts and connections
Non-static vacuums (chassis, power supplies, fans)
Cleaning monitors
Cleaning removable media devices
Ventilation, dust, and moisture control on the PC
hardware interior.
(continued)
Identify the purpose of various types of preventive
maintenance measures, products and procedures,
and when and how to use them. Content may
include the following: (continued)
Hard disk maintenance (defragging, ScanDisk,
CHKDSK)
Verifying UPS (Uninterruptible Power Supply)
and suppressors
Cleaning is a major part of keeping a
computer system healthy. Therefore, the
technician’s tool kit should also contain a
collection of cleaning supplies. Along
with hand tools, it will need a lint-free,
soft cloth (chamois) for cleaning the
plastic outer surfaces of the system.
TEST TIP
Know the common methods of cleaning
various computer components.
TEST TIP
Know what types of materials/techniques
can be used to clean different areas of the
computer system.
Figure 8-1: How to Handle a PC Board
The environment around a computer system
and the manner in which the computer is used
determine greatly how many problems it will
have. Occasionally dedicating a few moments of
care to the computer can extend its Mean Time
Between Failures (MTBF) period considerably.
This activity, involving maintenance not
normally associated with a breakdown, is called
Preventive Maintenance (PM).
TEST TIP
Know what environmental conditions
or activities are most likely to lead to
equipment failures.
TEST TIP
Be aware of the effect that missing
expansion slot covers have on the
operation of the system unit.
TEST TIP
Know that computer vacuums have special
grounding to dissipate static buildup that
can damage computer devices.
CHALLENGE #1
When you arrive at a customer’s office to repair one
of their key computers, you trace the problem down
to a defective microprocessor. The system has
apparently been upgraded several times over its
lifetime  there are several open expansion slots
and loose cables inside the system unit. There is also
a layer of dust on all the internal components. What
should you tell the manager when she asks you
about what you have found?
Figure 8-2: Caution Areas Inside the Monitor
Figure 8-3: Proper
Packing of a Hard
Drive for Shipment
Figure 8-4: Data Backup Systems
TEST TIP
Be aware of the precautions that
should be employed with storing
system backups.
Unlike hard-disk drives, tape drives, floppy
drives, and CD-ROM/DVD drives are at least
partially open to the atmosphere, and their
media may be handled on a regular basis. This
opens these drive units up to a number of
maintenance concerns not found in hard disk
drives. Also, the removable cartridges, disks,
or discs can be adversely affected by extremes
in temperature, exposure to magnetic and
electromagnetic fields, bending, and airborne
particles that can lead to information loss.
Figure 8-5: Cleaning the R/W Heads
Input peripherals generally require very
little in the way of preventive maintenance.
An occasional dusting and cleaning should
be all that’s really required. There are,
however, a few common-sense items to
keep in mind when using an input device
that should prevent damage to the device,
and ensure its longevity.
TEST TIP
Remember that dust can settle into
the keyboard through the cracks
between the keys.
Figure 8-6: Cleaning the Keyboard
Figure 8-7: Cleaning the Rollers in a Mouse
TEST TIP
Know how to clean a trackball
mouse.
Because printers tend to be much more
mechanical than other types of computer
peripherals, they require more effort to
maintain. Printers generate pollutants, such
as paper dust and ink droplets, in everyday
operation. These pollutants can build up on
mechanical parts and cause them to wear.
As the parts wear, the performance of the
printer diminishes. Therefore, printers
require periodic cleaning and adjustments
to maintain good performance.
WARNING
Cleaning the printer and its mechanisms
periodically adds to its productivity by
removing contaminants that cause wear.
Vacuum the inside of the unit, after applying
antistatic solution to the vacuum’s hose tip.
Wipe the outside with a damp cloth, also using
antistatic solution. Brush any contaminant
buildup from the printer’s mechanical
components, using a soft-bristled brush. Never
lubricate the platen assembly of the printer.
The steps to cleaning a dot-matrix printer are
described in the following steps:
1
Adjust the printhead spacing.
2
Check the tension on the printhead positioning
belt.
3
Clean the printer and its mechanisms.
4
Clean the printer’s roller surfaces.
5
Clean the surface of the platen.
6
Clean the surface of the dot-matrix printhead.
(continued)
The steps to cleaning a dot-matrix printer are
described in the following steps: (continued)
7
Clean the paper-handling motor’s gear train.
8
Apply light oil to the gears using a swab.
9
Turn the platen to distribute the oil.
10 Apply a light coating of oil to the rails.
11 Move the carriage assembly to distribute
the oil.
The steps to cleaning an ink-jet printer are
provided by the following steps:
1
Adjust the printhead spacing.
2
Check the tension on the printhead positioning
belt.
3
Clean the printer and its mechanisms.
4
Clean the printer’s roller surfaces.
5
Clean the surface of the platen.
6
Clean the surface of the ink-jet printhead.
(continued)
The steps to cleaning an ink-jet printer are
provided by the following steps:
7
Clean the paper-handling motor’s gear train.
8
Apply light oil to the gears using a swab.
9
Turn the platen to distribute the oil.
10 Apply a light coating of oil to the rails.
11 Move the carriage assembly to distribute
the oil.
TEST TIP
Remember acceptable methods for
cleaning laser printers.
Steps to cleaning a laser printer are described
in the following hands-on activity:
1
Remove dust build-up, and excess toner, from
the interior.
2
Clean the laser printer’s rollers.
3
Clean the paper-handling motor’s gear train.
4
Apply light oil to the gears, using a swab.
5
Distribute the oil throughout the gear train.
6
Clean the corona wires.
Check for other sources of heat buildup
around the computer and its peripherals.
These sources include:
Direct sunlight from an outside window
Locations of portable heaters in the winter
Papers/books piled up around the
equipment
Identify various types of preventive maintenance
measures, products, and procedures and when
and how to use them. Content may include the
following:
Verifying UPS (uninterruptible power
supply) and suppressors
Avoid power variations  Digital
systems tend to be sensitive to power
variations and losses. Even a very short
loss of electrical power can shut a digital
computer down, resulting in a loss of any
current information that has not been
saved to a mass storage device.
Typical power supply variations fall into two
categories:
Transients — an over-voltage condition, while sags
are an under-voltage condition. Over-voltage
conditions can be classified as spikes (measured in
nanoseconds), or as surges (measured in
milliseconds).
Sags — can include voltage sags and brownouts. A
voltage sag typically lasts only a few milliseconds,
while a brownout can last for a protracted period
of time.
TEST TIP
Be aware of how undervoltage and
overvoltage situations are
categorized (i.e., time lengths).
There are two factors to consider when
choosing a surge suppresser:
Clamping speed
Clamping voltage
TEST TIP
Know what type of devices will
protect systems from minor power
sags and power surges.
TEST TIP
Know what type of device prevents
power interruptions that can corrupt
data.
Uninterruptible power supplies are
battery-based systems that monitor
the incoming power and kick in
when unacceptable variations occur
in the power source. The term UPS
is frequently used to describe two
different types of power backup
systems.
Figure 8-8: UPS System
Figure 8-10: Connecting the UPS in the System
NOTE
High-power-consumption peripheral
devices, such as laser printers, should
not be connected directly to the UPS.
These devices can overload the UPS
and cause data loss.
If the UPS system does not come on, there are
several items that should be checked. These
items include:
The On/Off switch
The commercial AC power supply
The UPS input circuit breaker
The UPS battery connector
TEST TIP
Remember that nonessential
peripheral devices should not be
connected to UPS supplies.
NOTE
Of course, you should be aware that
any UPS testing should be performed
while the computer is not in active
use and when there is no chance of
data loss due to unexpected test
results.
TEST TIP
Be aware that the UPS system will
not be able to control the operation
of the computer without the serial
communications cable installed.
TEST TIP
Know that the best device for
transporting computer equipment
is the original manufacturer’s
packaging, including the antistatic
foam and bags used to pack it.
Identify various safety measures and procedures,
and when/how to use them. Content may include
the following:
Situations that could present a danger
or hazard
Potential hazards and proper safety
procedures relating to:
High-voltage equipment
Power supply
CRTs
In most IBM compatibles, there are only
two potentially dangerous areas. One of
these is inside the CRT display, and the
other is inside the power supply unit. Both
of these areas contain electrical voltage
levels that are lethal. However, both of
these areas reside in self-contained units,
and you will usually not be required to
open either unit.
TEST TIP
Be aware of the voltage levels that
are present inside a CRT cabinet.
Laser printers contain many hazardous
areas. The laser light can be very
damaging to the human eye. In addition,
there are multiple high-voltage areas in
the typical laser printer, and a hightemperature area to contend with as well.
TEST TIP
Know the areas of the computer
system that are dangerous for
personnel and how to prevent
injury from these areas.
TEST TIP
Remember the type of fire
extinguisher that must be used with
electrical systems, such as a PC.
Identify environmental protection measures
and procedures, and when/how to use them.
Content may include the following:
Special disposal procedures that comply
with environmental guidelines
Batteries
CRTs
Chemical solvents and cans
MSDS (Material Safety Data Sheet)
Most computer components contain some
level of hazardous substances. Printed
circuit boards consist of plastics, precious
metals, fiberglass, arsenic, silicon, gallium,
and lead. CRTs contain glass, metal,
plastics, lead, barium, and rare earth
metals. Batteries from portable systems
can contain lead, cadmium, lithium,
alkaline manganese, and mercury.
TEST TIP
Remember that toner cartridges
from a laser printer should be
recycled.
TEST TIP
Remember that the proper disposal
method for batteries is to recycle
them.
The MSDS contains information about:
What the material is
Its hazardous ingredients
Its physical properties
Fire and explosion data
Reactivity data
Spill or leak procedures
Health hazard information
Any special protection information
Any special precaution information
Identify various safety measures and
procedures, and when/how to use them.
Content may include the following:
ESD (Electrostatic Discharge)
precautions and procedures
What ESD can do, how it may be
apparent or hidden
Common ESD protection devices
What is ESD  Electrostatic Discharges
(ESD) are the most severe form of
Electromagnet Interference (EMI). The
human body can build up static charges that
range up to 25,000 volts. These buildups can
discharge very rapidly into a electrically
grounded body, or device. Placing a 25,000volt surge through any electronic device is
potentially damaging to it.
TEST TIP
Remember what the acronym
ESD stands for.
The most common causes of ESD are:
Moving people
Improper grounding
Unshielded cables
Poor connections
Moving machines
Low humidity (hot and dry conditions)
TEST TIP
Memorize the conditions that
make ESD more likely to occur.
TEST TIP
Be aware that compressed air can be
used to blow dust out of components
and that it does not create ESD.
TEST TIP
Memorize conditions and actions that
produce electrostatic discharge.
You may be a little confused by the fact that we warn
you about the lethal 25,000 volts present inside the
monitor and then say that the 10,000 to 25,000 volts
of ESD is not harmful to humans. The reason for this
is the difference in current-delivering capabilities
created by the voltage. For example, the circuitry in
the monitor and the power supply is capable of
delivering amps of current, while the currentproducing capability of the electrostatic charge is
less than a thousandth of that. Therefore, the 120
Vac, 1-amp current produced by the power supply
unit is lethal, while the 25,000 Vdc, microamps of
current produced by ESD is not.
TEST TIP
Remember that the current capabilities
of electrical devices establish the
potential danger levels associated with
working around them.
CHALLENGE #2
You have been asked to consult on the design of
your company’s new repair facility near Phoenix,
Arizona. In particular, management wants to know
how to equip the work areas of their new facility.
You have not been to the site, but you know that it is
in a hot desert environment. Also, the building will
be air-conditioned. How should you advise them
about precautions that should be taken with the
work area?
Figure 8-10: Typical Antistatic Devices
TEST TIP
Know when not to wear an
antistatic wrist strap.
To avoid damaging static-sensitive devices, the
following procedures will help to minimize the
chances of destructive static discharges:
Before touching any components inside the system,
touch an exposed part of the chassis or the power
supply housing with your finger. Grounding yourself
in this manner will ensure that any static charge
present on your body is removed. This technique
should be used before handling a circuit board or
component. Of course, you should be aware that this
technique will only work safely if the power cord is
attached to a grounded power outlet. The ground plug
on a standard power cable is the best tool for
overcoming ESD problems.
(continued)
Figure 13-8: Discharging Through the Power Supply
Unit
To avoid damaging static-sensitive devices, the
following procedures will help to minimize the
chances of destructive static discharges:
(continued)
Do not remove ICs from their protective tubes
(or foam packages) until you are ready to use
them. If you remove a circuit board or
component that contains static-sensitive devices
from the system, place it on a conductive surface,
such as a sheet of aluminum foil.
(continued)
To avoid damaging static-sensitive devices, the
following procedures will help to minimize the
chances of destructive static discharges:
(continued)
In the event that you have to replace a hard-soldered IC,
you may want to install an IC socket along with the chip.
Be aware that normal operating vibrations and
temperature cycling can degrade the electrical
connections between ICs and sockets over time. This
gradual deterioration of electrical contact between chips
and sockets is referred to as chip creep. It is a good
practice to reseat any socket-mounted devices when
handling a printed circuit board. Before removing the IC
from its protective container, touch the container to the
power supply of the unit in which it is to be inserted.
(continued)
TEST TIP
Be aware of the effects that
temperature cycling can have
on socket-mounted devices.
To avoid damaging static-sensitive devices, the
following procedures will help to minimize the
chances of destructive static discharges:
(continued)
Use antistatic sprays or solutions on floors,
carpets, desks, and computer equipment. An
antistatic spray or solution, applied with a soft
cloth, is an effective deterrent to static.
Install static-free carpeting in the work area. You
can also install an antistatic floor mat as well.
Install a conductive tabletop to carry away static
from the work area. Use antistatic tablemats.
(continued)
To avoid damaging static-sensitive devices, the
following procedures will help to minimize the
chances of destructive static discharges:
(continued)
Use a room humidifier to keep the humidity
level above 50% in the work area.
The term ground is often a source of
confusion for the novice, because it
actually encompasses a collection of
terms. Generically, ground is simply any
point from which electrical
measurements are referenced. However,
the original definition of ground actually
referred to the ground. This ground is
called earth ground.
Figure 8-12: Power Transmission System
Figure 8-13: Grounds on IC Chips
TEST TIP
Remember that ESD is destructive
and EMI is not.
TEST TIP
Know the best protection for
a computer system during an
electrical storm.
There are basically three types of viruses, based
on how they infect a computer system:
A boot-sector virus - This type of virus copies
itself onto the boot sector of floppy and hard
disks. The virus replaces the disk’s original
boot-sector code with its own code. This allows
it to be loaded into memory before anything else
is loaded. Once in memory, the virus can spread
to other disks.
(continued)
There are basically three types of viruses, based
on how they infect a computer system:
(continued)
A file infector - File infectors are viruses that add their
virus code to executable files. After the file with the
virus is executed, the virus spreads to other executable
files. A similar type of virus, called a macro virus,
hides in the macro programs of word processing
document files. These files can be designed to load
when the document is opened or when a certain key
combination is entered. In addition, these types of
viruses can be designed to stay resident in memory
after the host program has been exited (similar to a
TSR program), or they might just stop working when
the infected file is terminated.
(continued)
There are basically three types of viruses, based
on how they infect a computer system:
(continued)
A Trojan horse - This type of virus appears to be
a legitimate program that might be found on
any system. Trojan horse viruses are more likely
to do damage by destroying files, and they can
cause physical damage to disks.
TEST TIP
Know how the different types
of viruses attack the system.
Typical virus symptoms include the following:
Hard disk controller failures.
Disks continue to be full even when files have been
deleted.
System cannot read write-protected disks.
The hard disk stops booting and files are corrupted.
The system will boot to floppy disk, but will not
access the HDD.
An Invalid Drive Specification message usually
displays when attempting to access the C: drive.
(continued)
Typical virus symptoms include the following:
(continued)
CMOS settings continually revert to default
even though the system board battery is good.
Files change size for no apparent reason.
System operation slows down noticeably.
Blank screen when booting (flashing cursor).
Windows crashes.
The hard drive is set to DOS compatibility, and
32-bit file access suddenly stops working.
Network data transfers and print jobs slow
down dramatically.
TEST TIP
Know how viruses are spread.
CHALLENGE #3
A customer calls you to his site complaining that the
wide-carriage, dot-matrix printer in his accounting
department is running very slowly and that they
cannot get all their invoices printed for today’s
shipping purposes. When you check their print
queue, you see that the print jobs for the invoices
are stacked up in the queue but are being processed.
The accounting manager tells you that they typically
don’t have any problems getting their invoices
printed and that nothing out of the ordinary has
been done to the computer to make it slow down.
What items should you check to determine the cause
of the slowdown?
KEY POINTS
Cleaning is a major part of keeping a
computer system healthy. Therefore, the
technician’s tool kit should also contain a
collection of cleaning supplies. Along with
hand tools, it will need a lint-free, soft cloth
(chamois) for cleaning the plastic outer
surfaces of the system.
KEY POINTS
The environment around a computer system
and the manner in which the computer is
used determine greatly how many problems it
will have. Occasionally dedicating a few
moments of care to the computer can extend
its Mean Time Between Failures (MTBF)
period considerably. This activity, involving
maintenance not normally associated with a
breakdown, is called Preventive Maintenance
(PM).
KEY POINTS
Unlike hard-disk drives, floppy drives are at
least partially open to the atmosphere, and
they may be handled on a regular basis. This
opens floppy-disk drives to a number of
maintenance concerns not found in hard-disk
drives. Also, the removable disks are subject
to extremes in temperature, exposure to
magnetic and electromagnetic fields, bending,
and airborne particles that can lead to
information loss.
KEY POINTS
Input peripherals generally require very little
in the way of preventive maintenance. An
occasional dusting and cleaning should be all
that’s really required.
KEY POINTS
Because printers tend to be much more
mechanical than other types of computer
peripherals, they require more effort to
maintain. Printers generate pollutants, such
as paper dust and ink droplets, in everyday
operation. These pollutants can build up on
mechanical parts and cause them to wear. As
the parts wear, the performance of the printer
diminishes. Therefore, printers require
periodic cleaning and adjustments to
maintain good performance.
KEY POINTS
Digital systems tend to be sensitive to power
variations and losses. Even a very short loss
of electrical power can shut a digital
computer down, resulting in a loss of any
current information that has not been saved
to a mass storage device.
KEY POINTS
Uninterruptible power supplies are batterybased systems that monitor the incoming
power and kick in when unacceptable
variations occur in the power source. The
term UPS is frequently used to describe two
different types of power backup systems.
KEY POINTS
In most IBM compatibles, there are only two
potentially dangerous areas. One of these is
inside the CRT display, and the other is inside
the power supply unit. Both of these areas
contain electrical voltage levels that are
lethal. However, both of these areas reside in
self-contained units, and you will usually not
be required to open either unit.
KEY POINTS
Laser printers contain many hazardous
areas. The laser light can be very damaging
to the human eye. In addition, there are
multiple high-voltage areas in the typical
laser printer, and a high-temperature area
to contend with as well.
KEY POINTS
Most computer components contain some
level of hazardous substances. Printed circuit
boards consist of plastics, precious metals,
fiberglass, arsenic, silicon, gallium, and lead.
CRTs contain glass, metal, plastics, lead,
barium, and rare earth metals. Batteries from
portable systems can contain lead, cadmium,
lithium, alkaline manganese, and mercury.
KEY POINTS
Electrostatic Discharges (ESD) are the most
severe form of Electromagnet Interference
(EMI). The human body can build up static
charges that range up to 25,000 volts. These
buildups can discharge very rapidly into a
electrically grounded body or device. Placing
a 25,000-volt surge through any electronic
device is potentially damaging to it.
KEY POINTS
In general, MOS devices are sensitive to
voltage spikes and static electricity
discharges. This can cause many problems
when you have to replace MOS devices,
especially Complementary-Symmetry Metal
Oxide Semiconductor (CMOS) devices. The
level of static electricity present on your body
is high enough to destroy the inputs of a
CMOS device if you touch its pins with your
fingers.
KEY POINTS
The term ground is often a source of
confusion for the novice, because it actually
encompasses a collection of terms.
Generically, ground is simply any point
from which electrical measurements are
referenced. However, the original definition
of ground actually referred to the ground.
This ground is called earth ground.
REVIEW QUESTIONS
1
List the two most dangerous areas of
a typical microcomputer system, and
explain why they are so dangerous.
REVIEW QUESTIONS
2
Name three devices used to minimize
ESD in the repair area.
REVIEW QUESTIONS
3
The best general-purpose cleaning tool
for computer equipment is _______.
REVIEW QUESTIONS
4
List at least three environmental
conditions that can adversely affect
microcomputer equipment.
REVIEW QUESTIONS
5
A short undervoltage condition, lasting
milliseconds, is called ________.
REVIEW QUESTIONS
6
Are there any restrictions on disposing
of a spent toner cartridge?
REVIEW QUESTIONS
7
Which type of IC device is most likely
to be damaged by ESD?
REVIEW QUESTIONS
8
Can an effective ESD strap be constructed
by simply wrapping a grounded bare wire
around your wrist?
REVIEW QUESTIONS
9
What is the most effective method
of dealing with EMI problems?
REVIEW QUESTIONS
10 The best method of protecting
computer equipment from a
thunderstorm is to _______.
REVIEW QUESTIONS
11 The best method for transporting
electronic devices is _________.
REVIEW QUESTIONS
12 List computer-related PM items
that should be performed annually.
REVIEW QUESTIONS
13 Name two characteristics that should
be checked carefully before purchasing
a UPS for a given computer system.
REVIEW QUESTIONS
14 Describe the normal duration of a
voltage spike.
REVIEW QUESTIONS
15 Once a virus has infected a computer,
where does it normally reside?
EXAM QUESTIONS
1
Do viruses normally attack the system’s CMOS
settings?
a. Yes, this is how a virus attacks most
computers.
b. No, viruses do not normally attack CMOS
settings.
c. Yes, this is how viruses attack all computers.
d. No, viruses never attack CMOS settings.
EXAM QUESTIONS
2
How are most computer viruses spread from
computer to computer?
a. By downloading programs from networks
b. By sharing infected files between
individuals
c. By not formatting disks before use
d. By transferring files over modems
EXAM QUESTIONS
3
What is the most common cause of ESD
in microcomputer systems?
a. Moving people
b. High humidity
c. Rubber mats
d. Grounded power supply cables
EXAM QUESTIONS
4
Where would it be inappropriate to use
an ESD wrist strap?
a. While working on hard-disk drives
b. While working on system boards
c. While working on CRT video monitors
d. While working on printers
EXAM QUESTIONS
5
What is one of the best methods for
protecting computer systems from ESD?
a. A water and fabric softener solution
b. A water and ammonia solution
c. A water and bleach solution
d. A hydrogen tetrachloride solution
EXAM QUESTIONS
6
A short overvoltage occurrence
(nanoseconds) is called ________.
a. a spike
b. a surge
c. a brownout
d. a sag
EXAM QUESTIONS
7
ESD is most likely to occur during
periods of _________.
a. low humidity
b. high humidity
c. medium humidity
d. rain
EXAM QUESTIONS
8
The best protection against powerfailure data loss is _______.
a. a tape backup
b. a surge suppresser
c. a UPS
d. a line filter
EXAM QUESTIONS
9
Define a voltage sag.
a. An overvoltage condition that lasts for a few
milliseconds
b. An undervoltage condition that lasts for an
extended period
c. An overvoltage condition that lasts for an
extended period
d. An undervoltage condition that lasts for a
few milliseconds
EXAM QUESTIONS
10 The most effective grounding system for
a microcomputer is _________.
a. an ESD wrist or ankle strap
b. the safety ground plug at a commercial
AC receptacle
c. the ground plane of the system board
d. the chassis ground provided by brass
standoffs