Diagnosing faults and defects

1. DIAGNOSING FAULTS
AND DEFECTS OF
CONTROL MODULES
Allan Gilbert Jain
Teacher III

2. Lesson Objectives
At the end of the lesson, learners are expected to:
1. What are Programmable Logic Controllers as a Control Module.
2. Identify the five (5) typical parts of PLC and their functions.
3. Enumerate common faults and defects by diagnosing PLC indicators.
4. Present service information by completing a Job or Service Sheet.

3. Activity
Answer the following question in your notebook.
•In your own words, what can you say about PLC based in
the diagram and description below?
Figure 1 shows a light connected to a switch. In general, the light operates under two conditions – ON
and OFF. Using PLC in the middle, you can turn on the light 30 seconds after the switch is turned ON.

4. PLC stands for “Programmable Logic Controller”. A PLC is a computer specially
designed to operate reliably under harsh industrial environments – such as extreme
temperatures, wet, dry, and/or dusty conditions. PLCs are used to automate industrial
processes such as a manufacturing plant’s assembly line, an ore processing plant, or a
wastewater treatment plant.
PLCs were invented by Dick Morley in 1964. Since then, PLC has
revolutionized the industrial and manufacturing sectors. There is a wide range of PLC
functions like timing, counting, calculating, comparing, and processing various analog
signals.
What are Programmable Logic Controllers as a Control
Module?

5. Five Typical Parts of PLC And Their Functions
Programmable Logic Controllers continuously monitor
the input values from various input sensing devices (e.g.
accelerometer, weight scale, hardwired signals, etc.) and
produce corresponding output depending on the nature of
production and industry. A typical block diagram of PLC
consists of five parts namely:
1. Rack or Chassis
In all PLC systems, the PLC rack or chassis forms the most
important module and acts as a backbone to the system. PLCs
are available in different shapes and sizes. When more
complex control systems are involved, it requires larger PLC
racks.
2. Power Supply Module
This module is used to provide the required power to the
whole PLC system. It converts the available AC power to DC
power which is required by the CPU and I/O module. PLC
generally works on a 24V DC supply. Few PLC uses an
isolated power supply.

6. Five Typical Parts of PLC And Their Functions
3. CPU Module and Memory
CPU module has a central processor, ROM & RAM memory. ROM
memory includes an operating system, drivers, and application
programs. RAM memory is used to store programs and data. CPU is
the brain of PLC with an octal or hexagonal microprocessor.
4. Input and Output Module
Have you ever thought about how to sense physical parameters like
temperature, pressure, flow, etc.? using PLC? Of course, PLC has an
exclusive module for interfacing inputs and output, which is called an
input & output module.
5. Communication Interface Module
To transfer information between CPU and communication networks,
intelligent I/O modules are used. These communication modules help
to connect with other PLCs and computers which are placed at a
remote location.

7. Learning Task 1
PLC as a Control Module uses a program to manipulate and control a system.
What similarities in function and features do you see compare to computers?

8. Self-Check 3.3-1
Five typical parts of PLC and their functions
Learning Task 2: True or False. Tell whether the given statement describes the parts and corresponding
functions of PLC. Write (T) if the statement is True and (F) if the statement is False. Write your answer in
your notebook.
_____ 1. Power supply module used to provide the required power to the whole PLC system.
_____ 2. When more complex control systems are involved, it requires smaller PLC racks or chassis.
_____ 3. Communication Interface Module helps to connect with other PLCs and computers which are
placed at a remote location.
_____ 4. CPU is the heart of PLC with an octal or hexagonal microprocessor.
_____ 5. Input and output modules have the capability to sense physical parameters like temperature,
pressure, flow in the interface.

9. LED status indicators can provide much information about field devices, wiring, and
I/O modules. Most input/output modules have at least a single indicator—input modules
normally have a power indicator, while output modules normally have a logic indicator.
For an input module, a lit power LED indicates that the input device is activated and
that its signal is present at the module. This indicator alone cannot isolate malfunctions to
the module, so some manufacturers provide an additional diagnostic indicator, a logic
indicator. An ON logic LED indicates that the input signal has been recognized by the logic
section of the input circuit.
If the logic and power indicators do not match, then the module is unable to
transfer the incoming signal to the processor correctly.
Common Faults And Defects By Diagnosing PLC Indicators

10. This indicates a module malfunction. An output module’s logic indicator
functions similarly to an input module’s logic indicator. When it is ON, the logic LED
indicates that the module’s logic circuitry has recognized a command from the
processor to turn ON.
In addition to the logic indicator, some output modules incorporate either a
blown fuse indicator or a power indicator or both. A blown fuse indicator indicates the
status of the protective fuse in the output circuit, while a power indicator shows that power
is being applied to the load.
Like the power and logic indicators in an input module, if both LEDs are not
ON simultaneously, the output module is malfunctioning.
Common Faults And Defects By Diagnosing PLC Indicators

11. 1. How important LED indicators in diagnosing early faults or defects in the Control
Module of a PLC system?
For more information visit:
YouTube.https://www.youtube.com/watch?v=GDJQ6-coD34&feature
Learning Task 3