The document provides a comprehensive overview of Programmable Logic Controllers (PLCs), detailing their structure, functions, advantages over traditional relay systems, and programming methods. It covers the basic operation of PLCs, including input/output processing, control applications, and the programming languages standardized by IEC 61131, particularly focusing on ladder diagrams. The importance of PLCs in industrial automation and their capacity for enhanced reliability, flexibility, and cost-effectiveness is emphasized.

1. MECHATRONICS: Programmable logic controller

2. TEXT BOOKS
• Mechatronics Electronic control system in Mechanical and Electrical Engineering, W Bolton,
Pearson Education, 1st Ed., 2005.
REFERENCE BOOKS:
• Mechatronics by HMT Ltd. - Tata McGrawHill, 1st Edition, 2000
Further Reference:
National Programme on Technology Enhanced Learning (NPTEL)
https://nptel.ac.in/courses/112103174/1 by Dr. S. N. Joshi (IITG)

3. • Define what a programmable logic controller (PLC) is and list its advantages over
relay systems.
• Identify the main parts of a PLC and describe their functions
• Outline the basic sequence of operation for a PLC
Learning Objectives

4. Module 4
Introduction to PLCs: Basic structure of PLC, Principle of operation, I/p & O/p
processing, PLC programming language, ladder diagram, ladder diagrams circuits,
timer counters, internal relays, master control, jump control, shift registers, data
handling, and manipulations, analogue input and output, selection of PLC for
application.
Application of PLC control: Extending and retracting a pneumatic piston using
latches, control of two pneumatic pistons, control of process motor, control of
vibrating machine, control of process tank, control of conveyer motor etc.

5. Programmable Logic Controller
• A programmable logic controller (PLC) is an industrial grade computer that is
capable of being programmed to perform control functions.
• A digital computer designed for use in machine control.
• It has been designed to operate in the industrial environment, and is equipped
with special input/output interfaces and a control programming language.
Introduction

6. Programmable Logic Controller
• Initially the PLC was used to replace relay logic, but
its ever-increasing range of functions, it is found in
many and more complex applications.
• The structure of a PLC is based on the principles as
employed in computer architecture.
• It is capable not only of performing relay switching
tasks but also of performing other applications such
as timing, counting, calculating, comparing, and the
processing of analog signals.
Introduction

7. Programmable Logic Controller
• Microprocessor-based controller, uses programmable memory to store
instructions to implement functions like, logic, sequence, timing, etc. to control
machines and processes
Introduction

8. Programmable Logic Controller
Introduction
( a ) Relay based control panel ( b ) PLC-based control panel
All the logic is contained in the PLC’s
memory
• Relays are the switches, aim at closing and opening the circuits electronically as well as electromechanically.
• It controls the opening and closing of the circuit contacts of an electronic circuit.
• When the relay contact is open (NO), the relay isn’t energize.
• If it is closed (NC), the relay isn’t energize given the closed contact.
• when energy (electricity or charge) is supplied, the states are prone to change.

9. Programmable Logic Controller
Introduction
Relationships between the inputs and outputs are
determined by the user program

10. Programmable Logic Controller
Reasons why PLCs are being widely used
• Rugged and designed to withstand Industrial conditions (vibrations,
temperature, humidity and noise)
• User friendly, fast and easy to operate
• Eliminate the need for hard wired relay logic.
• Its input and output modules can be extended depending on the requirements

11. Programmable Logic Controller
Reasons why PLCs are being widely used
• Communications Capability. A PLC can communicate
with other controllers or computer equipment to
perform functions like; supervisory control, data
gathering, monitoring devices and process
parameters, download and upload of programs
Communication Module

12. Programmable Logic Controller
Reasons why PLCs are being widely used
• Faster Response Time: Designed for highspeed and
real-time applications.
• Programmable controller operates in real time, means
an event taking place in the field will result in the
execution of an operation or output.
• Machines that process thousands of items per second
and objects spend only a fraction of a second in front
of a sensor require the PLC’s quick-response
capability.
High speed Counting

13. Programmable Logic Controller
Reasons why PLCs are being widely used
• Easier to Troubleshoot: PLCs have resident diagnostics and override functions,
allow users to easily trace and correct software and hardware problems.
• To find and fix problems, users can display the control program on a monitor
and watch it in real time as it executes
Control program can be displayed on the monitor in real time

14. Programmable Logic Controller
Advantages
• Less wiring: Wiring between devices and relay contacts are done in the PLC
program.
• Easier and faster to make changes.
• Troubleshooting aids make programming easier and reduce downtime.
• Reliable components make these likely to operate for years before failure.

15. Programmable Logic Controller
Advantages
• Increased Reliability: Once a program has been written and tested it can be
downloaded to other PLCs.
• Since all the logic is contained in the
PLC’s memory, there is no chance of
making a logic wiring error.

16. Programmable Logic Controller
Advantages
• More Flexibility: OEMs can provide system updates for a process by simply
sending out a new program.
• It is easier to create and change a program in
a PLC than to wire and rewire a circuit.
• End-users can modify the program in the field.

17. Programmable Logic Controller
Advantages
• Lower Costs: Originally PLCs were designed to replace relay control logic.
• Generally, if an application requires more
than about 6 control relays, it will usually be
less expensive to install a PLC.

18. Programmable Logic Controller
Manufacturers
The manufacturers of PLCs include:
• Allen Bradley
• ABB
• Siemens
• Mitsubishi PLC
• Hitachi PLC
• Delta PLC
• General Electric (GE) PLC
• Honeywell PLC

19. Basic Structure

20. Basic Structure
Expansion Module

21. Programmable Logic Controller
Basic Structure
Sensors & Transducers Actuators
Logic
• Solenoid valve
• Motors
• LED display
• Sounds / Alarm

22. Programmable Logic Controller
Basic Structure
1. Rack / Chassis
2. Power supply Module
3. Central processing unit (CPU)
4. Storage / Memory
5. Input/output interface circuit
6. The function module
7. The communication module
8. Programming unit

23. Programmable Logic Controller
Basic Structure
There are two ways in which I/Os (Inputs/Outputs) are incorporated into the PLC
1. Fixed
2. Modular

24. Programmable Logic Controller
Basic Structure
Fixed I/O
• Typically small PLCs, comes in one package with no
separate, removable units.
• The processor and I/O are packaged together
• The I/O terminals will have a fixed number of connections
built in for i/p & o/p.
Advantage: Lower cost.
Disadvantage:
• Lack of flexibility: you are limited, the quantities and types
dictated by the packaging.
• If any part in the unit fails, the whole unit has to be replaced.

25. Programmable Logic Controller
Basic Structure
Modular I/O
• Divided by compartments, in which separate modules can be plugged.
• The basic modular controller consists of a rack, power supply, processor module
(CPU), input/output (I/O modules), and an operator interface for programming and
monitoring.
• The modules plug into a rack.
• When a module is slid into the rack, it makes an electrical connection with a
series of contacts called the backplane, located at the rear of the rack.
• The PLC processor is also connected to the backplane and can communicate with
all the modules in the rack.

26. Programmable Logic Controller
Basic Structure
Modular I/O
Modular I/O configuration

27. Basic Structure
• 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.
• Modular type rack PLC, accepts different types of I/O
modules with sliding and fit in concept.
• All I/O modules will be residing inside this rack/chassis.
Rack / Chassis

28. Basic Structure
• PLC power supply converts a line voltage, i.e. AC into a DC voltage, required by the CPU
and I/O module in the rack.
• In Modular PLC racks, the power supply is the part of rack.
• A battery backup, to provide energy to the memory of the PLC in case of a power supply
failure
Power supply

29. Programmable Logic Controller
CPU Module
• Control centre of the PLC
• CPU module has CPU, ROM & RAM memory.
• Rom includes an OS, drivers and application programs.
• RAM for storing Programs and Data
• Performs the routine check
• It controls and processes all the operations within the PLC
Processor Module
• Microprocessor for implementing the logic and controlling the
communications among the modules.

30. Programmable Logic Controller
• The PLC program is executed as part of a repetitive
process, referred as scan.
• PLC scan starts with the CPU reading the status of
inputs, then, the application program is executed.
• Once the program execution is complete, the CPU
performs internal diagnostic and communication
tasks.
• Next, the status of all outputs is updated.
• This process is repeated continuously as long as the
PLC is in the run mode.
PLC scan cycle

31. Programmable Logic Controller
Memory: The memory elements available in PLC are;
• ROM: Permanent storage for the OS and fixed data.
• RAM: For user's program.
• Programs in RAM can be changed by the user.
• To prevent the loss of these programs, when the supply is switched off, a
battery is provided in the PLC to maintain the RAM contents for a period of time.

32. Programmable Logic Controller
lnput / Output (l/O) circuitry
• I/O unit provides the interface between the system
and outside world.
• Programs are entered into using the input unit.
• The programs, can also be entered by means of PC,
with an appropriate software package.
• Input devices can be start and stop pushbuttons,
switches etc.
• output devices can be an heater, valves, relays etc.

33. Programmable Logic Controller
• The I/O channel provides signal conditioning
and isolation functions so that sensors and
actuators can be directly connected to them
without the need for other circuitry
I/O circuitry

34. Programmable Logic Controller
• Inputs might be limit switches which are
activated when some event occurs.
• The outputs might be motor, starter coils,
solenoid valves, etc.
• Electrical isolation from the external world is
usually by means of optical isolators.
I/O circuitry

35. Principles of Operation

36. Principles of Operation
• Consider the simple process control problem
illustrated in Fig.
• A mixer motor is to be used to automatically stir
the liquid in a vat When the temperature and
pressure reaches a preset values.
• A direct manual operation of the motor is provided
by means of a separate pushbutton station.
• The process is monitored with temperature and
pressure sensor switches that close their
respective contacts when conditions reach their
preset values.
Mixer process control problem

37. Principles of Operation
• This control problem can be solved by relay method for motor control, shown
in the relay ladder diagram (Fig.)
• The motor starter coil (M) is energized when both the pressure and
temperature switches are closed or when the manual pushbutton is pressed.
Process control relay ladder diagram

38. Principles of Operation
• PLCs might be used for this
application.
• The input field devices (pressure
switch, temperature switch, and
pushbutton) are used.
• These devices would be hardwired to
an appropriate input module as shown
in fig.
Wiring connections for a 120 VAC modular configured input module

39. Principles of Operation
• The output field device (motor, starter
coil) would also be used.
• This device would be hardwired to an
appropriate output module
Wiring connections for a 120 VAC modular configured output module

40. Principles of Operation
• The PLC ladder logic program would be constructed and entered into the memory of the CPU.
• A typical ladder logic program for this process is shown in Fig.
Process control PLC ladder logic program

41. Principles of Operation
• The format / instruction is similar to the layout of the hardwired relay ladder circuit.
• The individual symbols represent instructions, the numbers represent the instruction
location addresses.
• To program the controller, enter these instructions one by one into the processor
memory from the programming device.
• Each input and output device is given an address, which lets the PLC know where it
is physically connected.
• The I/O address format will differ, depending on the PLC model and manufacturer.

42. Principles of Operation
• Instructions are stored in the user program portion of the processor memory.
• During the program scan, the controller monitors the inputs, executes the control
program, and changes the output accordingly.
• During each operating cycle, the controller examines the status of input devices,
executes the user program, and changes outputs accordingly.

43. Principles of Operation
• Each symbol ‘II’ can be thought of as a set of
normally open (NO) contacts.
• The symbol –( )- represents a coil, when energized,
will close a set of contacts.

44. Principles of Operation
• The coil O/1 is energized, when contacts I/1 and I/2 are
closed or when contact I/3 is closed.
• Either of these conditions provides a continuous logic path
from left to right across the rung that includes the coil.

45. Principles of Operation
The RUN operation for the process control scheme is described by the following
sequence of events:
1. First, the pressure switch, temperature switch, and pushbutton inputs are
examined and their status is recorded in the controller’s memory.
2. A closed contact is recorded in memory as logic 1 and an open contact as logic 0.
3. The ladder diagram is evaluated, with each internal contact given an OPEN or
CLOSED status according to its recorded 1 or 0 state.
4. When the states of the input contacts provide logic continuity from left to right
across the rung, the output coil memory location is given a logic 1 value and the
output module interface contacts will close.
5. When there is no logic continuity of the program rung, the output coil memory
location is set to logic 0.

46. Principles of Operation
• The completion of one cycle of this sequence by the controller is called a scan.
• Scan time: the time required for one full cycle, provides a measure of the speed of
response of the PLC.
• Generally, the output memory location is updated during the scan but the actual output is
not updated until the end of the program scan during the I/O scan

47. Principles of Operation
Typical wiring required to implement the process control scheme
PLC ladder logic program for the
modified process

48. PLC Programming Language

49. PLC Programming Language
• The term PLC programming language refers to the method by which the user
communicates information to the PLC.
• The standard IEC 61131 was established to standardize the multiple languages
associated with PLC programming by defining the five standard languages:

50. PLC Programming Language
1. Ladder Diagram (LD): A graphical representation of a process with rungs of logic,
similar to the relay ladder logic schemes that were replaced by PLCs.
2. Function Block Diagram (FBD): A graphical representation of process flow using
simple and complex interconnecting blocks.
3. Sequential Function Chart (SFC): A graphical depiction of interconnecting steps,
actions, and transitions.
4. Instruction List (IL): A low-level, text-based language that uses mnemonic
instructions.
5. Structured Text (ST): A high-level, text-based language such as BASIC, C, or
PASCAL specifically developed for industrial control applications.

51. PLC Programming Language
Standard IEC 61131 languages associated with

52. PLC Programming Language

53. Ladder Logic / Ladder Program / Ladder diagram (LD)

54. • Used to program a PLCs.
• It is a graphical programming language which expresses logical operations with
symbols and notation using ladder diagrams.
• Used to execute logical, sequential, counting, timing and arithmetic tasks in order
to carry industrial automation applications.
• Ladder logic programming is still used today because the core fundamental logic
principles for machine and process control are still the same.
Ladder Logic / Ladder Program
Concept of Ladder Logic

55. • In the earlier days, machine and process automation
was accomplished using a hard wired control system
known as relay logic.
• Ladder logic was originally designed to replace the
use of hard wired relay logic circuits for machine
control.
• The ladder logic programming code resembles as of
an electrical schematic drawing.
Ladder Logic / Ladder Program
Concept of Ladder Logic

56. • In PLC programming, ladder logic is a programming language, used for
developing logical expressions in order to automate tasks / process.
Ladder Logic / Ladder Program
Concept of Ladder Logic

57. • Ladder logic is used extensively for programming PLCs in industrial automation
applications.
• E.g. : Material Handling Conveyor System / Pallet Packing and Strapping.
• Ball Mill Lubrication System / Logistics Package Conveying and Sorting.
• Cement Batching / Beverage Bottling and Labelling.
• Hopper and Tank Level Control / Air / Liquid Flow and Pressure Control.
Ladder Logic / Ladder Program
Concepts Ladder Logic

58. • A ladder diagram is a type of schematic diagram used in industrial automation that represents
logic control circuits.
• Ladder diagrams are composed of two vertical power rails and horizontal logic rungs to form what
looks like a ladder. The control logic in a ladder diagram is contained within the rungs.
Ladder Logic / Ladder Program
What is a Ladder Diagram?
• The name “ladder diagram” is derived
from the program’s resemblance to a
ladder with two vertical rails and a series
of horizontal rungs between them.
• The rails are called “power rails” in the
ladder diagram.

59. • The reason is because the early control system designers were accustomed to
relay logic control circuits and ladder diagrams closely mimic these.
• The person / staff already knows how to read relay control circuits, so using
ladder diagrams for programming a PLC.
• Also, able to troubleshooting control system problems easily.
Ladder Logic / Ladder Program
Why is a ladder diagram used for PLC programming?

60. Ladder diagram / Logic
• Ladder diagram (LD): official name given in the international PLC programming
standard IEC-61131. (International Electrotechnical Commission)
• Symbols represent opening and closing relays, counters, timers, shift registers,
etc.
• Symbols are arranged in the desired program routine.
• Rules in ladder logic are termed “rungs.”
• Each rung has a single output.

61. The Logic Behind The Ladder
Seven basic steps of a ladder diagram.
1. Rails: Two rails (power rails) in a ladder diagram, represented by vertical lines.
• The power flows from the left hand side to the right hand side.
2. Rungs: Horizontal lines, connects the rails to the logic expressions.

62. The Logic Behind The Ladder (SCAN)
(a) Relay circuit (b) Drawing an electric circuit, (c) Rung in a ladder program.
The sequence followed by a PLC when executing a program.
1. Scan the inputs associated with one rung of the ladder program.
2. Solve the logic operation involving those inputs.
3. Set/reset the outputs for that rung.
4. Move on to the next rung and repeat operations 1, 2, 3.
5. Until the end of the program with each rung of the ladder program scanned in turn.
6. The PLC then goes back to the beginning of the program and starts again.
if A and B are both closed then a solenoid (output) is energised.

63. The Logic Behind The Ladder
3. Inputs: Inputs are sensors and transducers.
E.g. Push button, limit switch etc.,
• Inputs are hardwired to the PLC terminals.
• Represented in the ladder diagram by a Normally open (NO) or Normally
closed (NC) contact symbol.

64. The Logic Behind The Ladder
4. Outputs: Outputs are external devices (Actuators).
E.g. Turn on and off an electric motor or a solenoid valve/coil.
• The outputs are hardwired to the PLC terminals.
• Represented in the ladder diagram by a relay coil symbol.

65. The Logic Behind The Ladder
5. Logic Expressions: The logic expressions are used in combination with the
inputs and outputs to formulate the desired control
operations.
6. Address Notation: Address notation describes the input, output, logic expression,
memory addressing structure of the PLC.
• Tag names: descriptions allocated to the addresses.

66. The Logic Behind The Ladder
7. Comments:
• Important part of a ladder diagram.
• Comments are displayed at the start of each rung.
• Used to describe the logical expressions and control operations of that rung.
• Understanding ladder diagrams are easier by using comments.

68. How to Read Ladder Logic
Microprocessors operates on the binary concept.
‘Binary’: principle, is that the event/s can be thought of in one of two states.
The states can be defined as:
• 1 or 0
• True or False
• On or Off
• High or Low
• Yes or No

69. How to Read Ladder Logic
• Ladder logic uses symbolic expressions and a graphical editor for reading and
writing code making it easier.
• If real world event is translated into ladder logic, it symbolically expressed in the
form of a normally open (NO) contact.
E.g. events like a button being pushed or a limit switch being activated.

70. How to Read Ladder Logic
Example
• Consider event ‘A’, has one of two states, TRUE or FALSE (1 or 0).
• Event is associated with the normally open (NO) contact can be TRUE or FALSE.
• If the event is TRUE, highlighted in green.
ladder logic truth table

71. How to Read Ladder Logic
• A normally open (NO) contact alone cannot decide what action to take to
automate the event
• It merely tells, what is the state of the event.
• Logic is the ability to decide what action needs to be taken depending on the
state of one or more events.
• Logic concept – IF, THEN logic functions.

72. Ladder Logic Functions
• Consider an event = A. Allocated to normally open (NO) contact.
• In ladder logic, the events are defined as PLC inputs.
• Let the result of the logic function = ‘Y’.
• The result of a rung logic function is defined as a PLC output.
• The two fundamental elements on a rung in a ladder diagram is first line of code.

73. Ladder Logic Functions
Two possible logic iterations:
• IF A = FALSE THEN Y = FALSE
• IF A = TRUE THEN Y = TRUE
Ladder Logic Basics – In Built Functions

74. Ladder Logic Functions
Two possible logic iterations:
• IF A = FALSE THEN Y = FALSE
• IF A = TRUE THEN Y = TRUE
Ladder Logic Basics – In Built Functions
Ladder logic diagram expressed symbolically in the
form of a normally open (NO) contact for the input and
the output relay coil.

75. In ladder logic fundamental logic functions are;
1. AND
2. OR
3. NOR
4. NAND
5. XOR
Ladder Logic Functions

76. Logic functions
(a) AND, (b) OR, (c) NOR, (d) NAND, (e) XOR

77. Ladder Logic AND Functions

78. Ladder Logic OR Functions

79. Ladder Logic
The sequence followed by a PLC when carrying out a program
1. Scan the inputs associated with one rung of the ladder program.
2. Solve the logic operation involving those inputs.
3. Set/reset the outputs for that rung.
4. Move on to the next rung and repeat operations 1, 2, 3.
5. Move on to the next rung and repeat operations 1, 2, 3.
6. So on until the end of the program with each rung of the ladder program.
The PLC then goes back to the beginning of the program and starts again.

80. Ladder Logic

81. Timers
• A motor or a pump need to be controlled to operate for a particular interval of time
or to be switched on after some time interval.
• PLCs have timers as built-in devices.
• Timers count seconds or fractions of seconds using the internal CPU clock.
Example

82. Timers
• The timers behave like relays with coils that when energized, result in the
closure or opening of contacts after some preset time.
a) The timer is treated as an output for a rung, with control being exercised over
pairs of contacts.
b) Timer as a delay block, inserted in a rung, delays signals in that rung from
reaching the output.
a b

83. Forms of Timers
There are a number of different forms of timers found with PLCs:
1. on-delay
2. Off-delay
3. Pulse
With small PLCs, just one form of timers are used i.e. the on-delay timers
As per IEC, the symbols denote:
• TON is used to denote on-delay, TOF off-delay, and TP pulse timers.
• On-delay is also represented by T-0 and off-delay by 0-T.

84. Timers
The IEC symbols for timers
IEC 1131-1 standards
• IN is the Boolean input. Q is the Boolean output.
• ET is the elapsed time output.
• PT is the input used to specify the time delay or pulse duration required

85. Timers
On-Delay Timers
All PLCs have on-delay timers.
The Allen-Bradley timer symbol shows the type of timer concerned with;
• The timer address, and the time base that indicates the increments by which
the timer moves to the preset value, such as 0.001 s, 0.01 s, 0.1 s or 1 s.
• The preset value (PRE) is the number of time increments that the timer must
accumulate to reach the required time delay.
• The accumulator (ACC) indicates the number of increments that the timer has
accumulated while the timer is active and is reset to zero when the timer is
reset
(useful if a program needs to record how long a particular operation took).

86. Timers
On-Delay Timers
The Allen-Bradley timers have three Boolean bits for ladder logic control:
• A timer enable bit (EN), which goes on when the timer accumulator is Incrementing
• A timer done bit (DN), which goes on after the set time delay.
• A timer timing bit (TT) is on when the accumulator is incrementing and remains on
until the accumulator reaches the preset value.

87. Timers

88. Internal relays
• Internal Relays are elements, used to hold data, i.e. bits, and
behave like relays, being able to be switched on or off and switch
other devices on or off. Hence the term internal relay.
• The internal relays do not exist as real-world switching devices but
are bits in the storage memory, behave in the same way as relays.
• For programming, they can be treated as an external relay output
and input.
• Thus inputs to external switches can be used to give an output
from an internal relay.
• This results in the internal relay contacts being used, in
combination with other external input switches to give an output,
e.g. activate a motor.

89. Internal relays
• In using an internal relay, it has to be activated on one rung of a
program and then its output used to operate switching contacts
on another rung, or rungs, of the program.
• Internal relays can be programmed with as many sets of
associated contacts as desired.
• To distinguish internal relay outputs from external relay outputs,
they are given different types of addresses.
• Different manufacturers use different terms for internal relays and
have different ways of expressing their addresses.
Example: Mitsubishi uses the term auxiliary relay or marker and the
notation M100, M101

90. Counters
• Counters are provided as built-in elements in PLCs and allow the number of
occurrences of input signals to be counted.
• Example: Items have to be counted as they pass along a conveyor belt / the
number of revolutions of a shaft / number of people passing through a door.
• A counter is set to some preset number value, when this value of input pulses has
been received, it will operate its contacts. Normally open (NO) contacts would be
closed, normally closed (NC) contacts opened.

91. Counters
There are two basic types of counter:
1. Down-counters
2. Up-counters.
1. Down-counters count down from the preset value to zero, i.e., events are
subtracted from the set value. When the counter reaches the zero value, its
contacts change state. Most PLCs offer down-counting.
2. Up-counters count from zero up to the preset value, i.e. events are added until
the number reaches the preset value. When the counter reaches the set value, its
contacts change state.
Forms of Counters

92. Counters
• PLCs offer the facility for both down and up-counting.
Figure shows the IEC symbols for such counters.
Forms of Counters
(a) The pulses at CD are counted.
When the counter goes from the start
PV value to 0, Q is set to 1 and the
counting stops. An input to LD clears Q
to 0.
(b) The pulses at CU are counted.
When the counter reaches the PV
value, Q is set to 1 and the counting
stops. An input to R clears Q to 0.
(c) The up-down counter has
two inputs CU and CD and
can be used to count up on
one input and down on the
other
IEC symbols for counters: (a) down-counter, (b) up-counter, and (c) up-down

93. Counters
Forms of Counters
Different counter representations.
(a) Counter as coils with contacts in another rung, RST is reset.
(b) The IEC 1131-3 representation as an element in a rung.

94. JUMP
A function often provided with PLCs is the conditional jump, described as:
IF (some condition occurs) THEN
perform some instructions
ELSE
perform some other instructions
• JUMP facility enables programs to be designed such that if certain conditions are
met, certain events occur, and if they are not met, other events occur.
Example: we might need to design a system, if the temperature is above 60C, a fan is
switched on, and if below that temperature no action occurs.

95. JUMP
Fig. illustrates JUMP concept.
• When there is an input to Input 1, its contacts close and there is
an output to the jump relay.
• This results in the program jumping to the rung in which the jump
end occurs and skipping the intermediate program rungs.
• In this case, when there is an input to Input 1, the program jumps
to rung 4 and then proceeds with rungs 5, 6.
• When there is no input to Input 1, the jump relay is not energized
and the program then proceeds to rungs 2, 3.
JUMP

96. Shift Register
• Registers, used for storing data that originate from input sources.
• A register is a number of internal relays grouped together, normally 8, 16, or 32.
• Each internal relay is either open or closed, these states being designated 0 and 1.
• The term bit is used for each such binary digit.
• If we have eight internal relays in the register, we can store eight 0/1 states.
• For internal relays each relay might store an on/off signal
state of the register
8-bit register

97. Shift Register
• With the shift register it is possible to shift stored bits.
• Shift registers require three inputs:
1. To load data into the first location of the register.
2. Command to shift data along by one location.
3. To reset or clear the register of data.

98. Shift Register
Example
8-bit register
If we receive the input signal 0. This is an input signal to the first internal relay
If we also receive the shift signal, the input signal enters the first location in the register, and all the bits shift
along one location. The last bit overflows and is lost.
The grouping together of internal relays to form a shift register is done automatically by a PLC when the
shift register function is selected.

99. Data Handling
Data handling consists of operations involving moving or transferring numeric
information stored in one memory word location to another word in a different
location, comparing data values, and carrying out simple arithmetic operations.
Data Handling
• The following are examples of data-handling instructions to be found with PLCs.
• Data Movement
• Data Comparison
• Data Selection

100. Data Handling
For moving data from one location or register to another.
Figure illustrates a common practice of using one rung of a ladder program for
each move operation.
Form used by three manufacturers: Mitsubishi, Allen-Bradley, and Siemens.
Data Movement

101. Data Handling
Form used by Allen-Bradley
Data Movement
When there is an input to | | in the rung, the move occurs from the designated source address to the
designated destination address.

102. Data Handling
• The data comparison instruction gets the PLC to compare two data values.
• It might be to compare a digital value read from some input device with a
second value contained in a register.
Data Comparison
An illustration, in structured text
(*Check that boiler pressure P2 is less than pressure P1*)
Output : = P2 < P1;
In ladder programs, for data comparison, the typical
instruction will contain the data transfer instruction to
compare data, the source (S) address from which the data is
to be obtained for the comparison, and the destination (D)
address of the data against which it is to be compared.
Greater than comparison

103. Data Handling
• There are a number of selection function blocks available with PLCs.
• Figure shows the standard IEC symbols.
Data Selection
Figure: IEC symbols: (a) selection, (b) maximum, and (c) minimum

104. Concept of Latching
• There are situations where it is necessary to hold a coil energized, even when the input which
energized it ceases.
• The term latch circuit is used for the circuit which carries out such an operation.
• It is a self-maintaining circuit, after being energized, it maintains that state until another input is
received.

105. Concept of Latching
• When Input 1 is energized and closes, there is an output. However, when there is an output, a
set of contacts associated with the output is energized and closes. The contacts is in OR the
Input 1 contacts.
• Even if Input 1 contacts open, the circuit will still maintain the output energized.
• The only way to release the output is by operating the normally closed contact Input 2

106. Concept of Latching
An example of a latch circuit: consider the requirement for a PLC to control a motor
• We require is a system that will still stop if a failure occurs in the stop switch.
• The program now has the stop switch as open contacts. However, because the hard-wired stop
switch has normally closed contacts, then the program receives the signal to close the program
contacts.
• Pressing the stop switch then opens the program contacts and stops the system
Stop system: (b) safe

107. Selection of a PLC
The following criteria need to be considered:
1. Types of inputs/outputs required, like;
- Isolation
- Out-board power supply for inputs/outputs
- Signal conditioning
2. lnput/Output capacity required
3. Size of memory required: linked with no. of I/O and complexity of program used
4. Speed and power required for CPU: linked to the no. of types of instructions,
handled by a PLC.

108. End