The document discusses programmable logic controllers (PLCs) and their components and applications. It defines a PLC as a solid-state industrial controller that performs logic functions to control machines and processes. The key components of a PLC include input/output modules that interface with field devices, a processor with memory and programming capabilities, and a power supply. PLCs can be unitary or modular in design. They are widely used in industrial automation to control functions like conveyor systems. The document outlines the history, advantages, and types of PLCs as well as their programming and applications in industrial processes.

1. PLC AND SCADA
Subject code: 18MT61

2. Syllabus
 what is A PLC, Technical Definition of PLC, What are its
advantages,
 characteristics functions of A PLC, Chronological
Evolution of PLC, Types of
 PLC, Unitary PLC, Modular PLC, Small PLC, Medium PLC,
Large PLC, Block
 Diagram of PLC: Input/output (I/O) section, Processor
Section, Power supply,
 Memory central Processing Unit: Processor Software /
Executive Software, Multi
 asking, Languages, Ladder Language.

3. WHAT IS PLC
 PLC is a solid state/ computerised industrial controller that
performs discrete or sequential logic in a factory
environment.
 It was originally developed to replace mechanical relays,
timers and counters.
 Used to successfully execute complicated control operations
in a plant.
 A sequence of instructions is programmed by the user to the
PLC memory and when the programme is executed the
controller operates the system to correct operating
specifications.
 The PLC was introduced in late 1960s

4.  Today PLC is used as a stand alone control or a part of a
Complex Computer Integrated Manufacturing System.
 PLC may also provide an interface between a machine and
robots or a machine and colour graphics which are known
as Human Machine Interface (HMI).
 PLC is important in the world of industrial automation.

5. Technical Defination Of PLC
 Is currently defined by NEMA (National
Electrical Manufacturers Association) as a
 “Digital electronic Device that uses a
programmable memory to store
instructions and to implement the
specific functions such as logic,
sequence, timing, counting and
arithmetic operations to control
machine and processes”.

6. History and Origin:
• •Developed to replace relays in the late
1960s
• •PLC began in the 1970s, and has
become the most
• common choice manufacturing for
controls.
• •The PLC was invented in response to the
needs of
• the American automotive manufacturing
industry
• (primarily General motors).
• •Costs dropped and became popular by
1980s
• • Now used in many industrial designs

7. PLC Advantages
 Faster Scan time
 Intelligent I/O
 High speed Counters
 Supervised Control capability
 Reliability in operation
 Flexibility in programming and reprogramming in a plant
 Cost effective for controlling complex systems
 Flexibility in Control techniques
 Minimum Maintainance
 Small physical size
 Superior computational capabilities to execute complex
control techniques
 ASCII message handling capablity

8.  Ability to communicate with computer systems in a
plant.
 Programmable troubleshooting aids which reduces the
downtime.

9.  PLC DISADVANTAGES
 There's too much work required in connecting wires.
 It is difficult to find errors.
 Skillful workers are required.
 There’s difficulty with replacements.
 PLC devices are proprietary, which means that parts and
software from one manufacturer can’t easily be used in
combination with parts of another manufacturer, which
limits the design and cost
 options.

10. PLC APPLICATIONS
• PLC’s are generally used in industries for
controlling heavy
machinery and processes like conveyor system, CNC
machines
etc.
• PLC usage can also be observed in day to day life.
For eg : automatic washing machines, cars, video
cameras etc.
• PLC has it’s applications in almost every automatic
machine.

11. COMPONENTS OF PLC
1 . Processor.
2. Memory unit .
3. Power supply.
4. I/O Modules.
5. Programming device.

12. Characteristics of PLC
 PLC offers certain characteristic functions that make
it advantageous in an industrial environment.
 It is superior in modern industrial control and
communication.
 Some of the major characteristic functions of PLC
are:
1. It is field programmable by the User: It allows the
user to modify the programs as per the requirements
of a plant without having it to send the unit back to
the manufacturer for this purpose.

13. 2. It contains the programmed functions: PLCs
contain at least logic, timing, counting and
memory functions that the user can access
through some kind of control oriented
programming language.
3. It scans memory, inputs and outputs (I/O) in a
predetermined manner.
4. It provides error checking and diagnostics. A PLC
will periodically carry out internal tests of its
memory, processor and I/O systems through in
built program execution to ensure what it is
doing to the machine or system, against what it
was programmed to do.

14. 5. A PLC can provide some form of monitoring
capabilities either through indicating lamps that
shows the status of inputs and outputs or by an
external device that can display the program
execution status.
6. It is packaged in such a way that it can withstand
the temperature, humidity, vibration and noise
found in most factory environments.
7. A PLC can be effectively designed for a wide
variety of control tasks, although it can also be
dedicated only for a particular task.

19. TYPES OF PLC
 PLCs are of two types: 1. Unitary PLCs
2. Modular PLCs.
 1. Unitary PLC:
 Unitary PLC has a power supply, a CPU and a
limited number of inputs and outputs.
 It has 20 inputs- 12 outputs, 32 I/O
 It is sometimes called “shoebox type”.
 It is mainly used for the control of a small system.

21. Unitary PLC

22. 2. Modular PLC:
A modular PLC is one that it can be constructed
using separate modules of power supply, CPU,
inputs, outputs, timers, counters, ADC, DAC,
expansion modules.
These modular PLCs are sometimes called “rack
mounted type PLC”.
Modular PLCs can be sub-divided into the following
types
a) Small PLC
b) Medium PLC
c) Large PLC

23. Modular PLC

26. a) Small PLC:
PLC having less than 100 inputs and outputs are
designated as small PLCs.
Out of the I/Os, 20 inputs and 12 outputs are
mounted locally within the processor.
Additional I/Os can be added through remote I/O
racks to accommodate the extra inputs and outputs.
These PLCs generally have a memory from 2KB to
10KB to store the users logic program.

27. b) Medium PLC:
These have extended instructions sets that include
mathematical functions, file functions, PID process
control etc.
These PLC can have between 4000 to 8000 inputs and
outputs.
They are also made to support wide variety of special
modules such as ASCII communication modules, BASIC
programming modules, 16 bit multiplexing modules,
analog input and output modules (both voltage and
current), communication modules.

28. c) Large PLC
The purpose of introducing large PLC was to provide
enough memory space and I/O for complete factory
automation.
Major Disadvantage: Whole Factory may collapse if the
PLC starts malfunctioning.
Distributed Control: Introducing this enables to use the
LAN , where the small or medium PLCs are connected
together through an appropriate network.
Entire factory is brought under the control of a number
of PLCs, but failure in one system will not disturb any
other system.

29. PLC SYSTEM

32. Block Diagram of PLC
 PLC has three basic sections namely 1) I/O section 2)Processor
Section 3) Program loader/monitor section
 The input section converters convert the field signals to logic
level signals that the PLC’s processor can read directly.
 The processor section reads these inputs, processes the signals
executing the program and prepare the output signals.
 The output section converts the logic level output signals coming
from the processor section into high level signals and releases the
signals for various field devices.
 The program loader is to enter the user’s program into the memory
or change it and to monitor the execution of the program.

33. I/O section
 The input output section establish the interfacing
between the real world outside the PLC and the
digital arena inside the PLC.
 The signals from the machines or instruments of a
system are received through the input section by a
PLC.
 After processing the output signals are transmitted to
the machines or the instruments for the desired
control action through the output section.
 The tasks of the I/O section can be classified as:
 1. Conditioning 2. Isolation 3) termination 4)
Indication

37.  The field signal is 220V or 440 V.
 PLC can support a voltage of 0-10 volts, the field
signal has to be conditioned before it is applied to
the PLC.
 To protect the PLC from any incoming surges from
the field, isolating devices such as transformers or an
opto-isolator are used.
 The I/O modules provides a proper terminal.
 It indicates the status of the I/O terminal, i.e when
the voltage is off the status indicator is off.

38. Processor Section
 This section consists of four major elements.
 1.) Power supply 2.) Memory 3.) CPU 4.) I/O interface
 1) Power Supply:
 Basic function of the power supply is to supply conditioned power
to the components of a PLC.
 The power supply is one of the most critical components of the
PLC for two reasons:
 1. It is typically non redundant. Hence failure of PLC power
supply can cause entire control system to fail.
 2. It contains high voltage components. Hence an insulation
failure can be a serious potential for injury or fire.

39.  The following guidelines should be followed when
choosing the power supply of a PLC:
 1) The power supply should be packed properly, so
that the heat generated by the power supply can be
dissipated in order to prevent overheating. This
increases reliability.
 The power supply should be tested for temperature
and electrical insulation by a proper certification
agency.
 The power supply should meet at least one reputable
standard for noise Immunity.
 Suitable redundancy will increase reliability.

41. 2. Memory
 Two types of memory in a PLC: i) volatile memory ii)
non volatile memory.
 Non volatile memories: are generally used for storing
users programs so that they are preserved during
power cuts.
 The non volatile memory in a PLC uses some form of
error checking to ensure that memory has not
changed.
 To ensure to have a safe execution of the users
programs this error checking should also be done while
PLC is controlling a machine or a process.

42.  Different types of memory that are generally used in
a PLC are as follows.
 1. Battery backed up CMOS RAM : This is the
probably the most widely used type of memory.
 CMOS variety consumes such a small amount of power
that a small battery will retain memory during the
power losses.
 The battery used vary from short life primary cells
(alkaline or Hg) which require periodic replacement
after 6 months to a year.
 To a long life battery which may last up to 10 years
to rechargeable secondary cells.

43.  2. EPROM Memory:
 Electrically Programmable Read only Memory is
programmed using electrical pulses.
 Can be erased by exposing the circuit to UV light
also called UV-EPROM.
 3. EEPROM Memory:
 Similar to EPROM Memory.
 It can also be erased using electrical pulses.
 In some PLC’s use of EEPROM only alleviates the
need of an UV light source.
 In other PLC’s the EEPROM is the only type of
memory used.
 This allows the flexibility of reprogramming.

44.  Four sections memory is subdivided: Executive
memory, internal RAM, user memory and I/O image
memory.
 The executive memory holds executive software.
 User memory is meant for user program storage

45. 3. CPU
 The flexibility of a PLC is dependent on : i) How
CPU is constructed ii) Overall speed of the PLC
 The speed is expressed in terms of how fst the
PLC will scan a given amount of memory.
 This measure is called scan rate.
 It is expressed in milliseconds per thousand
words of memory.

47.  1. Processor:
 Processor is the central processing unit (CPU) of the
 programmable controller, it executes the various logic
 sequencing functions by operating on the PLC inputs to
determine
 the appropriate output signals.
 The CPU consists of one or more microprocessors
similar to that
 of PCs and other data processing equipment but are
designed to
 facilitate I/O transactions with different ranges with
corresponding
 clock speed.

48. Consists of a microprocessor
for implementing the logic and
controlling the communications
among the modulus.
Designed, so the desired circuit
can be entered in relay ladder
logic form.
The processor accepts the input
data from various sensing devices,
executes the stored user program
and sends appropriate output
commands to control devices.

49.  2. Memory:
 Memory unit is connected to the CPU, which contains the
 programs of logic, sequencing and I/O operations .
 It also holds data files associated with these programs, Including I/O
stations bits, counter and timer constants, and other variable and
parameter values.
 This memory unit is referred to as the user or application memory because
its contents are entered by the user and also directs the execution of
control program and coordinates I/O operations with the aid of system
memory.

50.  3. Power Supply:
 A power supply of 120v alternating current(ac) is typically
 used to drive the PLC (some units operate on 240 v ac).
 The power supply converts the 120v ac into direct current
(dc) voltages of ±5V.
 These low voltages are used to operate equipment that
many have much higher voltage and power ratings than the
PLC itself.
 The power supply often includes a battery backup that
switches in automatically in the event of an external power
source failure.

51.  Supplies DC power to other modules
 that plug into the rack.
 In large PLC systems, this power
 supply doe not normally supply power
 to the field devices.
 In small and micro PLC systems ,
 the power supply is also used to power field devices.`

52.  4. I/O Modulus:
 The input/output module provides the connections to
the
 industrial equipment or process that is not to be
controlled.
 Inputs to the controller are signals from limit-switches ,
pushbuttons,
 sensors , and other on/off devices.
 Outputs from the controller are on/off signals to
operate
 motors, valves, and other devices required to actuate
the process.
 Many PLCs are capable of accepting continuous signals
from

53.  Forms the interface by which input
 field devices are connected to the
 controller.
 The terms “field” and “real world”
 are used to distinguish actual external
 devices that exist and must be
 physically wired into the system.

54.  Forms the interface by which
 output field devices are connected
 to the controller.
 PLCs employ an optical isolator
 which uses light to electrically
 isolate the internal components
 from the input and output terminals.

55.  5. Programming Device:
 The PLC is programmed by means of a programming device.
 The programming device is usually detachable from the PLC cabinet
 so that it can be shared among different controllers.
 Different PLC manufacturers provide different devices , ranging
 from simple teach pendant type devices , similar to those used in
 robotics, to special PLC programming keyboards and CRT displays.
 PCs and laptops now widely used for programming a much faster
 and a PC / laptop are also smaller now.

56. The software allows users to create , edit
.document, store and
trouble shoot programs .
The personal computer communicates with the
plc processor via
serial or parallel data communications link

57.  Hand – held programming devices are sometimes used to
 program small PLCs.
 They are compact, inexpensive and easily to use, but are
not able
 To display as much logic on screen as a computer monitor.
 Hand-held units are often used on the factory floor for
trouble
 shooting, modifying programs and transferring programs to
multiple
 machines.

58. Languages
 Variables: These are the way in which the language
allows the user to access the I/O internal data.
 On line s. OFF line: On line language is the one that
can be programmed interactively with the PLC.
 OFF line: Requires that the program be generated
separately then downloaded to PLC.
 Flow Control: How easy to take decisions and to
control the program execution.
 Maintainability: How easy the language is to debug,
modify and teach to others.

59.  Functions: Some language provide minimum set of
logic, timing , counting and memory functions.
 Speed: Some language takes a longer time to execute
than others.
 Choosing a slow but powerful language in time critical
applications can make programming much more
difficult than using a fast but simple language.
 EFFECIENCY: This is the measure of how much
memory is required for a language to implement a
given function.

60. Ladder Language
 Electrical ladder diagram
 Relay logic
 Power flow through the ladder diagram
 Program is fast
 Readable program for sequence control

61. Multi Tasking
 Some PLC can execute multiple tasks within a single
processor.
 Two forms of multi tasking: Time driven and Event Driven
 Time Driven: The user writes programs and assigns a
separate I/O for each task.
 The user may be then allowed to configure the processor
to run each task at periodic time intervals.
 Event Driven: In the Event driven multitasking user
defines a particular event that causes each task to run.
 Some multitasking systems allows any task to access any
variable such as an I/O point.

62.  Configuration process typically consists of: Assigning
the I/O points to particular I/O racks.
 Telling the processor how much memory I/O it has.
 Assigning specific memory for tasks.
 Determining the fatal and non fatal faults.
 Assigning many other items interactively on a
program loader.
 Not all PLC can be configured, but the processor can
enhance the efficiency of the PLC if it can be
configured.

63. Processor Software/Executive
Software

65.  Executive software is a program that the PLC
manufacturer will provide internal to the PLC,
which executes the user program.
 Determines what functions are available to the
users program, how the program is solved, I/O
is serviced, what the PLC does during power
up, down and fault conditions.