The document discusses programmable logic controllers (PLCs), including their architecture, types, modules, configuration, scan cycle, capabilities, and selection criteria. It describes the main components of a PLC, including input and output modules, a central processing unit, memory, power supply, and programming devices. The three main types of PLC architectures are fixed, modular, and distributed PLCs, which vary in size, flexibility, customization, and suitable applications.

1. Chapter 2
Programmable Logic
Controllers
Prepared by
Harshal Vaidya
Assistant Professor
Dept. of Electrical Engineering

2. Contents
• Architecture of PLC
• Types of PLC
• PLC Modules
• PLC Configuration
• Scan Cycle
• Capabilities of PLC
• Selection criteria for PLC
• PLC Communication with PC and Software
• PLC Wiring
• Installation of PLC and its modules

3. Introduction
• A PLC is a specially designed digital controller that is microprocessor
based and can be easily programmed to perform complex control tasks.
• -built for industrial applications to provide control and automation to
machinery and processes with high accuracy and reliability.
• uses a programmable memory to store instructions and to implement
functions such as logic, sequencing, timing, counting and arithmetic to
control machines and processes.
• Programmable Logic Controller is more often abbreviated to PLC.

4. Brief History
• Father of the PLC :Richard E Morley. Commonly known Dick Morley,
• An American engineer expert in the field of computer design, artificial
intelligence and automation receiving numerous industry awards through
out his career.
• Born: Clinton, Massachusetts on December 1, 1932 & died on October
17, 2017, in New Hampshire.
• Dick Morley is credited with the invention of the PLC in 1968.
• Lead the team that spearheaded the development of the first PLC while
he was at Bedford and Associates.
• The invention of the PLC was fueled for the need to automate the
General Motors production facility.
• The first PLC : Modicon PLC.
• Shortened version of modular digital controller.
• Modicon was the company that created the Modicon PLC in 1968.

5. Characteristic Functions of PLC
• Field programmable by user allowing the user to modify the program as
per requirements.
• Contains programmed functions such as logic, timing, counting and
memory functions.
• Scans memory and I/Os in predetermined manner which allows the user
to determine the response of the m/c or process precisely.
• Provides error checking diagnostics where a PLC carries internal tests
of memory, I/O s/m, and processors through in-built program execution
ensuring the operation is in harmony with the programme.

6. Characteristic Functions of PLC
• Provide monitoring capabilities: through lamps or external display.
• Capable of withstanding temperature, humidity, vibration and noise.
• Can be designed for various control tasks, although dedicated for a parti
cular task.

7. Advantages
1. Compact and robust.
2. Extremely reliable operating system.
3. Fast processor execution time.
4. Virtually maintenance free.
5. Easily expandable due to its modular design.
6. Lower power consumption compared to relay systems.
7. Built in communication for remote I/O, instrumentation, other PLCs and
SCADA.
8. Can handle a large number of digital inputs and outputs.
9. Able to process analogue input signals and PID loops.
10. Multiple programming languages available.

8. Advantages
11. Large programming instruction set.
12. Easy to use programming interface via PC.
13. Control logic modifications easily done via software, no hard wire
modifications required.
14. Installation costs greatly reduced compared to relay systems.
15. Excellent documentation facilities.
16. Increased ability for fault finding and diagnostics..

9. Disadvantages
1. For simple applications where relay logic might suffice, using a PLC
might blow out costs due to the need to hire a programmer.
2. Math functions in a PLC are quite advanced, but when it comes to doing
large amounts of complex math computations then an industrial PC
might be better suited.
3. Certain robotic and positioning applications may require extremely high
speed execution which may not be able to be achieved form a PLC.
4. Can be expensive for automating an application with fixed parameters
for mass production as compared to a microcontroller.

10. PLC Architecture

12. PLC Architecture
• The main parts of PLC are divided into
1. Input Module
2. Output Module
3. Central Processing Unit (CPU)
4. Memory
5. Power Supply
6. Programming Device
• follows closed architecture (proprietary) system of hardware
• software components cannot (or is difficult to) connect to other
manufacturers components and software.

13. PLC Architecture
1. Input Module
• Serves as link between field input devices and PLC’S CPU
• Each modules has terminal block for connecting input from field devices
• Takes input from field devices (220V AC), convert it to a signal level (5V
DC), electrically isolate it and send the signal to CPU
• For isolation, opto-coupler is used.
• Example of field input devices: Sensors, Limit Switches, Selector
Switches, etc.

15. PLC Architecture
2. Output Module
• Serves as link between field output devices and PLC’S CPU
• Each modules has terminal block for connecting output to each field
devices
• Takes CPU’s control signal (5V DC), electrically isolate it and energize
or de-energize the modules switching device to turn ON or turn OFF
output field devices (220V AC).
• Example of Field Output Devices: Motors, Starters, Solenoid Valve, etc

16. PLC Architecture
3. Central Processing Unit
• Brain of PLC system
• It is microprocessor which controls and supervise the entire process
• Solves the users programs and updates the status of the outputs
• Executes the various logic and sequencing functions.
• During program execution, it reads the inputs, processes the signals
and according to control program, energizes or de-energize the outputs
and hence updates the outputs.
• Process of reading the inputs, executing the control application program
and updating output is called SCAN.

17. PLC Architecture
4. Memory
• The PLC memory is made up of program memory, data memory and
firmware.
• The PLC uses the memory to store the program for processing by the
CPU and to store data for input and output processing and execution.
• The size of PLC memory will vary depending on the processing power of
the CPU.
• Higher powered PLC models equipped with faster CPUs and will have
more On board memory.
• Data Memory is where inputs and outputs are declared and allocated to
memory locations.
• The data memory stores the status of the inputs and outputs and is
continually updated by the application program.
• Program Memory where application program (such as ladder diagram) is
stored and processed.

18. PLC Architecture
4. Memory
Memory
ROM RAM EPROM EEPROM

19. PLC Architecture
4. Memory
• Read Only Memory (ROM): Non-volatile – programmed only
once – Unsuitable and least popular
• Random Access Memory (RAM): Volatile – store user program –
battery back-up needed
• Erasable Programmable Read Only Memory (EPROM): Holds data
permanently – no battery back-up needed – UV rays used to
erase data
• Electrically Erasable Programmable Read Only Memory(EEPROM)
: EEPROM=RAM+EPROM – data erases using electrical pulses up
to an extent

20. PLC Architecture
5. Power Supply
• Converts AC line voltages to DC voltages, to power electronic circuits in
Programmable Controller systems.
• The main purpose of a PLC power supply is to convert the available power
supply voltage into a voltage that is usable by the PLC processor (CPU) and
other PLC modules.
• The most common PLC power supply input voltages are 120VAC, 240VAC, AC
voltage range (like 85-265VAC) and 24VDC. While the most common PLC
power supply outputs are 24V DC and 5V DC.

21. PLC Architecture
6. Programming Devices
• PLC is programmed with the help of programming device.
• Through programming device, programmer or operator can enter or edit
program instructions or data.
• Basic elements: keyboard, visual display, microprocessor and communi
cation cable.
• Normally connected only when entering or editing of program is required
otherwise it is disconnected from the system.
• Functions: 1) Create and transfer the user program to PLCs memory.
2) Debug the user program and control system start up
3) Perform installation diagnostics
• Most common programming Devices are :
1) Handheld programming device
2) Personal Computer (PC)/Laptops

22. PLC Architecture
1) Handheld programming device
• Inexpensive and portable units, hence used to
program small PLCs.
• Resembles portable calculators, but with larger
display and different keyboards.
• Keyboard contains alphanumeric keys,
programming instruction keys and special
function keys.
• Also used for testing, changing and monitoring
the program.

23. PLC Architecture
2) Personal Computer (PC)/Laptops
• Most commonly used programming
device nowadays
• It can be used for programming using
PLC Software available for particular
PLC.
• Software allows to create, edit,
document, store, transfer and
troubleshoot the program.
• As this device is having big display,
more logic can be displayed as
compared with handheld
programming device.

24. Types of PLC
• PLC has advanced tremendously since its inception of PLC back in the
1960s.
• The 3 distinct types of PLC architecture : fixed, modular and distributed
• Fixed PLCs used for smaller size, less complex application.
• Modular PLCs used for medium sized, more complex applications.
• Distributed PLCs for large sized, extensive application spread across
multiple location.

25. Types of PLC
1) Fixed PLC
• A fixed PLC employs single unit architecture
• All the hardware components like power supply, CPU, memory, input,
output and communication interfaces embedded into a single unit.
• Most common names; fixed, integrated, nano, micro, compact, small,
mini, basic, unitary, standard and brick.

27. Types of PLC
1) Fixed PLC
Advantages:
• specifically design to cater for smaller, low end automation projects.
• Small in size so they do not take up very much space in an enclosure.
• Quick and easy to mount.
• Low in cost so they are an economical solution for basic applications.
Disadvantages:
• CPU processing power is low and memory is small so complex tasks
can be difficult to realize.
• Inflexible because the number of input, output and communication
interfaces fixed.
• Only suitable for basic applications with small number of inputs and
outputs.

28. Types of PLC
2) Modular PLC
• separate module for each
of its hardware components
• Each PLC module
interconnected using a
common mounting system.
• The mounting system has a
certain amount of modules to
accommodate.
• modular PLC can be
configured for specific
application ..

30. Types of PLC
Advantages:
• larger memory
• higher performance processors
• larger number of input and outputs
• increased communication options
• fully customizable and are easily expanded
• Each hardware component is separate housed in a module which can
be replaced if it is faulty.
2) Modular PLC
Disadvantages:
• Large in size so take up more space in an enclosure than a fixed PLC.
• The mounting system is more complex than a fixed PLC.
• Higher in cost than a fixed PLC so may not be cost effective for smaller
applications.

31. Types of PLC
3) Distributed PLC
• a high end PLC system with modular architecture and the capability to
interconnect hardware components across different locations via high
speed communication links.
• Each location in the distributed PLC system contains multiple hardware
modules that are housed in a mounting system and are usually called a
node, rack or drop...

33. Types of PLC
Advantages:
• Plant wide control network with multiple processors and remote I/O
drops.
• High performance processor.
• Large program and data memory.
• Able to handle large volumes of I/O.
• Can handle large amounts of complex process control tasks.
• Ease of maintenance.
• Save time and money on installation costs.
3) Distributed PLC
Disadvantages:
• Large in size with bigger installation footprint.
• The mounting system is more complex than a fixed PLC.
• Higher in cost than other types of PLC so they may not be cost effectiv
e for smaller less complex applications.
• Higher level programming skills may be required.

34. PLC TYPES FIXED PLC MODULAR PLC DISTRIBUTED PLC
CPU Performance Low Medium to High High
Program & Data Memory
Size
Small Medium to Large Large
Power Supply Embedded Module Module
Input Interface Embedded Modules Modules
Output Interface Embedded Modules Modules
Communication Interface Embedded Modules Modules

35. Mounting System Single Unit
Rack, Back plane, Rail or Cha
ssis.
Rack, Back plane, Rail or Cha
ssis.
Physical Size Small Medium to Large Medium to Large
Flexibility No Yes Yes
Customizable No Yes Yes
Applications
Basic applications with small
number of inputs and outpu
ts.
Medium to high end applicat
ions with large number of in
puts and outputs.
High end applications and pl
ant wide control with a very
large number of inputs and
outputs.