This document discusses fundamentals of programmable logic controllers (PLCs). It defines a PLC as a digital electronic device that uses programmable memory to implement logic functions to control machines and processes. The document describes the basic architecture of a PLC system including the central processing unit, input/output modules, memory, power supply, and communication interfaces. It also discusses digital and analog input/output modules, their representation in PLC programming, rules of ladder logic programming, needs and advantages of PLCs, disadvantages, and applications.

1. NAME: PAVAN.D
SRN: R22EM807
SEM: 4TH
BRANCH: EEE
SUBJECT: FUNDAMENTALS OF PLC

2. FUNDAMENTALS OF PLC
WHAT IS PLC
PLC stands for “Programmable Logic Controller”.
PLC can be defined as digital electronic device that uses a programmable memory to store
instructions and to implement specific functions such as logic, sequencing, counting, timing
and arithmetic operations to control machine, and processes.
ARCHITECTURE OF PLC
The basic architecture of a Programmable Logic Controller (PLC) system consists of several
key components that work together to perform control and automation tasks.

3. 1. CPU (CENTRAL PROCESSING UNIT)
 The CPU is the brain of the PLC system. It processes instructions, performs
calculations, and controls the overall operation of the PLC.
 The CPU executes the control program stored in its memory, which consists of a
series of instructions that determine the behaviour and logic of the system.
2. INPUT AND OUTPUT MODULES(I/O)
 Input modules receive signals from sensors and field devices and convert them into
digital data that the CPU can process.
 Output modules receive digital data from the CPU and generate output signals to
control actuators, motors, and other devices.
 I/0 modules are connected to the PLC's backplane or rack, which provides power
supply and communication channels for data exchange.
3. MEMORY
 PLCs have different types of memory to store the control program, data, and system
parameters.
 Program memory stores the control program written by the programmer, which
determines the behaviour and logic of the PLC system
 Data memory holds variables and data used by the program during runtime.
System memory stores system parameters and configuration settings
4. POWER SUPPLY UNIT (PSU)
 The PSU supplies power to the PLC system, ensuring proper operation of all
components.
 It converts the incoming power supply (AC or DC to the voltage levels required by
the PLC system.
5. COMMUNICATION INTERFACE
 PLC systems often have communication interfaces to interact with external devices,
networks, or supervisory systems
 These interfaces enable data exchange, remote monitoring, and control of the PLC
system.

4. CLASSIFICATION OF PLC INPUT AND OUTPUT MODULES
1. Digital I/O Module
The digital module is also called Discrete Module.
In this module, the I/O signal work on the binary system i.e. only 0 or 1 value. For the digital
input module, only the 1-bit signal is used.
It is useful in the ON or OFF condition.
Based on Input and Output, the digital module is of two types.
 Digital Input Module
 Digital Output Module
The digital I/O signal gives status in the different form like –
1. High/Low, True/False and 1/0 for General Status
2. ON/OFF for Load Condition
3. Activated/Deactivated for Switching Mechanism
4. Close/ Open for the Switching Contact Status

5. Examples: Push switch, Toggle switch, Rocker switch, Selector
switch, Proximity switch, Limit switch and e tc are the example of
the Digital Input Signal.
Examples: Lamp, Coil, Buzzer, Relay, Motor, Fan, Heater,
Actuator, Solenoid Valve and etc are the example of the Digital
Output Signal.
2. Analog I/O Module
The analog module is called a Continous Module.
Usually, the voltage or current is given to the input module in the form of an analog signal.
For the analog input module, 12-bit or 13-bit signal is used.
Generally, analog input signals operate in the range of 4-20 mA, 0-20 mA, 1-5 V, etc.
Again, analog I/O modules are also of two types.
 Analog Input Module
 Analog Output Module
Examples:
Temperature detection switch, Pressure detection
switch, Flow detection switch, Level detection switch,
Limit detection switch, Position detection switch, PH
Level detection switch are the best example of the
Analog Input Signal.
Examples:
Temperature Transmitter, Thermocouples, Pressure Transmitter, Flow
Transmitter, Level Transmitter, etc., are the example of the Analog
Output Signal.
In the PLC system, we can use either digital or analog types of modules as per the project
requirements.

6. Representation of Input-Output Module In PLC Programming
In the PLC programming,
 Input modules are represented by the ‘I‘
 Output modules are presented by the ‘Q‘
For the Ladder Diagram (LD) programming language, normally open and normally closed
contact is used in the form of input. And the coil or lamp is used in the form of output.
The symbolic representation of I and O modules in the LD program.
RULES OF PLC LADDER DIAGRAM:
 Inputs can be used in Series as well as Parallel to form a connection
 Outputs (or coil) can be used only in Parallel
 One Input can be used in multiple times in one program
 One Output cannot be used multiple times in one program, except in Set/Reset and
Latch/ Unlatch functions
 Input Address cannot be used as an Output Address
 Outputs Address can be used as Inputs Address

7. NEED OF PLC:
 To get the automation in the process
 Controlling of equipment’s with just one click.
 Arranging so many controllers as per our requirements
 To make the efficient use of digital electronics and control system
 Real time application of microcontrollers, microprocessor and input-output modules.
ADVANTAGES OF PLC:
 The highlighting advantage of PLC is to increase the reliability and flexibility in
industrial automation.
 It is simple to operate and understand
 It requires very less operation time to perform any task.
 It gives a fast response to the connected system.
 It provides high accuracy as compared to the relay network.
 It needs low maintenance.
 It has remote control capability.
 It gives supervisory control capability.
 It can operate in extreme environments.
 PLC programs are easy to write and understand. There are many programming
languages used in PLC. Out of them, the ladder diagram is very popular and user-
friendly.
DISADVANTAGES OF PLC:
 You can operate one program at a time in a compact PLC.
 In the case of PLC, we can not use software and parts of one PLC manufacturer in
another PLC manufacturer.
 When power restores, PLC automatically starts. It can damage the system. To avoid
the damage, you can program the output to go to the fail-safe mode.

8. APPLICATIONS OF PLC:
 Smart Traffic Control Signal System.
 Smart Elevator Control System.
 Fire Detection and Alarm System.
 Water Tank Level Control System
 Car Washing and Parking System.
 Flashing Light Controlling System.
 Automatic Door Opening/Closing System.
 Steel Industry
 Glass Industry
 Paper industry