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List of PLC Components

List of PLC Components

PLCs (programmable logic controllers) are the main component in industrial automation. PLC is a digitally operating component. For storing the instructions, PLC has a program memory. These instructions are for certain functions such as arithmetic, logic, sequencing, etc. PLCs monitor inputs and depending upon their state it would make decisions based on its program to its output to automate machine or process.

Components of PLC

  • Power supply
  • Processor module-CPU, memory
  • Communication interface
  • I/O modules
  • O/P modules
  • Software

Power supply

The power supply is the main required component for the PLC. The PLC works either at 24 VDC or 240 VAC. PLC controllers have a separate module of electrical supply. It will monitor and regulate the supplied voltages and manages the CPU. It will provide, a well-regulated power and protection for other system components. The PLC power supply must be able to handle 10 -15% variation in the line of voltage.


This is the major part of the PLC. It will perform tasks for the functions of the PLC. CPU has a program based on the control instructions to execute a program. This will communicate with other PLCs, Input /Output devices, and programming devices. CPU is also called a PLC brain. So that it can process, controls all the operations within the PLC. The CPU consists of arithmetic and logical unit, program memory, internal timers, and counters.

Arithmetic logic unit

The main function of the ALU is to do Arithmetic and logical operations of the data. Here accumulator is a part of the ALU and this would store processed data and the result of an operation. The command has the operation part and the address part.

Operation part: it will indicate, which logic operation be carried out.

Address part: it defines the operands with which a logic operation is to be executed.


Functions of the CPU

  • Receives inputs from various sensing device and switches
  • Executes the user program
  • Capable of making decisions for controlling the operations of a process
  • It can do arithmetic and data manipulation functions
  • Capable of delivering the output to various load control devices such as relay coils and solenoids.

Input Module

Input devices like pushbuttons, sensors, potentiometers, pressure switches. The input module changes the voltages from high level to low-level voltages. The capability of the input module is to process the analog input and digital input. But in industries, digital inputs are been used. This analog input module would convert analog signals from analog devices. Analog devices such as temperature sensors, pressure sensors.

These analog signals are converted to digital with the help of an ADC. The analog signal voltage range from 0-12V. and these values are converted into integer values.

The input module would receive a signal from the machine or device and converts it into signals which can be used by the controller. In the input part, the signals can be filtered to remove noise. The input and the output modules can be located near the i/o devices by this the wiring connection can be reduced.

Types of I/O circuits

  • Discreet I/P
  • General-purpose outputs
  • Pilot duty outputs
  • Analog I/O
  • DC I/P module
  • AC I/P module

Output Module

The output devices are controller coils, solenoid coils, lamps, etc. output module amplifies the low-level logical signal it is generated by the CPU. It will convert the controller signals to external signals which are used to control the process. It is capable of converting the signals from the CPU into analog or digital values.


Memory is part of the PLC. It can store the data, information, and programs. Here, the process of adding information into memory is called writing. And the process of retrieving information from a memory called reading. Here, there are 2 types of major memories.

RAM(Random access Memory)-Reading and writing

ROM(Read-only Memory)-Only reading

System buses

System bus Is the internal path in the PLC. The path between the digital signals. The system buses consist of four main busses.

  • Data bus
  • Address bus
  • Control bus
  • System bus


PLC has 2 parts the operating system and the user program. The OS of the PLC provides support from the user program. A program scan cycle consists of sequential operations.

Components of PLC in Education

PLC Roles for different components are

  • School leaders
  • School staff developers
  • PLT leaders
  • Teacher leaders
  • PLT members
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