Programmable Logic Controller - PLC Tutorial for Beginners

PLC Tutorial for Beginners

PLC - Definitions:
         A programmable logic controller is a digitally operated electronic system.

  • It is designed for use in an industrial environment.
  • It uses a programmable memory for the internal storage of user oriented instructions for implementing specific functions such as logic sequencing, timing, counting and arithmetic to control etc.

A programmable Logic Controller accepts instructions from certain input devices, communicates to a central processing unit, which sends signals to certain output devices and controls based on already programmed instructions.

  • A PLC is an industrial computer.
  • Control devices like limit switches, push buttons, proximity or photo electric sensors, float switches, pressure switches provide incoming control signals to the unit.
  • Based on the instructions specified in the user program, the PLC interact with the input signals.
  • The user program also directs the PLC on how to control the field devices like motor starters, Pilot lights etc.

PLC Block Diagram:
The general Block diagram of the PLC is given below:
Programmable Logic Controller Block Diagram

A PLC is made up of following sections. Each section has a unique job in its operation.
Six major sections of PLC are:

  1. Sensing inputs or controlling hardware
  2. PLC input hardware
  3. The CPU or controller
  4. Hand held programming device or personal computer
  5. Output PLC hardware
  6. Hardware Output devices

The sensing section:
It is made up of limit switches, photo electric sensors etc. These sensing hardware devices provide input signals.

Input section:
The input section has two major areas.
The first portion is the physical screw terminals where incoming signals from field input devices like a limit switch are attached to PLC.
The second portion is internal conversion electronics.

  • Assume that a product is moved on a conveyor belt.
  • When the conveyor moves the product into position, the sensor will send an input signal to the input screw terminal.
  • The high voltage signal from the field device is converted to a low voltage and then applied to the CPU of PLC.


The CPU is nothing but a microprocessor. It controls or supervises the entire process. As per the user program the CPU updates the status of outputs.

It is the device that the programmer or operator can enter/edit program instructions/data. The programmer may be a personal computer, handheld unit (laptop) or an industrial computer programming terminal.

Output Section:
The result of the process whereby the CPU will read the input signal status and based on the program instructions gives series of outgoing signals from CPU to output section. For example the PLC program can be written to instruct the PLC to control the conveyor motor.

Field Hardware Device:
The PLC will control/stop the conveyor motor when the product is sensed.

Advantages of PLC:

  1. Consistency in manufacturing can be achieved
  2. Complete control of the manufacturing process
  3. Quality and accuracy can be improved
  4. It can work in difficult and hazardous environments
  5. Productivity is increased
  6. It reduce the rework, scrap and cost associated with them.
  7. It can easily control inventory
  8. It can offer greater product variety
  9. With the use of PLC, the products can be quickly changed.

PLC Memory System
The PLC memory is divided into two categories.

  • They are namely system memory and Application memory.
  • In a programmable Logic Controller, a combination of Battery backed Random Access Memory, Read Only Memory, Electrically erasable Programmable ROM are utilised for storing instructions and data.
  • The system memory provides the understanding to know what to do, how to understand and react to the instructions the user will program into its memory.
  • The system memory is transparent to the user.
  • Under normal operating conditions the user can not change it.
  • The ROM type memory is used for system memory.

The application memory is the portion of the memory that stores the user program. In addition to that, It stores internal data needed for program interaction.

  • The application memory is composed of two parts.
  • The first part is the memory area where the user ladder program will be stored.
  • This portion is completely separate section and it is exclusively reserved for user program storage.
  • The second part is the data memory portion.
  • It stores the information such as timer and counter information, input and output status files, bit files, whole numbers stored for data manipulation and recipe values.

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