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A Programmable Logic Controller can be defined in many ways.

It can be defined as a program controlled electronic equipment with a reprogrammable memory for storing a set of commands which is used for executing exact tasks like logical, arithmetic, timer, counter, and  furthermore controlling different processes or machines with the help of analog/digital through digital or analog inputs/outputs.

In other words, it can also be defined as :

The Programmable Logic Controller is fundamentally a programmable interfacing that links the i/p components (such as sensor, transducer, push-button, limit-switch etc) and o/p components (such as actuator, driver, solenoid-valve, transducer etc).

These interfacing links (PLCs) contains :

  • I/P component
  • Central processing unit
  • Reprogrammable memory
  • Bus-system
  • Standard electrical supply

 The block diagram for Programmable Logic Controller is  mentioned below.

Basic blocks of PLC system

If we compare Programmable logic controller with conventional devices, then it has the following advantages :

  1. It has more flexibility.
  2. It occupies Less space and is more compact as compared to conventional devices.
  3. Higher reliability (less mechanical parts).
  4. Modular, can be easily connected to computer systems and to hardware.

for further details and elaboration of advantages, visit

However, if we compare Programmable logic controller function with computers, it has following advantages and disadvantages :

Advantage :

  • many inputs and outputs, excellent for controlling and monitoring  many processes.
  • designed for industrial environments, very robust and reliable.
  • reprogrammable.
  • modular.
  • ideally suited to supervisory control.
  • easy to set up and good for FMS environment.

Disadvantage :

  • do not use the full capabilities of the microprocessor.
  • ladder logic programming style is cumbersome and prone to mistakes.
  • only good for yes/no decision.
  • do not handle continuous inputs and outputs. It is not good for “direct control” purposes.
  • inflexible (if compared to microprocessors)

The function of these Programmable Logic Controller blocks is a tricky and lengthy part which will be discussed in my next post.

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