What Is a Programmable Logic Controller (PLC)?

In today’s world, we rely on automated technology to carry out many of our most important industrial activities, from producing power to the weaving fabric. It allows us to produce and innovate at previously unimaginable levels. But without a crucial development known as the programmable logic controller, our level of automation today would not be possible (PLC).

Programmable Logic Controller – 

A programmable logic controller is a small computer that has inputs for data and outputs for sending operating instructions. A PLC’s primary duty is to regulate a system’s operations using the internal logic programmed into it.

Advantages – 

  • Flexible in nature: Depending on the situation, a single PLC model can be applied to a variety of tasks.
  • Simple to install and troubleshoot: Installation time for hard-wired relay-based systems is longer than for PLC-based control panels.
  • Large contacts are available: PLC programming tools have several internal contacts that can be used for any change induced in various applications.
  • Cost-effective: PLC is more affordable than a controller- or relay-based systems because of their advanced technology and widespread production.
  • Simulation feature: Simulation features are included by default in PLC programming tools.
  • Simple programming techniques: PLC is provided with simple programming methods to program the PLC like Ladder or Boolean type of programming.
  • Ease of maintenance: Relay-based or microcontroller-based control systems are more difficult to maintain than PLCs, which have lower maintenance costs.
  • Documentation: The programmer can program & print efficiently the programs of PLC for future use.


  • It has fixed circuit operation & offers only closed-loop architecture also too much work is required in connecting wires
  • PLCs are only partially capable of operating in high-temperature and vibrating environments.
  • Need extra security as some PLCs may cause an accident as they automatically turn on when power is restored
  • Some applications that carry out a single fuction are inefficient while using PLC.
  • PLC is designed by semiconductor, which depends on the thermal characteristics & Hold-up time is undefined, it is usually long
  • Several operational modules must be added to maximize flexibility & execution


If you are trying to comprehend a PLC system, it will be helpful to learn how a PLC controller functions once the program is written & the appropriate I/O modules are selected. The programmable logic controller will process the given data (inputs), execute the instruction from the program, and deliver the results (outputs) based on the provided data and written logic. PLCs function in a repeating loop once the user determines the inputs and outputs.

The four basics steps in the PLC function are:

  1. Input Scan – Detects the state of input devices connected to the PLC
  2. Program Scan – Executes program created by the user
  3. Output Scan – Operate all output devices connected to the PLC
  4. Housekeeping – Communicates with other devices and runs diagnostics

Types of PLCs

  • Unitary PLC
  • Modular PLC

How To Choose Which PLC Type?

Ask these questions yourself when purchasing a programmable logic controller:

  1. What type and how many I/O points does your system require?
  2. What programming language will you be using to dictate the PLC’s processes?
  3. How will your PLC communicate with other networks, systems, or field devices?
  4. How complex are the methods you’re looking to control?
  5. Are you implementing this PLC into a new or existing system?
  6. What type of environment will the PLC reside in?
















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