Introduction to PLC – Programmable Logic Controller

PLC Feature
PLC Feature

Introduction to PLC :

Introduction to PLC, the branch of instrumentation engineering has been evolving very much since past two-three decades.  In electronics, to control the flow of current, we use switches and thereby we control gadgets. But, switches require human interference to operate. So, to overcome this disadvantage of switches, relays were invented. Relays are the automatic switches which are mainly used in instrumentation engineering applications. But in industries, we require many relays to control ‘n’ number of processes. This problem created a necessity of a control unit which will control different processes without human interference, according to certain environmental conditions by using algorithms (programs) stored into its memory and executed by its micro-processors or micro-controllers. These control units can be called as Programmable Logic Controllers. Programmable Logic Controllers are gaining popularity in automation industries because of certain unique advantages like

  1. Easy programming and trouble shooting.
  2. Flexible and can be reprogrammed to control other systems, etc.
  3. Completely reliable regardless of I/O
  4. Easy communication with Computers
  5. Many such devices can be cascaded and used for large industries
  6. PLC – PLC communication is possible
  7. Can be interfaced with SCADA
  8. Has built-in resources like timers, ADC, DAC and many more


Programmable Logic Controllers are very expensive as compared to micro-processors or micro-controller based systems. Programming software is sold separately, and it is vendor specific software. That is if you purchase a Philips PLC you can program it using Philips Developments Software only. These limitations make PLC less feasible for use by small factories or industries.

Architecture of PLC:

PLC Architecture
Sketch of Architecture

The architecture of PLC is as same as any computer architecture. As shown in the diagram, it mainly consists of a CPU which comprises of its most vital part that is processing devices, two memories viz. program and data, a programming device which is used to store/read program in it, input module which takes inputs from various sensors, output module to which many output devices like motors, LEDs, buzzers etc. are connected and power supply which provides necessary power to the equipment.

Programming of PLCs:

The PLC programming is very interesting. Usually; ladder programming is used for programming. This ladder logic is based upon the relay logic wiring schematic which is user friendly and easy to understand. We will be discussing Ladder Logic Programming in Upcoming Articles, it’s and interesting topic hope you would love it.

Also See:

Electronic Voting Machine using 8051 Micro-Controller


Raw Data – Tanvi Shivgan


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