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Programmable Logic Controller- Computer Marvels

Updated on May 13, 2012

What are PLC

Programmable Logic Controllers or PLC are a type of minicomputers designed for industrial and automation purposes. They are used to control a variety of equipment such as robotic arms, escalators, elevator systems, power stations and in most of the processes that require precision and accuracy with zero error. So what is difference between a personal computer and a PLC. The answer can be found in the construction of one. PLC are highly durable and resistant to dust, mechanical shock, humidity, dirt, and moisture. This makes it an ideal candidate for industrial automation were it is extensively used.

Evolution-Necessity is the Mother of all Inventions

An industry manufactures goods and materials according to the demands of the user. For this it requires number of manufacturing equipment. These equipment should work in varying intervals for example, in an automobile industry, one machine makes the door of the car, the other brake system and yet another the engine. All these processes work in different time limits, either 10 minutes, half an hour or an hour. So how to set this specified time in the machines. Before the invent of the PLC, this was done with the help of Relays, Cam timers, Drum Sequencers etc.


Relays are electromagnetic switches. They come in two forms, Normally Open or Normally Closed.

Normally Open relays consist of two contacts separated by a metal strip, The metal strip in the initial state, is not connected to the contacts. It is however in close proximity to an electromagnet. When current is passed through the electromagnet, the magnet attracts the metal strip and connects it to the contact. Thus current starts flowing through the circuit, where the relay is connected. When the electromagnet is deenergized, the metal strip goes to its initial position, ending the connection.

In Normally Closed the only difference is in the initial state, the metal strip is connected to the contacts and on activation the strip is disconnected from the contact.

Cam Timers

A Cam timer consist of a cam shaft attached to an electric motor. When the motor rotates, the cam shaft also rotates. Attached to the Cam shaft are protrusions which in turn operates the electric switches. The protrusions has connection to cam followers which in turn are connected to the electric switches

However the electric connections consisting of Relays and Cam Timers were very complex, bulky and hard to maintain. Moreover commissioning of these devices were time consuming affecting overall productivity.

To remove the demerits, General Motors issued a tender to replace the complex relay circuits. The tender was awarded to Bedford Associates from Bedford, Massachusetts. They came up with a novel idea, which formed the base for the modern PLC. The first PLC was called MODICON which meant Modular Digital Controller. Modicon is now taken over by the French company, Schneider Electrical.

Components of a PLC

There are different players in the PLC market such as Allen Bradley, Siemens, Modicon, ABB, GE Fanuc, Mitsubishi etc. All PLC consist of these basic components

1.CPU- This is the brain of the PLC which performs arithmetic and logical operations, monitors input and outputs, communicates with memory, scanning of program etc

2. Memory- Memory is used to store data for use in programs or to change the state of input and ouput. There are two types of memory, RAM and ROM. RAM or Random Access Memory can be written, deleted and rewritten. It used to store Program Memory. ROM or Read Only Memory can only be written once and cannot be deleted. It is used to store the core programs of the PLC and cannot be altered by the user.

3. Inputs and Outputs- Inputs and Outputs are used to communicate the PLC with the outside world. Depending upon the type of PLC, the number of inputs and outputs vary. Modular type of PLC can increase the number of inputs and outputs depending upon the application at hand.

The Power Supply used in PLC are 100- 240V AC or 24V DC.

Communication Protocol

To download or upload a program and for communication between the personal computer and PLC, RS232 port is used. Some of the communications now are USB supported.

Programming Basics

All PLC are programmed either using

1.Ladder logic

2.Functional Block Diagram

In Ladder Logic, each program line occupies a rung in the Ladder. The CPU scans the first rung then the second and it goes on in the ascending order.

In Functional Block Diagram, each input and output is denoted by using Block diagram and logic gates. It is more simpler compared to Ladder Logic

A Technical Feat in Itself

PLC provides number of benefits and advantages over other competitors

1. It is economical

2. Provide easier installation and maintenance.

3. Simple for the end user to make correction or probe the system process.

4. Remote control and probing along with low power consumption.


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    • plcgurusdotnet profile image


      8 months ago

      Great article. Have you checked out PLCGurus.NET? It's the Internet's fastest growing community of professional engineers, technicians and technologists. We'd love to have you


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