Header Ads

What is PLC? How does it work? Industrial Automation, Ladder Diagram, PLC Circuit and PLC Design.

What is PLC? How does it Work? All information about PLC.

What is PLC?

A PLC (programmable logic controller) is a digital computerized system which is used for controlling lines of assembly, robots, industrial or factory based machinery equipment’s, and so on. In the example, SCADA and DCS are used in factory and power plants through computers to control the whole system from the operation room. There are various kinds of PLC, such as -
a) Siemens
b) Omron
c) Delta
d) Mitsubishi
and so on.
Whenever and wherever a specific PLC program is used, there must be that specific kind of cable otherwise program will not be uploaded or having a problem with uploading.



At the very beginning, it was developed for controlling industrial machinery equipment’s, such as motors, machines which were used by the replace of human beings. It is an example of a “real –time system” where output results must be produced in the response of input programs within a limited time along with designs.

Invention and Early Development

In 1968, the Automatic Transmission of General Motors, requested for an electronic replacement for harf-wired relay systems by the engineer Edward R. Clark. This proposal came from Bedford Associates Of Bedford, Massachusetts. The very first PLC designed 084 (eighty-fourth projects)  because it was the 84th projects of Bedford. Bedford started a new company which was developing, manufacturing, selling, and servicing this new product: Modicon, which stood for a modular digital controller.

Modicon History

Modicon created the first PLC in the United States in 1968. In 1969, Modicon introduced the devise whuch gave success to the industrial projects and completing projects as the program created and designed. PLCs intelligence to the machines and automated processes used widely in industry, infrastructure and buildings.

In !973 the “084th” upgraded into “184th”, then their offices were opened in many places, such as Cleveland, Los Angeles and Rochester.
In 1975, Modicon upgraded into “284th”, which used a microprocessor and distributed control system (DCS) and finally “384th” is the first PLC with algorithms and other digital controls.
In 2000, Modicon created Web Automation which can be control through their IP address and Google Chrome(In Mobile), Seimens LOGO PLC 1200 and 1500, which has so many outputs and web access control system.

How does it work

The PLC is connected to the computer(input) and output is connected to the motors (3-phase) so that it can be controlled the speed of Motor, and the program which covers connection from “Star” to “Delta” fro “star-Delta” connection for “Stepping up” (230V AC to 417V AC).

plc architecture
PLC Diagram

Input and output – the CPU of the PLC receives data which programmed in the computer (Industrial PCs, Microcontrollers, and so on) through an online test. These input/output modules are those which provides information to the CPU and trigger specific results (according to the programs sent down to the PLCs).
Communications – According to the input and output systems, PLC can be controlled DCS (Distributed Control System) and SCADA (Supervisory Control And Data Acquisition). DCS system widely used in Garments, industry, factory, etc. and SCADA specifically used in “Transmission and Distribution” Power Plants Engineering computers through the control room.
HMI – HMI is Human Interface Machine which can control many outputs, likewise 128,256 and 1024 switches. In the display, on/off, cooling down, temperature control and so on can be controlled easily. This is an operator, which display shows and indicates everything in a way simple way,  with a text-readout and
keypad, or a touch-screen panel for controlling everything.


There are multiple ways to program a PLC, such as –
a) Ladder Diagram
b) Function Block Diagram
c) Statement List (C/C++ programming)
d) Logic Functions
Ladder Diagram mainly used by the Electrical and Mechanical Engineers and Statement List used by the Computer Science Engineers.

Ladder Diagram -

These diagrams can also be line diagram from “Power Rail” or “Bus-Bar”. Circuit controlling is very much difficult, that is why ladder diagram is used instead of circuit diagram. It is much easier because just make or break contact, relay, virtual memory and others set up in position then draw line each other’s from “Power Rail”.

ladder diagram
Ladder Diagram

A simple relay controlling ladder-logic
A simple relay controlling ladder-logic

Simple PLC Ladder Diagram for “latching, on/off valve or motor”.
Simple PLC Ladder Diagram for “latching, on/off valve or motor”.

Function Block Diagram -

After using “Ladder Diagram”, “Function Block Diagram” is used widely. It is simple to use in a graphical way to program any functions altogether in the PLC program.
As one of the official PLC programming languages, where the diagram is IEC 61131-3,  this diagram is sort of basic level diagram for beginners.

Function Block Diagram
Function Block Diagram

Statement List -

This kind of programming code for “Siemens”, it is easy to use in “Siemens” programming. It is more likely “C/C++” programming,  and it is defined in the IEC 61131-3 specification. Sadly to say, it is more harder than “FBD or Ladder” diagram, it  needs much practice to understand and memorize the patterns.

Logic Function -

It is more likely “Digital Logic Design”, uses AND, NAND, OR, XOR, NOR, XNOR. When a condition needed to make happen, then this kind of “Logic Functions” will be certainly helpful.
The main advantage is that PLC is much easier than using Relay, these programs can be download, upload, save and copy/paste into another computer for the same purpose.


After understanding properly the program, purpose and operation  of a PLC, it can be considered to spend time on programming, testing and debugging PLC programs. PLC systems, hard-wire tools, PLC boards are not cheap, it is heavily costing. If the program is wrong or misunderstanding, then it will happen to catch fire and damage all the equipment’s and it will turn the industry down, though it can save time and easy to control all the machinery equipment. So, there must be safety about all the equipment’s, the wire must be connected correctly

PLC and other components compare

PLCs are fully adapted for the range of automation tasks. There are typically industrial processes in manufacturing where the total cost is high to maintain the quality for better work and their long-lasting life. PLCs contain input and output devices compatible with the industrial program for devices and control, the design of the little electrical is required for that. That design is meant for the centre of problems on expressing the desired sequence of operations. PLCs applications are actually higher than others for customizing them correctly. Though it is a high cost but comparatively lower than building specific equipment for specific functions or programs. For simply fixed automation tasks, different techniques can be used,  like a cheap consumer dishwasher can be controlled by an electro-mechanical cam timer, that costing would be very little only a few dollars.
A micro-controller based design will be appropriate instead of using and producing hundreds of thousands of units to be produced for a specific purpose, and indeed it will bring the cost level down.
Very complex processing controls, like used in the chemical industry, may require algorithms and beyond performance capability for producing, sometimes even more powerful than high-performance  PLCs, or very high-speed or precision controls may require customized solutions, like aircraft flight controls.
Programmable controllers are widely used in motion in wood or plastic or metallic industrial designs in CNC (computerized numerical control) machines. In CNC machines, G-code and M-code are used widely.



Main differences between G-code and M-codes are -

Usage of G-code :

  • Rapid Movements
  • A feed is controlled in a straight line or arc
  • Dwell
  • Pain selection
Usage of M-code :

  • Spindle ON/OFF
  • Coolant ON/OFF
  • Open workstation door
  • Custom functions for good user creations. 
Discrete (Digital) and Analog signals of a PLC

Digital signals behave like switches, like binary ones, in example 1 or 0, like logic. On the other hand, analog signals are like 24V DC I/O, AC temperature, and so on. It actually works like a volume controlling, from “-32,768” to “32,767”. Pressure, temperature, flow and weight are often represented by analog signals.

PLCs are like the forefront of manufacturing automation. When an engineer works in any industry factory or any other workshops, s/he encounters PLCs, machines and computers for controlling and coding for the output. In reality, every single electrical students must know the very  basic of the PLCs, basic programs, how to run them, how to download-upload-test through the IP address and PLC Board-Computer connections through IP Address.

Written by
Wasimul Bari,
BSc in Electrical Engineering.

Thanks for reading this article. Hope it will be very helpful for your PLC research life. Share this on your social platform.

No comments

Powered by Blogger.