Working Principle Of [PLC]Programmable Logic Controller And Best Features

Definition of Programmable Logic Controller? 

PLC (Pro­gram­ma­ble Log­ic Con­troller) is a dig­i­tal­ly pow­ered elec­tron­ic device. It records the instruc­tions giv­en in its pro­gram­ma­ble mem­o­ry and con­trols var­i­ous machines or process­es through dig­i­tal or ana­log I / O mod­ules, exe­cut­ing log­ic, sequenc­ing and actions accordingly 

PLC (Programmable Logic Controller) 

PLC is used to trou­bleshoot all of the above faults that occur in a hard wire con­trol circuit.

A PLC per­forms relay switch­ing oper­a­tions such as count­ing, com­par­ing and calculations. 

It also process­es dig­i­tal and ana­log sig­nals. This PLC con­trol sys­tem is very flexible. 

That is, you can eas­i­ly mod­i­fy the con­trol log­ic in less time by chang­ing the pro­gram of the PLC. 

With this, the fault in the cir­cuit can be detect­ed very eas­i­ly in a short time. Thus PLC can be used for many types of process control. 

PLC is very use­ful for con­trol­ling the repet­i­tive process­es that take place in the indus­try, in sequence. 

Cur­rent­ly all types of man­u­fac­tur­ing plants and all the divi­sions there use PLC tech­nol­o­gy. Com­put­ers act as the heart of a PLC. 

But not all com­put­ers can func­tion as PLC. This is because PLC is a spe­cial com­put­er designed to per­form spe­cial functions. 

PLCs are pro­grammed with relay schemat­ic dia­grams. Com­put­er lan­guages ​​like Basic and For­tran are not used in these 

Block Diagram of Programmable Logic Controller

Programmable Logic Controller
Block dia­gram of Pro­gram­ma­ble Log­ic Controller







The input mod­ule con­verts high pow­er sig­nals (220 V AC or DC) received from input devices such as push but­tons, lim­it switch­es, lev­el sen­sors and prox­im­i­ty sen­sors into low pow­er sig­nals that are com­pat­i­ble with dig­i­tal cir­cuits in the CPU. 

Such an input mod­ule con­verts all input sig­nals into bina­ry for­mat with address. It then records all the input data in the input image table in RAM memory. 


The CPU is con­sid­ered to be the brain of the PLC. It exe­cutes and con­trols all the oper­a­tions of the PLC in accor­dance with the giv­en pro­gram instruc­tions. To exe­cute a pro­gram instruc­tion, the proces­sor first obtains input data from the input image table, solves its log­ic instruc­tion­al­ly, and records the solu­tion in the out­put image table. This process is called scan. 

Output module

The Ouput Mod­ule acts as a sig­nal ampli­fi­er. It con­verts the low pow­er sig­nals avail­able from the out­put image table into high pow­er sig­nals suit­able for oper­at­ing out­put devices such as con­tac­tor, sole­noids, relay and indi­ca­tor lamps. 


sys­tem Mem­o­ry (ROM) per­ma­nent­ly stores infor­ma­tion nec­es­sary for the oper­at­ing sys­tem. Appli­ca­tion mem­o­ry con­sists of two parts (1) User Pro­gram Mem­o­ry (2) vari­able Data Mem­o­ry. User pro­gram mem­o­ry is used to record the user’s pro­gram instructions. 

Data Mem­o­ry is used to record the infor­ma­tion file of the timer and counter, the sta­tus file of the input and out­put, and the val­ues ​​of the phys­i­cal vari­ables used in the process being controlled. 


Pro­gram­ming device is used to record the user pro­gram in the PLC’s mem­o­ry. There are two types of pro­gram­ming devices. One of them is a hand­held device with an LCD display. 

These types of devices dis­play only one line of the pro­gram at a time. On the oth­er hand, a key­board 6, Cath­ode Ray Tube (CRT) 20 6001 SUULL sys­tem is used for programming. 

This will dis­play 4 or 6 lines of the pro­gram. Some­times a per­son­al com­put­er (PC) is used to pro­gram in a PLC 

WORKING PRINCIPLE OF PLC (Programmable Logic Controller ) 

When the pow­er sup­ply is sup­plied to the PLC and it is con­vert­ed to RUN mode, the proces­sor starts scan­ning the con­trol pro­gram in the pro­gram memory. 

The con­troller first obtains the sta­tus of all the inputs from the input image table and then exe­cutes the log­ic func­tion accord­ing to the con­trol programs. 

It then reg­is­ters the solu­tion in bina­ry for­mat in the out­put image table. This out­put infor­ma­tion is then sent to the out­put mod­ules. Out­put mod­ules con­vert bina­ry for­mat out­put data to fit out­put devices and run them 


A PLC pro­gram is an orga­ni­za­tion that con­tains one or more instruc­tions for per­form­ing a spe­cif­ic task. These pro­grams are stored in the PLC’s mem­o­ry with the help of a pro­gram­ming device 

There are three types of pro­gram­ming meth­ods for pro­gram­ming a PLC

  1. Lad­der log­ic dia­gram method 
  2. State­ment list­ing method 
  3. Func­tion block dia­gram method

1. Ladder Logic Diagram Method 

Ladder Logic Diagram
Lad­der Log­ic Diagram

It is the most wide­ly used pro­gram­ming lan­guage in PLCs. Its struc­ture is sim­i­lar to that of the line dia­gram used in hard-wired con­trol circuits 

The ver­ti­cal line to the left of the lad­der dia­gram indi­cates pow­er or ener­gized con­duc­tors. As such, the ver­ti­cal line to the right indi­cates the neu­tral or return path 

The lad­der dia­gram is first read from left to right, from top to bot­tom. The hor­i­zon­tal lines in the lad­der dia­gram are called rung or net­work. In a net­work (Rung), many types of con­trol ele­ments can be used. But only one out­put ele­ment should be used 

Each rung must have an input ele­ment, fol­lowed by an input or con­trol ele­ment, fol­lowed by an out­put ele­ment. A lad­der dia­gram can have mul­ti­ple rungs. Each of its input and out­put ele­ments is giv­en a sep­a­rate num­ber and alpha­bet­i­cal address 

Usu­al­ly, the address of the input ele­ment is pre­ced­ed by the let­ter 1 fol­lowed by the num­bers. Sim­i­lar­ly, in the address of the out­put ele­ment, the let­ter 0 is giv­en first fol­lowed by the numbers 

I: 0/0, I: 01, I: 02 and I: 03 are the inputs in the above mod­el. These can be the switch or push but­ton or the con­tact of the con­trac­tor. O: 0/0 and O: 01 are the out­puts. These can be coils of relay or con­trac­tors The first rung con­tains instruc­tions for address­es such as I: 0/0, I: 01 and O: 0/0. It is con­nect­ed in AND log­ic mode. That is, both I: 0/0 and I: 01 are close but only the out­put of O: 0/0 is energized 

The sec­ond rung is I: 02, I: 03 and O: 01. It is con­nect­ed in OR log­ic mode. So either out­put I: 02 or I: 03 is close but even out­put relay O: 01 is energized 

2. Statement Listing Method 

All actions that need to be per­formed in this man­ner are, in turn, referred to as state­ments. The actions to be tak­en are indi­cat­ed on the left side of the state­ment. These are called OP code. All the infor­ma­tion required for the action is giv­en on the right hand side of the state­ment. These are called operand 

Net­work 1 Oper­a­tion Performed 

LD I:0/0 AND 

A I:0/1

= O:1/0

Net­work 2 

LD I:0/2

O I:0/3 OR

= O:0/1

3. Function Block Diagram Method 

Function block diagram Method
Func­tion block dia­gram Method 

In this mode, each rung is rep­re­sent­ed by a box. Each action is named after its func­tion. The process­es are also referred to as rec­tan­gu­lar shapes. Inputs are indi­cat­ed on the left side of the rec­tan­gle and out­puts on the right side. The func­tion block dia­gram shown in the fig­ure above rep­re­sents the lad­der dia­gram and the same func­tions men­tioned in the state­ment listing


1.The size is small. 


3.Easy to install. 

4.With mod­ern technology

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