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The Working Principle Of [PLC]Programmable Logic Controller

  • Definition of PLC? 

    A PLC (Programmable Logic Controller) is a digitally powered electrical device. It employs digital or analogue I/O modules to control various machines or processes, carrying out logic, sequencing, and actions as necessary. The instructions are stored in its programmable memory.

    PLC (Programmable Logic Controller)


    • PLC is used to explore all of the aforementioned problems that occur in a hard wire control circuit.


    • A PLC handles relay switching activities like counting, comparing, and calculating.


    • It also manages digital and analogue signals. This PLC control system can be used in a variety of ways.


    • In other words, you can rapidly and simply change the control logic by modifying the PLC's programme.


    • This makes it quick and simple to find the circuit issue. Therefore, PLC can be used for a range of process control techniques.
    • PLC is very useful for regulating recurring industrial operations consecutively.


    • Currently, PLC technology is utilized in all types of manufacturing facilities and departments. A computer is the beating heart of a PLC.


    • However, PLC functionality is not available on all machines. This is the case due to


    • PLCs are special computers designed to do particular tasks.


    • PLCs are programmed using relay schematic diagrams. These avoid using programming languages like Basic and Fortran.


    Block Diagram of a Programmable Logic Controller

    Block diagram of the Programmable Logic Controller


    2. CPU
    4. MEMORY


    Examples of input devices include pushbuttons, limit switches, level sensors, and proximity sensors. The input module converts the high power (220 V AC or DC) signals coming from these devices into low power signals that are compatible with the CPU's digital circuitry.


    All input signals are converted by such an input module into accessible binary representations. The input data is subsequently stored in the input picture table in RAM memory.



    The CPU is regarded to be the "brain" of the PLC. It manages and carries out each PLC function in accordance with the supplied programme instructions. Before executing a programme instruction, the processor first receives input data from the input image table, solves its logic instructionally, and saves the result in the output image table. Scanning is the term for this action.

    3.Output module

    The output module acts as a signal amplifier. It converts the low power signals from the output image table into high power signals that, among other output devices, can power relays, solenoids, contactors, and indicator lamps.


    The information that the operating system needs to function is permanently stored in system memory (ROM). The two parts of application memory are user programme memory and variable data memory. User programme memory is where the user's programme instructions are kept.


    The information file for the timer and counter, the status file for the input and output, the values of the physical variables used in the process being regulated, and all of these are kept in data memory.



    Using a programming tool, the user programme is put into the PLC's memory. The two types of programming tools are as follows. A handheld device with an LCD screen is one of them.


    These devices only display one line of the programme at a time. On the other hand, programming is carried out using a CRT 20 6001 SUULL system and a keyboard 6.


    This will display the first four or six lines of the programme. On occasion, a computer is used to programme a PLC (PC).

    WORKING PRINCIPLE OF PLC (Programmable Logic Controller ) 

    As soon as the PLC obtains power and is set into RUN mode, the processor starts scanning the control programme in the programme memory.


    The controller first identifies the status of each input from the input image table before executing the logic function in line with the control programmes.


    The response is then recorded in binary format in the output image table. After that, the output modules get this output data. Output modules convert output data into binary representation to fit output devices.


    A collection of one or more instructions for carrying out a certain action makes up a PLC programme. These programmes are stored in the PLC's memory using a programming tool.

    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 

    • It is the most popular programming language used by PLCs. It has a similar architecture to the line diagram used in hard-wired control circuits.


    • The vertical line to the left of the ladder diagram indicates power or energised conductors. The vertical line to the right indicates the neutral or return course as a result.


    • First, read the ladder diagram from top to bottom and from left to right. The horizontal lines in a ladder diagram are referred to as rungs or networks. In a network, a wide range of control elements can be used (Rung). However, you ought to use just one output element.


    • Each rung must contain an input element, an input or control element, and an output element. A ladder diagram could have numerous rungs. Its input and output components each have an alphabetical address and a special number.


    • Before the address of the input element, a letter and a series of numbers are customarily placed after each other. In a manner similar to this, the address of the output element starts with the letter 0 and is followed by the numbers.


    • I: 0/0, I: 0/1, I: 0/2, and I: 0/3 are the inputs used in the model above. These could be a switch, push button, or the contractor's contact details. O: 0/1 and O: 0/0 are the outputs. These could be relay coils or contractors. The first rung contains instructions for locations like I: 0/0, I: 01, and O: 0/0. It has logic AND wired into it. In other words, only O: 0/0's output gets energised even though I: 0/0 and I: 0/1 are close to one another.


    • The second rung is made up of I: 02, I: 03, and O: 1. It connects with OR logic. Because of this, even though outputs I: 02 or I: 03 are nearby, output relay O: 01 is powered.

    2. Statement Listing Method

    All actions that must be performed in this manner are described using statements. The required steps are mentioned on the left side of the statement. They are known as OPcodes. The right half of the statement contains all the information needed for the action. They are referred to as operands.


    The Network 1 Operation was finished.


    I : 0 LD AND


    A I:0/1


    = O:1/0


    Network A2


    LD I:0/2


    O I:0/3 OR


    = O:0/1

    3. Function Block Diagram Method 

    In this mode, a box represents each rung. The name of each activity is determined by its objective. The methods are also referred to as rectangle shapes. Both the left and right sides of the rectangle list inputs and outputs, respectively. The function block diagram in the image above represents the ladder diagram and the identical functions stated in the statement listing.

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