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What is a PLC? Logic Controller (Programmable)

  • An industrial facility uses programmable logic controllers, often known as PLCs or programmable controllers, to control machinery.

    PLCs can control a wide range of equipment, just like industrial facilities themselves. You name it, and there's definitely a PLC out there operating it. Examples include utility plants; batch control applications; chemical processing; conveyor systems; food processing equipment; auto assembly lines, etc.

    Programmable Logic Controller (PLC)

    • In a conventional industrial control system, each control device is connected to each other directly in accordance with the system's intended use. But with a PLC system, the PLC takes the place of the wire connecting the devices.
    • As a result, all equipment is wired to the PLC rather than directly to each other. The "wiring" link between the devices is then provided by the control program inside the PLC.
    • The computer programme that is stored in the PLC's memory and instructs the PLC on what should be happening in the system is known as the control programme. Softwiring refers to the use of a PLC to provide the wiring connections between system components.


    In a conventional industrial control system, each control device is connected to each other directly in accordance with the system's intended use. But with a PLC system, the PLC takes the place of the wire connecting the devices.

    As a result, all equipment is wired to the PLC rather than directly to each other. The "wiring" link between the devices is then provided by the control program inside the PLC.

    PLC Advantages

    In addition to the previously mentioned programming freedom, PLCs provide further benefits over conventional control systems.

    These benefits consist of:

    • highly trustworthy
    • minimal space needed
    • calculating abilities
    • lower prices
    • resistance to extreme conditions.
    •  expandability

    A PLC basically consists of two elements:

    1.Central processing unit

    1. Input/output system


    The Central Processing Unit

    • A programmable controller's central processing unit (CPU) is the component that retrieves, decodes, stores, and processes information.
    • Furthermore, it runs the control program that is kept in the PLC's memory. The "brains" of a programmable controller are, in essence, the CPU.
    • With the exception of the fact that it employs unique instructions and coding to carry out its tasks, it operates in a manner similar to how the CPU of a typical computer does.

    The CPU has three parts:


    • The processor
    • The memory system
    • The power supply


    The part of the CPU that codes, decodes, and computes data is known as the processor.The portion of the CPU known as the memory system is where the control software and data from the equipment connected to the PLC are stored.The PLC's power supply component supplies it with the voltage and current it needs to function.

    The Input/Output System

    The PLC module to which all field devices are attached is called the input/output (I/O) system.The I/O system can be compared to the limbs and legs of a PLC if the CPU is the PLC's brain.The control commands from the software stored in the PLC's memory are physically carried out by the I/O system.

    The I/O system consists of two main parts:


    • The Rack
    • I/O modules


    The rack, which is attached to the CPU, is an enclosure containing slots.Field devices are wired to I/O modules, which are objects with connection terminals. The interface between field devices and the PLC is formed by the rack and the I/O modules.

    There are two categories into which all field devices linked to a PLC can be placed:

    • inputs 
    • Outputs



    • Devices that provide a signal or data to a PLC are called inputs.


    • Pushbuttons, switches, and measurement devices are common types of inputs. In essence, a PLC is informed by an input device that "Hey, something's going on here." You should investigate this to determine how it impacts the control software.



    • Devices known as outputs wait for a signal or data from the PLC in order to carry out their control operations. A few examples of output devices are lights, horns, motors, and valves.


    • These devices remain stationary and uninvolved until the PLC instructs them to turn on, open up their valve a little more, etc.


    Input and output devices can be divided into two categories:


    • Discrete
    • Analog


    • Inputs and outputs that are discrete devices have just two states: on and off.
    • They do this by sending and receiving basic signals to and from a PLC. These signals are just made up of 1s and 0s. A 1 indicates that the gadget is on, whereas a 0 indicates that it is off.


    • Inputs and outputs of analogue devices can exist in an unlimited number of states. These gadgets can be barely on, almost fully on, almost off, etc. in addition to being on and off.
    • To and from a PLC, these devices transmit and receive complicated signals. They use a variety of signals in their communications, not only 1s and 0s.


    • We briefly addressed the control programme previously. In the memory of the PLC is a software programme called the control programme. It is what gives a programmed controller control.


    • The person who creates the control software is typically the user or the system designer. Instructional elements make up the control programme. In essence, instructions are little computer programmes that cause inputs and outputs to follow your wishes in order to get the desired outcome.


    • There are many different types of instructions that may be used to programme a PLC to perform almost anything (add and subtract data, time and count events, compare information, etc.)

    Various PLCs provide various types of instructions. That contributes to the distinctiveness of each form of PLC. But all PLCs employ two fundamental categories of instructions:

    • Contacts
    • Coils


    • Contacts are instructions that make reference to the input programme circumstances, or to the data provided by the input field devices. Each control programme contact keeps an eye on a particular field equipment.


    • Depending on the type of contact, the contact waits for the input to do a certain action (e.g., turn on, switch off, etc.).


    • The contact then informs the PLC's control programme, "The input device just performed as intended. If any of the output devices are supposed to be affected by this, you should check.


    • Coils are instructions that relate to the control program's outputs, i.e., what each specific output device in the system is expected to perform.


    • Each coil, like a contact, keeps an eye on a certain field device. A coil, on the other hand, monitors the PLC control programme and then instructs the field device what to do, in contrast to a contact, which monitors the field device and then instructs the PLC.

    PLC Memory

    • The intricate memory structure of a PLC enables it to retain data on both the control programme and the state of all the inputs and outputs. It uses a technique known as addressing to keep track of all this data.


    • An address is a label or number that identifies the location of a certain piece of data in a PLC's memory.


    • A device or piece of data's address indicates where information about it is stored in the PLC's memory, just like your home address indicates where you reside in your city.


    • In this manner, a PLC will know where to search to find information about a field device if it needs to.


    • Information regarding the state of specific field devices can be found in some addresses. Data that is the output of control programme calculations is stored at other addresses.


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