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    What is a PLC?

    A programmable logic controller (PLC) is a digital arithmetic operation electronic system specially designed for application in an industrial environment. It uses a programmable memory in which the instructions for performing operations such as logical operations, sequence control, timing, counting and arithmetic operations are stored, and various types of mechanical equipment or production processes are controlled through digital or analog input and output. ATO online store offers you Delta's DVP series programmable logic controllers with high speed, stability and reliability, applicable for various industrial automation machines. 

    Programmable logic controller appearance


    A PLC controller is a kind of computer dedicated to industrial control, and its hardware structure is basically the same as the microcomputer. The programmable controller is composed of a CPU, memory, input/output interface, power supply, digital-analog conversion and other functional units.

    • Power supply: The power supply is used to convert the alternating current into the direct current required inside the PLC. Most of the programmable logic controllers are powered by switching regulated power supply.

    • Central processing unit (CPU): The central processing unit (CPU) is the control center of PLC and the core component of PLC, and its performance determines the performance of PLC. The central processor is composed of a controller, an arithmetic device and a register. These circuits are concentrated on a chip and are connected to the input/output interface circuit of the memory through the address bus and the control bus. The role of the central processing unit is to process and run user programs, perform logical and mathematical operations, and control the entire system to make it coordinated.

    • Memory: Memory is a semiconductor circuit with a memory function, its role is to store system programs, user programs, logic variables and some other information. Among them, the system program is the program that controls the PLC to achieve various functions, written by the PLC manufacturer, and solidified into the read-only memory (ROM), and the user cannot access.

    • Input unit: The input unit is the input interface connected between the PLC and the controlled device, and is the bridge for the signal to enter the PLC controller. Its role is to receive the signal from the master component and the detection component. Input types include DC input, AC input, and AC-DC input.

    • Output unit: The output unit is also the connection between the PLC and the controlled device, its role is to transmit the output signal of the PLC to the controlled device, that is, the weak current signal sent by the central processor is converted into a level signal, and the actuator of the controlled device is driven. The output types are relay output, transistor output, and gate output.

    Working Principle

    At their core, PLCs interface with input devices, such as sensors and switches, to collect data about the state of a process. They then use this data to control output devices, like motors and valves, to manage and regulate the industrial processes they oversee.

    PLCs operate on a continuous scan cycle, where they read input data, execute programmed control logic (often using Ladder Logic), update internal memory values, and send commands to output devices. This cyclic operation ensures real-time control and monitoring, crucial for industrial applications. PLCs utilize Boolean logic principles, employing logical operators like AND, OR, and NOT to create conditional control logic, enabling decision-making based on input conditions. Additionally, PLCs offer timer and counter functions for managing time-dependent actions and tracking events within the control system. They also feature fault detection and handling mechanisms to identify and respond to system anomalies, ensuring safe and efficient operation. Many PLC systems include a Human-Machine Interface (HMI) that provides operators with a graphical interface for monitoring processes, adjusting control logic, and receiving status updates and alerts. These principles collectively underpin the design, programming, and operation of PLCs, making them indispensable tools in industrial automation and control.

    Functional Characteristics

    1. High reliability: Because PLC mainly uses a single-chip microcomputer, the integration is high, coupled with the corresponding protection circuit and self-diagnosis function, improving the reliability of the system.

    2. Easy programming: PLC programming uses a relay control ladder diagram and command statements, the number is much less than the microcomputer instructions, in addition to the middle and high-end PLC, the general small PLC is only about 16. Because the ladder diagram is simple and vivid, it is easy to master, easy to use and can be programmed without computer expertise.

    3. Flexible configuration: Because the PLC adopts the building block structure, the user only needs to combine simply, and can flexibly change the function and scale of the control system, so it can be applied to any control system.

    4. Complete input/output function modules: One of the biggest advantages of PLC is that for different field signals (such as DC or AC, switching quantity, digital quantity or analog quantity, voltage or current, etc.), there are corresponding templates that can be directly connected with industrial field devices (such as buttons, switches, sensing current transmitters, motor starters or control valves, etc.), and connected to the CPU motherboard through the bus.

    5. Easy installation: Compared with computer systems, the installation of PLC does not require a dedicated computer room, nor does it require strict shielding measures. When used, only the detection device and the I/O interface terminals of the actuator and PLC are correctly connected to work normally.


    Programmable logic controllers are essential components in industrial automation, enabling precise control and monitoring of machinery and processes. PLCs come in various types, each suited to specific applications and requirements. The types of PLCs can be broadly categorized into three main types: integral, modular, and stacked.

    1. Integrated PLC: Integrated PLC is the power supply CPU, input/output interface and other components are centralized in a chassis, with compact structure, small size, and low price characteristics.

    2. Modular PLC: Modular PLC is the PLC components that are made into a number of separate modules, such as CPU module, input/output module, power module (some included in the CPU module) and various functional modules.

    3. Stackable PLC: The characteristics of integrated PLC and modular PLC are combined to form the so-called stacked PLC. Stacked PLC CPU, power supply, input/output interface, etc., are also independent modules, but they are connected by cable, and each module can be stacked layer by layer. This system can not only be flexibly configured but also can be made small.

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