PLC (programmable logic control)

What is PLC (programmable logic control) – Working, Structure & Types


Efficient control systems are vital in enhancing productivity and streamlining processes in the automation industry. One such control system that has gained immense popularity is PLC (programmable logic control).

Whether manufacturing, power generation, or process control, PLCs have become the backbone of modern industries.

This blog aims to comprehensively understand PLCs, delving into their working principles, structure, and various types. You can have an idea about the core components of a PLC system, the programming languages used, and the benefits it offers in industrial automation. 

Additionally, we will touch upon the limitations of PLCs and discuss future trends in this rapidly evolving field. So, let’s explore the world of PLCs and unravel the mysteries behind their working, structure, and types. 

Working of PLC

A Programmable Logic Controller (PLC) is a specialized computer-based control system that monitors and controls various industrial processes. It consists of several vital components that work together to execute tasks efficiently.

Input Modules

These modules receive signals from sensors, switches, and other devices in the field. They convert these signals into digital data that the PLC can process. Common input types include digital inputs (e.g., on/off signals) and analog inputs (e.g., temperature or pressure readings).

Output Modules

These modules are responsible for sending signals to actuators, motors, and other devices in the field. They convert digital data from the PLC (programmable logic control) into physical output signals. Output modules can control devices such as solenoid valves, motors, relays, and indicators.

Central Processing Unit (CPU)

The CPU is the brain of the PLC. It processes data, executes control algorithms, and communicates with other components. The CPU performs calculations, monitors inputs, manages the user-programmed logic, and generates outputs accordingly.


PLCs have two primary types of memory: program memory and data memory. The program memory stores the user-programmed instructions and logic, while data memory holds temporary data for calculations and storage during program execution.

Input and Output Modules

Input and output modules serve as the interface between the external world and the PLC. They allow the PLC to receive sensor signals and send control signals to actuators. These modules connect to the field devices through discrete or analog channels.

Discrete Input Module

These modules receive digital signals, typically representing on/off or high/low states. They interface with limit switches, push buttons, and proximity sensors.

Discrete Output Modules

These modules send digital signals to control devices like relays, solenoids, and motor starters. They convert the digital outputs from the PLC into physical actions in the field.

Analog Input Modules

Analog input modules convert continuous physical signals (e.g., temperature, pressure, or flow) into digital values that the PLC can process. These modules often include analog-to-digital converters (ADCs) to convert analog signals into digital data.

Analog Output Modules

Analog output modules receive digital values from the PLC and convert them into continuous physical signals (e.g., voltage or current). They can control devices like variable frequency drives (VFDs), proportional valves, or motor speed controllers.

Programming Languages Used in PLC

PLCs support various programming languages to develop the control logic:

Ladder Logic (LD)

Ladder Logic is the most common and widely used programming language in PLCs. It uses ladder diagrams, which resemble electrical schematics. Ladder Logic represents control logic using contacts (inputs), coils (outputs), and

Function Block Diagram (FBD)

FBD is a graphical programming language representing control logic using interconnected function blocks. Function blocks are pre-defined software modules that perform specific operations or calculations.

Structured Text (ST)

Structured Text is a high-level programming language that resembles structured programming in other programming languages. It uses a text-based syntax similar to programming languages like C or Pascal. ST allows more complex and flexible programming constructs.

Sequential Function Chart (SFC)

SFC is a graphical programming language used for complex control sequences. It represents control logic as a series of interconnected steps or states, with transitions between them based on specific conditions.

Understanding the working principles, components, and programming languages of a PLC (programmable logic control) provides a solid foundation for designing effective control systems in industrial applications. With the ability to receive and process inputs, execute programmed logic, and generate outputs, PLCs are vital in improving efficiency, accuracy, and safety in various industries.

Structure of PLC

A Programmable Logic Controller (PLC) has a specific physical structure designed to accommodate its components and facilitate installation in industrial environments. 

While the exact appearance may vary depending on the manufacturer and model, the following are common features found in the physical structure of a typical PLC:


PLCs are housed in sturdy enclosures that protect the internal components from environmental factors such as dust, moisture, and temperature fluctuations. The enclosure is typically made of durable materials like metal or high-grade plastic.


PLC enclosures often feature mounting options, such as DIN rail mounting, which allows for easy installation in control panels or other equipment.

Front Panel

The front panel of a PLC enclosure usually contains various ports, indicators, and buttons for convenient access and monitoring. These may include programming ports, communication ports (such as Ethernet or serial ports), status LEDs, and a display for system information or diagnostics.

I/O Expansion Modules

PLCs can support additional input and output modules beyond their built-in capacity. These expansion modules connect to the main PLC unit, allowing for increased flexibility in handling more field devices.

Specialty Modules

Some PLCs provide specialty modules to cater to specific requirements or applications. These may include modules for high-speed counting, analog I/O, motion control, or safety functions.

Networking Capabilities

PLCs can be integrated into a networked system to exchange data with other devices or systems. This allows for centralized monitoring, control, and data sharing. PLCs may support networking protocols like Ethernet/IP, Profinet, or Modbus TCP.

Networking capabilities also enable remote access and control, facilitating maintenance and troubleshooting tasks from a central location. The structure of a PLC, with its robust enclosure, various modules, and expansion options, ensures reliability, flexibility, and scalability in industrial control systems. 

Types of Programmable Logic Control (PLC)

PLCs are of various types based on size, processing capabilities, and complexity. Here are some commonly encountered types:

Compact PLCs

Compact PLCs are small-sized units designed for applications with limited space or where fewer I/O points are required. They are cost-effective and offer basic control functionalities. Compact PLCs are suitable for small-scale automation projects or where space is a constraint.

Modular PLCs

Modular PLCs consist of separate modules that can be interconnected to customize the system according to specific application requirements. They offer scalability and flexibility, allowing for easily expanding I/O points and functionality. Modular PLCs are widely used in medium to large-scale automation projects.

Rack-mounted PLCs

These types of PLCs are larger units typically mounted on a standard 19-inch rack. Rack-mounted PLCs deliver high-performance processing capabilities and extensive I/O capacity. Rack-mounted PLCs are suitable for complex automation applications that require a large number of I/O points and advanced control features.

Programmable Automation Controllers (PACs)

PACs are advanced PLCs that combine the capabilities of traditional PLCs with those of industrial computers. You can have powerful processing, advanced networking, and extensive programming options. PACs are commonly used in demanding automation applications that require complex control algorithms, data processing, and integration with higher-level systems.

Bottom Line

As technology advances, PLC (programmable logic control) is poised to evolve further, offering more advanced features, increased processing power, and improved connectivity. 

By understanding the working, structures, and types of PLCs from this blog, you can use them more beneficially. However, the continued development and adoption of PLCs from ICS will contribute to industrial automation’s ongoing growth and innovation, enabling industries to achieve greater efficiency, safety, and productivity.

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