PLC: Programmable Logic Controller – Basics, Working, Programming & Future Trends

Table of Contents

• What Is a Programmable Logic Controller (PLC)?
• How Does a PLC Work?
• Main Types of PLCs
• Key Benefits of Using PLCs
• PLC Programming Languages (IEC 61131-3)
• Role of PLCs in SCADA & HMI Systems
• PLC Integration with Ignition Software
• PLCs and the Industrial Internet of Things (IIoT)
• The Future of PLCs

What Is a Programmable Logic Controller (PLC)?

A Programmable Logic Controller (PLC) is an industrial digital computer used to control and automate machines, equipment, and industrial processes. PLCs continuously monitor input and output signals to ensure smooth and reliable operation of field devices like ON-OFF Valve, Control Valve, VFD, Heaters etc..

PLCs are available in different sizes — from compact units for small machines to modular rack-mounted systems for large industries. They can be customized with additional I/O modules, communication interfaces, and display units to meet specific process requirements.

How Does a PLC Work?

The working principle of a PLC involves three key steps: Input Scanning, Program Execution, and Output Control. A PLC reads signals from input devices (ON and OFF feedback, trip feedback, run feedback, Emergency feedback, selection feedback auto manual feedback thermal trip feedback etc..) processes them according to the program logic, and sends appropriate output commands to actuators or machines.( On the Valve if digital CMD then directly open and analog CMD then % open as per compatible instrument/ Valve. Heater as per start CMD, VFD as per start CMD, Dol-Stator as per  start CMD) 

• Input Scanning: Reads data from sensors, switches, and transmitters.
• Program Execution: Executes logic to determine system outputs.( as per logic design) 
• Output Control: Sends control signals to devices such as motors, relays, and coil.

PLCs operate in a continuous scan cycle, updating input/output status multiple times per second to achieve real-time control and monitoring.

Types of PLC Inputs and Outputs

• Digital I/O: For ON/OFF devices like switches, solenoid valves, and relays.
• Analog I/O: For variable signals like pressure, flow, or temperature sensors.

Main Types of PLCs

PLCs are generally categorized into Fixed PLCs and Modular PLCs.

1. Fixed PLCs

• Compact design with a fixed number of inputs and outputs.
• Cost-effective and easy to install for small applications.
• Limited expansion capability and smaller memory.

2. Modular PLCs

• Scalable design with interchangeable I/O and communication modules.
• Ideal for large or complex automation systems.
• Easy maintenance, better fault isolation, and upgrade options.

Comparison Between Modular PLCs and Fixed/Compact PLCs

Feature	Modular PLCs	Fixed/Compact PLCs
Scalability	Highly scalable — add I/O, communication, or specialty modules as systems expand	Limited — fixed I/O; often requires replacing the whole unit for expansion
Flexibility	Choose only the modules needed (I/O mix, communication protocols, motion, safety)	One-size-fits-all design, less customizable
Upgrade Path	Replace or upgrade a single module instead of the entire PLC	Typically replace the entire controller for upgrades
Initial Cost	Higher upfront investment	Lower upfront cost
Complexity	More complex configuration, wiring, and setup	Easier to install and program for smaller applications
Maintenance	Faulty modules swapped individually, minimizing downtime	Entire unit may need replacement if failure occurs
Best Use Case	Medium-to-large plants, phased expansions, long-term flexibility	Small machines, standalone systems, low-budget installs

Key Benefits of Using PLCs

Before PLCs, industries relied on complex relay-based control systems. PLCs revolutionized automation with advanced flexibility and efficiency.

• Flexible Programming: Easily reprogram for new processes without rewiring.
• High Reliability: Withstands vibration, dust, and extreme temperatures.
• Real-Time Monitoring: Provides instant data and performance feedback.
• Low Maintenance: Reduces downtime and repair costs.
• Compact Design: Saves space and supports modular expansion.

PLC Programming Languages (IEC 61131-3 Standard)

PLCs are programmed using standardized languages defined in the IEC 61131-3 standard. The five main PLC programming languages are:

1. Ladder Logic (LD): Graphical language resembling electrical circuit diagrams.
2. Function Block Diagram (FBD): Block-based logic representation.
3. Sequential Function Chart (SFC): Used for step-by-step control processes.
4. Structured Text (ST): High-level text-based programming language.
5. Instruction List (IL): Low-level, assembly-like code (less commonly used today).

A PLC’s Role in SCADA & HMI Systems

In modern automation, PLCs act as the hardware backbone of SCADA (Supervisory Control and Data Acquisition) and HMI (Human Machine Interface) systems. They connect field devices with SCADA/HMI software for real-time monitoring and control.

Applications include conveyor systems, robotic cells, temperature control, and process automation etc. 

How to Use PLCs With the IIoT (Industrial Internet of Things)

The Industrial Internet of Things (IIoT) enhances PLC communication by connecting edge devices and cloud systems. While traditional PLCs use the poll-response method, modern IIoT setups use MQTT protocol with Sparkplug B specification for efficient, real-time data sharing.

Edge gateways such as Ignition Edge IIoT help convert legacy PLC data into IIoT-compatible formats, enabling seamless cloud integration and analytics.

The Future of PLCs

The future of PLCs is evolving with technologies like AI, cloud computing, and edge analytics. Programmable Automation Controllers (PACs) are bridging the gap between PLCs and PCs, offering advanced processing capabilities.

Despite new innovations, PLCs will remain essential in industrial automation due to their simplicity, cost-effectiveness, and robustness. With tools like Ignition and IIoT connectivity, PLCs continue to play a vital role in Industry 4.0 and smart manufacturing.