What Is a Programmable Logic Controller (PLC)?


Human beings are prone to making mistakes when carrying out reiterative tasks. This further causes the companies various kinds of losses like monetary losses, human resources losses, etc. These limitations gave rise to the innovation of PLC programming, increasing the efficiency of businesses by automating industrial processes.

This is also why the global programmable logic controller market is speculated to be valued at $15.5 billion by 2026. However, in case you do not understand the meaning of PLCs, read on till the end.


What is a PLC?

A Programmable Logic Controller is commonly known as a PLC. It is a specialized digital computer designed to control and automate industrial processes. Its primary function is to receive information from connected sensors or input devices, process the data, and trigger outputs based on pre-programmed parameters.

PLC programming is typically done on a computer and then downloaded to the controller. This includes the usage of languages like Ladder Logic or “C” due to its visual representation resembling circuit diagrams.

PLC programming follows IEC 61131-3 standard. The IEC 61131-3 standard sees itself as a guideline for PLC programming, not a rigid set of rules. PLC programming as per IEC 61131-3 could be implemented in textual programming languagessuch as Ladder Diagram [LD], Instruction List [IL], Structured Text [ST] and Functional Block Diagram [FBD]. Instruction List [IL] is programming language closer to machine code whereas structured Text [ST] is a higher-level language, LD is most suitable for Boolean logic operations.

Additionally, the description language Sequential Functional Chart [SFC] could be used to describe the structure and flow of a PLC program by displaying its sequence and parallel execution. The various subdivisions or tasks can be programmed using any of the IEC 61131-3 programming languages.

Key Features of PLC

PLCs distinguish themselves from other industrial control solutions through several key features, including:

1. I/O (Input/Output)

The CPU of a PLC stores and processes program data. Whereas input and output modules connect the PLC to the machinery. These modules further facilitate the flow of information between the CPU and the physical components of the system, allowing for various configurations to suit different applications.

Various I/O modules are installed on a CPU panel back rack mounting, depending on an application solution different I/O modules are selected by control design engineers, some of the basis I/O modules are digital input modules which connects to field sensors and field instruments status conditions to CPU programs, digital out modules are used where the CPU program needs to control or turn ON/OFF any controls like pumps, valves, start stop some process operation. There are I/O modules which are analog input and output types which provides continues position, temperature or sensor values to the CPU program in real time, these analog inputs are either 4-20mA or 0-10 VDC signal being continuously fed to these analog modules.

2. Communications

PLCs often need to interact with external systems. Thus, they offer various ports and communication protocols for this purpose. This connectivity ensures seamless integration with supervisory control and data acquisition (SCADA) systems or other devices.

In the PLC world of communication, most of data exchange happens using industrial protocols like in case of Allen Bradley PLC using Ethernet/IP, Siemens PLC using ProfiNet/IP or ProfiBUS protocols and others using MODBUS RTU / TCP mode of communication.

Communication protocol used in PLC controlled system are different the standard Ethernet TCP/IP communication protocol. And at times extra demand on controls engineers to handle data exchange between various PLC manufacturers or for that matter between control systems.

A common method of communication or data exchange between PLC control systems is called Produce and consume method, where in each PLC system shared its own PLC tags or data place holders to be shared with intended target PLC system.

3. HMI (Human Machine Interface)

HMIs can range from simple text-readout displays to sophisticated touchscreen panels, providing an intuitive interface for operators. PLCs incorporate HMIs for real-time interaction. This allows users to monitor and input information.

HMI allows users to perform changes in process variables per existing system operation, each process requires tunning per process conditions, and these are implemented by changing data variables with the PLC program via the HMI user screens. HMI provides an good interface to monitor trend within a process system.

Advanced Features of PLCs


In the era of the Industrial Internet of Things (IIoT) and Industry 4.0, PLCs have evolved to meet new challenges. Today, its advanced features include the following:

1. Web Browser Communication

PLCs have seamlessly integrated web browser communication, establishing a dynamic channel for data exchange. This innovative feature enhances accessibility and control by allowing users to interact with the PLC through familiar web interfaces.

2. Database Connectivity through SQL Integration

The convergence of PLCs with databases through SQL integration signifies a paradigm shift in data management within industrial automation. This feature facilitates seamless communication between the PLC and databases, enabling efficient storage, retrieval, and data manipulation. Moreover, this integration not only streamlines information handling but also provides a foundation for advanced analytics and reporting

3. Cloud Integration

PLCs also boast the capability of cloud integration to ensure better connectivity. This transformative feature empowers PLCs to establish connections with cloud platforms, facilitating remote monitoring and control. Therefore, this integration further contributes to heightened efficiency, predictive maintenance, and the realization of Industry 4.0 principles.

Types of PLCs

Numerous types of PLCs are used to monitor different industrial processes. Some of its types are:

1. Modular PLCs

Modular Programmable Logic Controllers (PLCs) stand out for their scalability and flexibility, presenting a tailored solution for industrial processes. These systems allow users to customize control systems based on specific operational requirements, providing adaptability in a dynamic industrial landscape.

2. Compact PLCs

Compact PLCs are designed to cater to smaller-scale applications without compromising functionality. Therefore, these systems are space-efficient and cost-effective in nature. This makes them a pragmatic choice for scenarios where spatial constraints and budget considerations are paramount.

3. Rack-Mount PLCs

Rack-Mount PLCs must be the preferred choice for large-scale industrial operations, as their configuration aligns with the demands of expansive and complex automation setups. These controllers facilitate the management of numerous inputs and outputs, offering extensive connectivity options.

4. Programmable Automation Controllers (PACs)

Programmable Automation Controllers (PACs) represent an intersection of enhanced computing capabilities, blurring the traditional boundaries between PLCs and industrial PCs. Therefore, these systems are better suited for handling complex, data-intensive automation tasks. Moreover, the versatility of PACs makes them an ideal choice for applications demanding a higher degree of computational power and adaptability.

Get a Free Consultation