A Guide to PLC Programming Languages for Controls Engineers

Programmable Logic Controllers (PLC) are the lifeblood of modern industrial automation systems. The international standard known as IEC 61131 — published in December 1993 by the International Electrotechnical Commission (IEC) and currently in its third edition — delineates the basic software architecture and programming languages of the control program within PLC.

As a controls engineer, you’ll be expected to have at least a working knowledge of the individual strengths and use cases of each programming language in industrial automation. Let’s take a look at the essentials of the five standards that make up IEC 61131-3.

Ladder Diagram (LD)

One of the most widely recognized PLC programming languages, Ladder Diagram (LD) is modeled after electrical relay logic. Its graphical interface uses symbols resembling those in electrical schematics, making it highly intuitive for electricians transitioning to automation as well as those looking for troubleshooting and debugging. LD is ideal for straightforward, binary logic tasks like switches and actuators, but it can also handle more complex control logic.

Function Block Diagram (FBD)

Function Block Diagram (FBD) provides a more modular approach. Each block in the control logic represents a function, like triggering an output based on inputs or adding two values. FBD’s block-based structure results in highly reusable code, reducing development time for engineers with a background in systems engineering. If you work with analog values, PID (Proportional—Integral—Derivative) controllers, or other complex processes that require continuous signal processing, then FBD is for you.

Sequential Function Charts (SFC)

The visual nature of this language suits industrial settings like conveyor systems or production lines in which operations must occur in a specific order. Sequential Function Charts (SFC) break down processes into steps and transitions, each representing a distinct phase in the operation. Supporting parallel processing and synchronization, SFC enables engineers to clearly depict a sequence of operations and manage complex automation workflows efficiently.

Instruction List (IL)

Instruction List (IL) is similar to assembly language. This low-level text-based language uses concise commands for logic, arithmetic, and data manipulation to give granular control over a PLC’s operations with compact code. Performance and efficiency are the hallmarks of IL, but it can be difficult to read and debug in more complex programs. Nonetheless, engineers can rely on its precision to give them direct control over every aspect of the PLC’s functioning.

Structured Text (ST)

Finally, there’s a high-level text-based language that resembles modern programming languages like C or Pascal: Structured Text (ST). Use ST for complex algorithms, calculations, and processes that require loops, conditionals, and data manipulation. It exceeds the capabilities of graphical languages like LD or FBD to serve as a solid choice for intricate control systems and tasks requiring large amounts of data processing.

In summary,

• Ladder Diagram is good for simple binary tasks,
• Function Block Diagram excels in modular, reusable control logic,
• Sequential Function Charts can handle complex sequences,
• Instruction List offers fine control for performance-driven applications,
• And Structured Text specializes in sophisticated algorithms and data manipulation.

Understand the strengths of each of these languages so you can optimize your automation projects.