1 year warranty
Selecting the right approach to PLC programming for industrial automation is one of the more consequential decisions a maintenance or controls engineering team makes. The wrong choice affects system performance, troubleshooting time, and the ability to modify programs without introducing faults. PLC programming languages are standardized under IEC 61131-3, which defines five languages, each suited to different control tasks and engineering backgrounds. This blog demystifies those languages and explains how to choose the right one for your processes.
Why Does Choosing the Right PLC Programming Language Matter?
The language you choose directly affects how efficiently a PLC executes operations, how easily the program can be maintained, and how quickly faults can be diagnosed. The right language will ensure your PLC executes operations efficiently and effectively. There are many languages available, each with its own specific advantages and disadvantages.
For example, some require an understanding of electrical diagrams, while others are more flexible and can perform complex operations. Choosing the wrong language can lead to slow performance, errors, and difficulty troubleshooting. Ultimately, taking the time to research and select the appropriate language will save you time and money while ensuring your PLC operates at its fullest potential.
What are the Top 5 PLC Programming Languages to Know?
Now that you know why this decision is so vital, it's time to dive into the specifics of the most common PLC programming languages. This is what you need to know.
1. What Is Ladder Logic and When Should You Use It?
Ladder logic is the most widely used PLC programming language and the most accessible entry point for engineers with an electrical background. Ladder logic programming was the first standardized PLC language, developed in the early 1960s, and it gets its name from the visual resemblance of the rungs on a ladder. It is a graphical programming language that uses simple instructions and logic symbols to represent different processes. The program's layout looks like a ladder, with the left-hand side representing input signals and the right-hand side representing output signals.
One of the primary benefits of ladder logic is its ease of use. The graphical layout makes processes easy to understand and visualize. It's a practical choice for straightforward control tasks, such as starting and stopping a motor or controlling the speed of a conveyor belt. However, ladder logic does have limits in terms of complexity, and debugging can be challenging. It can also be difficult to modify existing programs, leading to downtime.
2. What Is Function Block Diagram (FBD) and What Is It Used For?
A Function Block Diagram (FBD) is a graphical PLC programming language that maps logic flow using interconnected blocks, similar to ladder logic in its visual approach, but more detailed and better suited to parallel processing and complex control applications. The blocks resemble flowcharts that connect in specific ways to represent the program's logic.
FBDs have significant advantages over ladder logic. They allow users to control tasks and perform parallel processing, thereby increasing the efficiency of control processes. FBDs are designed for advanced PID and motion-control applications, such as robotic arms, due to their versatility and detailed representation. However, they can be more difficult for beginner programmers to use.
3. What Is Structured Text (ST) in PLC Programming?
Structured text (ST) is a text-based PLC programming language modeled on high-level languages such as Pascal and C, designed for complex control tasks involving calculations, string handling, and data processing. Structured text PLC programming is more flexible than ladder logic and supports complex math, string, and array handling. This option also follows structured programming concepts, making the code easier to understand and modify.
ST is an excellent choice for control applications that require decision-making or heavy data processing. However, ST can be challenging for new users because it is a text-based language, and syntax errors can cause logic errors, leading to downtime. Ensure your technicians are trained in this language before asking them to write programs with it.
4. What Is Instruction List (IL) and Where Is It Applied?
Instruction List (IL) is a low-level PLC programming language that provides direct access to the processor architecture, making it best applied to situations requiring short reaction times and minimal memory usage. In an IL program, each command corresponds to one machine's code instruction. This language is particularly useful in firmware development and microprogramming.
IL programs are not human-readable; a specialized editor or conversion tool is required to write them. The commands in an IL program use a specific syntax, including the mnemonic instruction, operand, and target address. Despite its technical complexity, IL remains a relevant PLC programming language due to its efficient resource management.
5. What Is Sequential Function Chart (SFC) and How Does It Work?
A Sequential Function Chart (SFC) is a graphical PLC programming language designed for highly sequential control processes, combining elements of flowcharts and ladder logic to represent process behavior over time. SFC is used in automation projects that require specialized control structures and visually represent the logical and temporal behavior of processes.
The SFC has five basic elements: steps, transitions, actions, actions in steps, and actions on transitions. Steps are programmatic behaviors that activate once all previous steps have executed. Transitions indicate movement from one programmatic state to another. Actions are operations that execute in parallel with steps, either triggered by a step or independently when activity conditions are met.
How To Choose the Right PLC Programming Language for Your Application
Each PLC programming language performs best under specific conditions. Knowing which options are available is only part of the decision; understanding how to select the right one is equally important.
-
Consider your application. The type and complexity of your application will greatly influence the language you use. Simple applications such as conveyor belts, packaging equipment, and basic material handling are well-suited to ladder logic or FBD. More complex applications requiring advanced calculations or data handling calls for structured text PLC or SFC. Before selecting a language, consider both the complexity of the application and the team's programming expertise.
-
Mix and match languages. Each language has specific strengths and combining them is a practical approach. For example, building the base control program in ladder logic while using structured text for customized I/O modules or specialized functions.
-
There is no one-size-fits-all solution. Requirements vary by application. Choose a language that meets those requirements and ensures it is supported by the PLC platform in use, has a solid user community, and includes available documentation.
Conclusion
The five IEC 61131-3 PLC programming languages, ladder logic, FBD, structured text, instruction list, and sequential function chart, each serve different applications, skill sets, and performance requirements. Choosing correctly from the start reduces commissioning time, simplifies troubleshooting, and extends the useful life of your automation hardware. If you're sourcing surplus sealed, refurbished, or used PLC hardware compatible with your programming environment, contact us today to check availability and request a quote.
