At the core of industrial automation resides a device known as a programmable logic controller (PLC). It is a control system that automates processes, such as those found in manufacturing plants, or in other places where a series of simple or complex events must occur in a specific order and at particular times. For a long time, PLCs used ladder logic as their programming language; ladder logic was used because it was a simple, intuitive language that workers already knew. But now many more languages are available, and this blog will explore some of the most common ones.
Overview of Common PLC Programming Languages
Ladder Logic (Ladder Diagram - LD)
Ladder Logic is among the oldest and most popular programming languages for PLCs. Its appearance is similar to that of electrical relay logic diagrams, which makes it very straightforward for those with an electrical background to use and understand. It seems intuitive to electricians and technicians, and it enjoys widespread support across a diversity of PLC platforms. Ladder Logic is great for control systems that are uncomplicated and for implementing relay logic. When you think of its Common use scenario, it is perfect for controlling simple machines, making interlocks, doing sequential control, and implementing safety circuits.
The Function Block Diagram (FBD)
employs graphical blocks to represent functions and their interconnections. Each block signifies a distinct function (e.g., AND, OR, timer, counter), and the blocks are linked together to conceptualize the overall logic. Complex systems can utilize it with the facility of having code blocks that can be reused, making it easy to modify. The FBD's basic arrangement of functions allows it to visualize complex logic flows, which is a major reason for its popularity in process control. Typical use cases include process control, PID loops, mathematical operations of sufficient complexity to require assembly into "stories," and modular control systems of various kinds.
High-level, text-based programming languages like Structured Text (ST)
share an affinity with traditional programming languages such as Pascal or C. They are most powerful and most flexible when performing complex calculations, manipulating data, and handling control logic tasks. ST is well-suited to these scenarios. Its structure is straightforward, allowing for easy use in implementing complex algorithms. Like other high-level languages, it has many built-in functions that aid in performing mathematical operations and that facilitate working with large quantities of data. It is particularly useful for programmers who wish to maintain or develop systems that require nontrivial control logic tasks and (especially) systems that conform to the IEC 61131-3 standard.
The Instruction List (IL)
is a low-level, text-based language that bears a strong resemblance to assembly language. Each line of this concise and neat language represents a single operation or instruction. They are executed in the order they appear in the list unless directed otherwise by a control flow instruction. Operations take no more than a few machine cycles, and memory accesses are well-timed and concise. This makes the Instruction List an ideal language for "performance-critical" applications, especially those that are simple, repetitive, and time-memory constrained.
The Sequential Function Chart (SFC)
is a graphical programming language that models processes as a series of steps and transitions. Each step represents a certain state or action, while the transitions determine the conditions for moving from one step to another. SFC excels at modeling processes that are purely sequential—start-stop sequences, multi-step operations, and much of batch processing. The language's visual aspect makes the normal form of an SFC easy to follow, even for people who aren't particularly versed in programming. Use cases for the SFC include state machines, process control with distinct phases, and sequential machine operations.
Selecting the appropriate PLC programming language relies on multiple factors—namely, the straightforwardness and interlock nature of the control system, the complexity of the task given to the programmer (or the skill set of the programmer), and the specific requirements of the particular system. For tasks that revolve around simple, ordinary logic operations and interlocks, most people would recommend using Ladder Logic because it is easy for technicians to understand and has a wide-based appeal. SFC (Sequential Function Chart) and FBD (Function Block Diagram) are used more frequently in Europe than in the United States. However, both languages are extremely powerful and should be appealing to American engineers who are not confined to "ladder thinking."
Conclusion
Should your application call for intricate algorithms, manipulation of voluminous data, or interfacing with other systems, you need the power and flexibility of Structured Text. If maximum performance and memory efficiency are demanded by your application, you will use Instruction List. Knowing what each of these PLC programming languages is good for—and what it is not good for—is vital to the development of efficient, reliable control systems.
Regardless of whether you are handling a straightforward task involving the control of a machine or a multifaceted situation requiring process automation, choosing the appropriate language can significantly improve the performance and maintainability of your system and its scalability in the future. It is important to match the language to the clear needs of your application and the decidedly diverse skill set of your programming team. Get this right, and the payoff can be substantial.