IEC 61131-3 is one of those standards in industrial automation that is treated as something obvious. Designers reference it, integrators list it in documentation, and customers assume that if a system is “standard-compliant,” it will be correct and easy to maintain. In practice, this is very often not the case. The standard organizes how a PLC program is written, but it does not protect against architectural mistakes. And it is the code architecture that ultimately determines whether a system can be maintained once it is commissioned.
What does IEC 61131-3 actually standardize?
IEC 61131-3 defines a formal programming model for PLC controllers. It specifies what a resource, a task, and a program organization unit are, and it introduces a unified division into Program, Function Block, and Function. It also standardizes programming languages and their semantic behavior. As a result, projects can be readable across different platforms and engineering environments, and engineers do not have to relearn a completely new conceptual model each time.
At the same time, the standard does not impose any logical program structure. It does not say how responsibilities should be divided, where process state should be stored, how error handling should be organized, or how sequential logic should be designed. It allows both well-thought-out solutions and those that work only because no one has modified them yet. From the perspective of the standard, both cases are equally valid.
Program, Function Block, and Function in real-world projects
The theoretical POU split is familiar to every automation engineer, but in practice it is often misused. Very frequently, the Program becomes a single large container for all logic, Function Blocks are used like procedural macros, and process state is scattered across global variables. Such an arrangement usually works at the time of commissioning, but it quickly becomes problematic during modifications and diagnostics.
In projects that hold up well in operation, the Function Block serves as a true representation of a process object. A valve, drive, axis, or sequence segment has its own state, its own transition conditions, and a clearly defined interface. The Program stops being the place where detailed logic is implemented and instead becomes a coordination layer. The Function remains a tool for calculations and simple, stateless operations. This division does not follow directly from the standard, but without it, a PLC system quickly loses clarity.
Programming language versus maintainability
IEC 61131-3 allows several programming languages and permits mixing them within a single project. In practice, problems rarely stem from whether LD, FBD, or ST was used. Issues arise when there are no clear rules defining where and for what purpose a given language should be applied. Sequential logic implemented in ladder diagrams, algorithmic fragments scattered across different POUs, and inconsistent naming conventions all make the code difficult to analyze.
In emergency situations or during system upgrades, such projects require guessing the original author’s intent instead of performing a technical analysis. The standard allows such solutions, but it does not protect against their consequences. This is another example of how compliance with IEC 61131-3 is not equivalent to good engineering practice.
Where the standard helps—and where it creates a false sense of safety
IEC 61131-3 is very useful as a common foundation for industrial automation. It facilitates communication between teams, standardizes basic concepts, and makes it possible to read someone else’s project without constantly guessing intentions. In this respect, it fulfills its role very well.
The problem begins when the standard is treated as a guarantee of project quality. The standard does not assess program architecture, does not enforce limitations on global variables, and does not impose rules for error handling. It allows projects that are formally correct but, in practice, generate high maintenance costs. Without additional design rules, the standard provides only a sense of order that quickly disappears after the first major system modification.
Summary
IEC 61131-3 is a solid foundation for PLC programming, but it is not a recipe for a good automation project. The standard defines the language and the formal model, while system quality is determined by program structure and how the available mechanisms are used. In industrial practice, it is the code architecture—not the choice of language—that determines whether a system can be safely maintained and developed over the years. If the structure is poor, compliance with the standard will not save the project. If the structure is sound, the standard becomes simply a well-chosen tool rather than a liability.
