IEC 61131-3 PLC languages: which one to use

IEC 61131-3 defines five programming languages for PLCs, and nearly every shop uses at least two of them in the same project. The languages overlap deliberately — the same interlock can be written in any of them — so the choice is about readability, maintainability, and who has to troubleshoot the code at 3 a.m.

The five, briefly

• Ladder Diagram (LD) — relay-logic rungs. The universal language of electricians and maintenance. Best for discrete interlocks, permissives, and anything the night shift must read. Weak at math, string handling, and complex sequencing.
• Function Block Diagram (FBD) — wired blocks for signal flow. Natural for process control (PID loops, scaling, filtering) and reusable library blocks. Gets unwieldy for large sequential logic.
• Structured Text (ST) — Pascal-like text. Best for algorithms, data manipulation, recipes, state machines, and anything over a few dozen lines of logic. The default for engineers comfortable with text; opaque to pure electrical troubleshooters unless documented well.
• Sequential Function Chart (SFC) — steps and transitions for sequences. Excellent for batch phases, machine cycles, and startup/shutdown sequences — the process reads top to bottom. Each step's actions are usually written in LD, FBD, or ST.
• Instruction List (IL) — low-level accumulator language, deprecated in the third edition of the standard. You will meet it only in legacy programs; do not write new code in it.

How shops actually mix them

A healthy convention: LD for safety-adjacent interlocks and motor control, SFC for the machine sequence, FBD for analog processing and device blocks, ST for recipes, data handling, and communication. Put the language choice in the project standard so the next engineer finds LD where they expect LD.

Two portability notes. First, 61131-3 standardizes the languages, not the libraries or the IDE — moving a program between vendors is a rewrite, not a recompile. Second, PLCopen's motion control function blocks are the closest thing to portable motion code across vendors; if you do servo work on multiple platforms, learn that library once and reuse the pattern everywhere.

References

• PLCopen: motion, safety, and IEC 61131-3 libraries
• The Automation Blog — PLC tutorials and training
• ACC Automation: Master PLC & Industrial Automation