IEC 61131-3 languages

One standard to program PLCs

IEC 61131-3 is the part of the IEC 61131 standard that defines PLC programming languages. Its first edition dates from 1993, the third from 2013 and the fourth from 2025. Thanks to it, someone who learns Ladder or structured text on one brand can carry almost everything over to another.

The standard defines graphical and textual languages:

The same program in all five languages

This is the classic motor start with seal-in: START starts the motor, the motor keeps itself on and STOP stops it.

MOTOR := (START OR MOTOR) AND STOP;

LD   START
OR   MOTOR
AND  STOP
ST   MOTOR

Ladder (LD)

It was created so that electricians who knew relay panels could program PLCs without learning to program computers. Each rung is read from left to right like a circuit: if “current” reaches the coil, the output turns on.

• Strengths: very visual, ideal for bit logic and interlocks, and for troubleshooting in the plant by watching the program online.
• Weaknesses: awkward for calculations, data handling and processes with many stages.

It is the most used language in the Americas, and the first one this simulator implements.

Function block diagram (FBD)

The program is a network of blocks (AND, OR, timers, PID…) joined by lines that carry signals. It is very common in process control, because control loops are naturally drawn as connected blocks.

Structured text (ST)

A high-level language with IF, CASE, FOR, WHILE, maths expressions and functions:

IF Level < 20.0 THEN
    Pump := TRUE;
ELSIF Level > 80.0 THEN
    Pump := FALSE;
END_IF;

It is the best language for calculations, data handling and algorithms. Its use grows every year, especially in CODESYS-based environments and high-end equipment.

Instruction list (IL)

A sequence of simple instructions on an accumulator: load a value, operate on it and store the result. It was popular on older equipment, especially European.

Sequential function chart (SFC)

It describes a process as a sequence of steps joined by transitions (conditions to move on). Each step runs actions. It is ideal for staged processes, such as a batch mixer: fill, mix, heat, empty.

SFC organises the program; actions and transitions are written in another language (LD, FBD or ST). It derives from the French Grafcet standard, known today as IEC 60848.

Which one to use?

They can be mixed in the same project: for example, the sequence in SFC, interlocks in LD and calculations in ST. Each part of the program (a POU: program, function block or function) is written in the language that suits it best.

Summary

• IEC 61131-3 defines five languages: LD, FBD and SFC (graphical), ST and IL (textual).
• Ladder is the most visual and the most used for bit logic.
• ST is the best for calculations and data; its use is growing.
• SFC organises staged processes.
• IL is deprecated and was removed in the 2025 edition.