industrial//industrial automation//PLC//IEC 61131-3

IEC 61131-3 is the standard that defines the programming languages of a PLC, and three of them carry most of the industry.

IEC 61131-3 is the standard that defines the programming languages of a PLC, and three of them carry most of the industry.

Ladder Diagram (LD) looks like a relay circuit: rungs, contacts and coils. A rung such as Sensor_OK in series with Start, driving the coil Motor, reads as the motor runs while the sensor is healthy and start is pressed. Electricians read it without training, and it dominates discrete logic (interlocks, sequences, start and stop conditions).

Function Block Diagram (FBD) represents functions as connected blocks: timers, counters, PID blocks, comparisons. It is very common in process and control, where the program is a signal flow rather than a relay logic.

Structured Text (ST) is a textual language conceptually close to Pascal, for algorithms that would be painful as rungs:

TEXT Copy

IF temperature > 100 THEN pump := TRUE; END_IF;

The standard also defines Instruction List and Sequential Function Chart; a vendor's environment may mix languages in one project, one block per language.

The language expresses the logic; it decides nothing about timing. Whether a rung is evaluated every 2 ms or every 50 ms depends on the task it is assigned to and on the controller, which is why the same program can behave differently on two PLCs.

Whatever the language, the program is executed inside the scan cycle of the PLC.