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

The standard that defines the programming languages of a PLC. Three of them carry most of the industry.

The standard that defines the programming languages of a PLC. Three of them carry most of the industry.

Ladder Diagram (LD) looks like a relay circuit: rungs, contacts and coils. 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. 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:

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.

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