PLC programming
languages

From Ladder Diagram to Instruction List

PLC programming languages

PLC programming languages are distinguished in graphic and text languages. In one of our articles we already talked about PLCs and how you program a PLC, but we just mentioned which programming languages are the most used. Let us see, then, what are those programming languages and how they differ from one another.

Graphic Languages

As we already saw, programming languages fall under two categories: graphic languages and text languages, regulated by the CEI EN 61131-3 regulation. The three most used graphic languages are Ladder Diagram (LD), Sequential Functional Chart (SFC) and Function Block Diagram (FBD).

Particularly, Ladder Diagram was the most used PLC language until a couple of years ago and it resembles both the graphics and symbols used in electric circuits, so that even someone who never worked with programming languages, but has some electronic training, could intuitively understand how to program a PLC, using input and output symbols.

Sequentional Functional Chart, on the other hand, is the most currently used and it allows to easily integrate machines and robots to an automation plant.

Dulcis in fundo, we have Functional Block Diagram, a program very similar to the circuit diagrams used by electronic technicians.

Example Ladder Diagram

Text languages

The text languages are Structured Text (ST) and Instruction List (IL). They look more like the classic programming languages, to the point that Instruction List has replaced Ladder Diagram as the most used programming tool in system integration companies that specialise in PLC programs. Moreover, Instruction List can be “exported” by Ladder and allows for a more structured programming than Ladder Diagram, this also being one of the reasons why it’s preferred to the graphic language.

Instruction List program