Topic
Languages
The IEC 61131-3 languages and when each one fits: ladder, Structured Text, Function Block Diagram and Sequential Function Chart.
4 articles
Ladder logic to Structured Text: converters, rules and pitfalls
Most rungs convert mechanically: series contacts become AND, a parallel branch becomes OR in brackets, an OTE becomes an assignment that runs every scan, and OTL or OTU become an IF with no ELSE. Timers, counters and one-shots become function block instances that must also be called every scan. A converter handles the simple rungs; test timers, edges and rung order yourself.
10 min readThe five IEC 61131-3 languages, and when each is right
IEC 61131-3 defines Ladder Diagram, Function Block Diagram, Sequential Function Chart, Structured Text and Instruction List. Ladder dominates discrete machine control because maintenance staff read it; Structured Text suits calculation and data handling; SFC suits sequences with clear steps; FBD suits process and signal flow; Instruction List is deprecated and should not be used for new work.
7 min readStructured Text for people who think in ladder
Map the concepts directly: a rung becomes an assignment guarded by a boolean expression, a series of contacts becomes AND, a parallel branch becomes OR in brackets, and a latch becomes an IF block rather than an assignment. Timers become function block instances that must be declared, which is the one genuinely new idea.
7 min readSequential Function Chart: when your machine has steps
Use SFC when the process has distinct steps with clear transitions between them: batch processes, machine start-up sequences, and any cycle where 'where are we' is a question operators ask. It replaces the step-counter-and-comparison pattern that ladder forces, making the current state explicit and the transitions visible.
6 min read