PLC Software & IEC 61131-3
How industrial controllers are programmed — the standard languages, how to structure a program, state-machine design, and where safety application software fits.
Programming practice is where a good electrical design either comes to life or becomes unmaintainable. These pages cover the vendor-neutral model defined by IEC 61131-3 and the structuring practices that survive commissioning and the next engineer — implementations diverge, so always consult your platform’s documentation.
Languages Overview
The five IEC 61131-3 languages — LD, FBD, ST, SFC, IL — and when each is the right tool.
Ladder Logic
Contacts, coils, seal-ins, one-shots, scan order, and the discipline — I/O mapping, command vs feedback, alarm hysteresis — that keeps ladder maintainable.
Program Structure
POUs, tasks, variable scope, and modular reusable design — structuring for maintainability.
State Machines
Why explicit state machines beat sprawling ad-hoc logic, and three ways to implement them.
Algorithms & Equipment Staging
FIFOs, queues, shift registers, and the staging family — lead-lag, runtime equalization, demand staging, load shedding — for multi-unit equipment.
PackML, ISA-88 & ISA-95
The standard state, procedural, and enterprise-integration models — and how they wrap, not replace, the machine's own sequence.
Vendor Architectures
How Siemens OB/FB/DB, Rockwell tags and AOIs, and Beckhoff TwinCAT each implement the shared IEC 61131-3 model — a translation guide.
Safety Application Patterns
Safety-rated controllers, certified function blocks, and keeping the safety application separate and traceable.
Related
- Engineering toolkit — tag database and IEC 61131-3 identifier rules
- Wiring & installation guides — the hardware the software drives
- Functional safety standards
This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.
Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.