PLC Programming: IEC 61131-3 Masterclass

PLCs control the compressors, the motor drives, the utilities, and the package units that keep process plants running. Engineers across many roles write, review, or commission PLC programs — and most learned it on the job with no structured methodology underneath.

This course gives that methodology. It covers PLC hardware and the scan cycle, all five IEC 61131-3 languages, program organisation, and the practical programming of the interlocks, sequences and motor control that real industrial plant needs.

The work is anchored in IEC 61131-3, the standard that defines the five PLC languages, program organisation units, and data types — the common ground beneath every modern platform. The course applies it on Siemens TIA Portal and Allen-Bradley Studio 5000 so the methods transfer to the system on your bench.

It opens with what a PLC is and where it is used, then PLC hardware architecture: CPU, I/O, power, racks, and the scan cycle that governs how every program executes.

The IEC 61131-3 overview sets up the five languages and program organisation units, then ladder logic is taught across two lessons — contacts, coils and rungs first, then timers and counters (TON, TOF, CTU, CTD) and their applications.

Function Block Diagram and Structured Text follow, covering blocks and connections, then ST syntax, loops and conditionals — the languages that handle the logic ladder does badly.

Sequential Function Charts are taught as the tool for sequence and state-machine design — steps, transitions, and the structured approach to batch and start-up sequences.

A dedicated lesson on program organisation and best practice covers POUs, modularity, naming and documentation — the difference between code another engineer can maintain and code that becomes a liability.

The course is built by a practising engineer with 15+ years writing and commissioning PLC logic on oil and gas and energy projects — including the interlocks, permissives and motor control that protect plant and people. The platforms lesson and the section project apply the methodology to a real sequence on Siemens and Allen-Bradley.

If you write, review, commission or maintain PLC programs and you want to work to a methodology rather than by imitation — writing logic that is safe, readable and maintainable — start with the hardware and scan cycle and work through to the sequence design project.