PLC Programming — Industrial Control Logic

A PLC (Programmable Logic Controller) is a ruggedised industrial computer used to control manufacturing machinery and processes. PLC programming defines the control logic in one of five IEC 61131-3 languages: Ladder Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), Instruction List (IL), or Sequential Function Chart (SFC).

Why companies use it

·PLCs are the standard control hardware for industrial machinery, assembly lines, and process plants worldwide
·Hardened for industrial environments (vibration, temperature, electrical noise) where standard computers would fail
·IEC 61131-3 standardisation means logic written for one PLC platform can be adapted to another — partially
·Safety PLCs (IEC 62061, ISO 13849) are required for machinery safety functions — e-stop, light curtains, safety gates

What hiring managers look for

·Automation and controls engineers who cannot program a PLC cannot commission or maintain most industrial machinery
·Knowledge of major PLC platforms (Siemens TIA Portal / STEP 7, Rockwell Studio 5000, Beckhoff TwinCAT) is directly practical
·Safety PLC programming experience (SICK, Pilz, Siemens S7-300F) is a significant differentiator for safety-critical roles
·The ability to read and trace ladder logic to diagnose a machine fault is a core maintenance and commissioning skill

Typical interview questions

What are the five IEC 61131-3 programming languages and when would you use each?

How does Ladder Diagram logic differ from conventional programming languages and what are its advantages for industrial control?

What is a PLC scan cycle and how does it affect real-time control performance?

How do you implement a safety function (e-stop) in a PLC and what standards apply?

Describe a PLC programme you have written or maintained. What did it control and what challenges did you encounter?

Common mistakes

·Writing PLC logic without considering the scan cycle time — time-critical operations in a slow scan cycle produce incorrect timing
·Not implementing proper fault handling and alarming — PLC programmes that fail silently are dangerous in industrial environments
·Using global variables excessively instead of structured function blocks — this makes large PLC programmes unmaintainable
·Not version-controlling PLC programmes — a machine modified without version control leaves no recovery path if the change introduces a fault
·Confusing a standard PLC with a Safety PLC — safety functions require SIL/PLe-rated hardware and certified programming tools

Real engineering example

An automation engineer was commissioned to upgrade a packaging line PLC from Siemens S7-300 to S7-1500. The original programme had been written in Ladder Diagram over 12 years with no version control. The migration revealed 240 unused rungs and three latent bugs in the reject conveyor logic that had never triggered because a downstream sensor had always failed before the fault condition could develop. The migration was completed in structured text with full version control, and the three bugs were fixed before commissioning.

Topics covered

PLCIEC 61131-3ladder diagramSiemensRockwellautomationindustrial control

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