02 · LIC-101 · PLC control

PLC programming and control logic in Structured Text

Designing and programming control logic to IEC 61131-3: Structured Text and ladder, PID loops, interlocks and state machines, backed by field work on instruments and loop checks.

Send a request ST Lab

A good PLC program is not judged by whether it runs, but by what it does when something breaks. I write control logic in Structured Text with structure: clear state machines, safety interlocks, timers that stop chattering, and names the next engineer can read. I also built the ST Lab, which runs IEC 61131-3 programs in the browser.

What I do

Control logic in ST and ladder

Start-up sequences, interlocks, pump and motor control, and alarm handling with TON and TOF timers.

PID loops and tuning

Level, pressure, flow and temperature control, with tuning and integral anti-windup.

Field instruments and loops

HART transmitter calibration, loop checks, and wiring 4-20 mA signals to PLC inputs correctly.

Documentation and Siemens TIA export

Readable, documented programs, and ST exported to SCL ready for TIA Portal.

Standards

IEC 61131-3ISA-5.1ISA-88NAMUR NE43

Technologies & skills

Structured TextLadderFBDSiemens S7 · TIA PortalAllen-BradleySchneiderPIDHART

Try it yourself

WS-610

ST Lab

Write Structured Text, run it against a virtual tank, export to Siemens SCL.

Open tool FY-102

4-20 mA & loop calculator

Scale current to engineering units and check the loop voltage budget.

Open tool

Projects from the field

SC-401

Graduation project: telemetry & predictive maintenance

An engineering framework linking controllers over RS485 to automate fault detection and reduce downtime.

2026-09-20

The TON timer in Structured Text, in practice

How the on-delay timer works, why it is the standard cure for chattering alarms, and an example you can run in the ST Lab.

Terms

Programmable logic controller (PLC)Structured Text (ST)PID controller 4-20 mA signal HART protocol

FAQ

What is Structured Text?

A high-level textual language in IEC 61131-3 that resembles Pascal. It suits calculations, complex logic and state machines, is supported by most modern PLCs, and Siemens calls it SCL.

When should I use ST and when ladder?

Ladder is great for simple discrete logic that maintenance technicians follow on screen. ST is better for maths, loops, arrays and state machines. Many projects use both.

What is the PLC scan cycle?

The PLC reads all inputs, executes the program top to bottom, writes the outputs, and repeats every few milliseconds. Understanding it prevents classic bugs such as reading a value from the previous scan.

Can I test a PLC program without hardware?

Yes. The ST Lab in the workshop runs your program against a virtual tank on a 50 ms scan and exports it to SCL.

Have a project in this area?

Write a few lines about the site and what you need; your request reaches me immediately.

Send a request