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What is Structured Text, and when to use it instead of ladder

What Structured Text is, where it sits in IEC 61131-3 among the five PLC languages, and when ST beats ladder logic.

Lesson 1 of 349 minIEC 61131-3

A text language inside the controller

Structured Text (ST) is a high-level text language for programmable logic controllers. You write lines such as:

Pump := (Start OR Pump) AND NOT Stop;

instead of drawing a contact and a coil on a rung. It reads much like Pascal: expressions, IF, CASE, FOR and WHILE.

Where it sits in the standard

IEC 61131-3 is the international standard that made PLC programming look the same across vendors. It defines five languages:

Language

Kind

Best for

LD · Ladder Diagram

graphical

contacts, interlocks, maintenance teams

FBD · Function Block Diagram

graphical

signal flow and analogue loops

SFC · Sequential Function Chart

graphical

step sequences

ST · Structured Text

text

maths, loops, state machines, data handling

IL · Instruction List

text

legacy, removed in the third edition

Why ST

  • One line replaces ten rungs. Scaling, filtering or a runtime counter is two lines.
  • It travels between vendors. The same idea runs on Siemens, CODESYS, Rockwell, Beckhoff, Schneider and Omron, with only naming differences.
  • It diffs and reviews. Text goes into Git and you can see what changed between versions; a drawing cannot.
  • It scales with repetition. A FOR loop over 64 motors instead of copying the logic 64 times.

When ladder is still better

When the logic is simple interlocks that a maintenance technician reads standing in front of the panel at night. The common practice in plants is ladder for interlocks and safety, ST for maths and sequences, both in the same project.

How you will learn it here

Every lesson has a complete program you open in the ST lab on this site: an editor, a virtual tank with a pump and a valve, a watch table and an export to Siemens SCL. No PLC and nothing to install.

Try it in the lab

The lab runs in your browser: an ST editor, a virtual tank with a pump and a valve, a watch table, and export to Siemens SCL.

What to try

  1. Press "Download & run", then press the green START button.
  2. Watch the tank level and the Pump value in the watch table.
  3. Press STOP and see the pump stop.

Terms in this lesson

Structured Text (ST)IEC 61131-3Programmable logic controller (PLC)Ladder logic (LD)

People find this lesson by searching

what is structured textstructured text plcIEC 61131-3 languagesST vs ladder logicplc programming languageslearn structured textstructured text tutorial

Course outline

  1. 01What is Structured Text, and when to use it instead of ladder
  2. 02How a PLC runs your code: the scan cycle and the process image
  3. 03Program structure: POUs, PROGRAM, FUNCTION and FUNCTION_BLOCK
  4. 04Your first program: starting a pump with seal-in logic
  5. 05Data types: BOOL, INT, REAL, TIME and the rest
  6. 06Declaring variables: VAR, VAR_INPUT and RETAIN
  7. 07Literals and constants: 16#FF, 2#1010 and T#5s
  8. 08Type conversion and scaling a 4-20 mA signal
  9. 09Assignment and arithmetic in ST: := and operator precedence
  10. 10Boolean logic, comparisons and bit operations
  11. 11Standard functions: ABS, SQRT, MIN, MAX, LIMIT, SEL, MUX
  12. 12IF, ELSIF and ELSE: the rules and when to avoid them
  13. 13The CASE statement and your first state machine
  14. 14The FOR loop and the watchdog
  15. 15WHILE, REPEAT, EXIT, CONTINUE and RETURN
  16. 16Edge detection: R_TRIG, F_TRIG and the "it fired every scan" bug
  17. 17Timers: TON, TOF and TP
  18. 18Counters: CTU, CTD and CTUD
  19. 19SR and RS flip-flops: which one wins
  20. 20Writing your own function
  21. 21Writing a function block
  22. 22Arrays in Structured Text
  23. 23Structs and enumerations
  24. 24References and pointers
  25. 25The state machine pattern: a batch sequence that cannot get stuck
  26. 26Alarm management to ISA-18.2: delay, deadband, latch, acknowledge, first-out
  27. 27Analogue signal processing: counts to engineering units, NE43 and filtering
  28. 28Closed-loop control in ST: hysteresis first, then a practical PID
  29. 29Motors and valves in practice: permissives, feedback and travel time
  30. 30The same ST on every platform: SCL, Studio 5000, CODESYS, TwinCAT
  31. 31Testing and debugging ST: the watch table, forcing and simulation
  32. 32An ST style guide: naming, comments, constants and version control
  33. 33Final project: a complete tank station with a state machine and alarms
  34. 3420 interview questions on ST and PLC programming, with answers