How a PLC runs your code: the scan cycle and the process image

The loop that never stops

A PLC does not run your program once. It repeats it thousands of times a minute. Each repetition is a scan cycle with three phases:

Read inputs — the controller takes a snapshot of every field input into memory called the input image.
Execute — your program runs from the first line to the last, reading the input image, not the terminal.
Write outputs — the output image is transferred to the field terminals in one go.

Then it starts again. The lab on this site uses a fixed 50 ms cycle; a real PLC is typically 1 to 20 ms.

Why the image matters

An input value cannot change mid-scan. If you read Start at the top of the program and again at the bottom, you get the same value in that scan even if the operator pressed the button in between. That is what makes behaviour predictable.

Order matters

The program runs top to bottom, so the last assignment wins:

Pump := TRUE;
Pump := FALSE;   (* this is what reaches the output; the line above is lost *)

Scan time and the watchdog

Every PLC has a watchdog: if one scan takes longer than allowed, the controller stops and raises a fault. So:

Never write a WHILE whose condition may never become false.
Never put a million-iteration FOR in one scan.
Spread heavy work across several scans.

Tasks

A modern PLC lets you put your program in a cyclic task every 10 ms, and slow work such as communications in a 100 ms task. Timers count real time, so they stay correct whatever the scan time is.

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

Run the program and watch the SCAN counter at the top.

Press "One scan" instead of running, and see the counter advance by one.

Swap the last two lines and see how the result changes.

Course outline

01What is Structured Text, and when to use it instead of ladder

02How a PLC runs your code: the scan cycle and the process image

03Program structure: POUs, PROGRAM, FUNCTION and FUNCTION_BLOCK

04Your first program: starting a pump with seal-in logic

05Data types: BOOL, INT, REAL, TIME and the rest

06Declaring variables: VAR, VAR_INPUT and RETAIN

07Literals and constants: 16#FF, 2#1010 and T#5s

08Type conversion and scaling a 4-20 mA signal

09Assignment and arithmetic in ST: := and operator precedence

10Boolean logic, comparisons and bit operations

11Standard functions: ABS, SQRT, MIN, MAX, LIMIT, SEL, MUX

12IF, ELSIF and ELSE: the rules and when to avoid them

13The CASE statement and your first state machine

14The FOR loop and the watchdog

15WHILE, REPEAT, EXIT, CONTINUE and RETURN

16Edge detection: R_TRIG, F_TRIG and the "it fired every scan" bug

17Timers: TON, TOF and TP

18Counters: CTU, CTD and CTUD

19SR and RS flip-flops: which one wins

20Writing your own function

21Writing a function block

22Arrays in Structured Text

23Structs and enumerations

24References and pointers

25The state machine pattern: a batch sequence that cannot get stuck

26Alarm management to ISA-18.2: delay, deadband, latch, acknowledge, first-out

27Analogue signal processing: counts to engineering units, NE43 and filtering

28Closed-loop control in ST: hysteresis first, then a practical PID

29Motors and valves in practice: permissives, feedback and travel time

30The same ST on every platform: SCL, Studio 5000, CODESYS, TwinCAT

31Testing and debugging ST: the watch table, forcing and simulation

32An ST style guide: naming, comments, constants and version control

33Final project: a complete tank station with a state machine and alarms

3420 interview questions on ST and PLC programming, with answers

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