Program structure: POUs, PROGRAM, FUNCTION and FUNCTION_BLOCK

The POU

Everything you write for a PLC lives in a POU (program organization unit). The standard defines three, and the difference comes down to one question: does it keep its memory between scans?

Kind	Memory	Outputs	Example
`FUNCTION`	none	one value	`SQRT`, a 4-20 mA scaler
`FUNCTION_BLOCK`	kept per instance	several	`TON`, a motor block
`PROGRAM`	kept	bound to a task	the station's main program

FUNCTION

Values go in, one value comes out, and nothing is remembered. The same inputs always give the same result. Use it for maths.

FUNCTION_BLOCK

This is the key idea in IEC 61131-3. A block has memory per instance. When you write

VAR
  T1 : TON;
  T2 : TON;
END_VAR

you created two instances of TON, each with its own elapsed time and output. That is why a timer must be declared in VAR and cannot be used as a throwaway function. The instance is what remembers.

PROGRAM

The top level: bound to a task and executed cyclically. In this lab every example is one program, and its variables are wired to the process by name.

Declaration sections

VAR_INPUT  … END_VAR   (* comes from outside, read-only *)
VAR_OUTPUT … END_VAR   (* goes to the outside *)
VAR        … END_VAR   (* internal, kept between scans *)

In the lab, variables named Start Stop Level LSH LSL are wired to inputs, and Pump Valve Lamp HiAlm to outputs.

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 T1.ET and T2.ET in the watch table.

They are independent even though they share a type: each instance has its own memory.

Delete the declaration of T2 and run: you will get "undeclared variable".

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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