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

The problem

Panel buttons are momentary: they give a signal only while your finger is on them. But the pump must keep running after you let go. The classic answer is seal-in (also called latch).

One line

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

Read it as: the pump runs if start was pressed or it is already running, and at the same time stop is not pressed. The OR Pump part is the seal: the output feeds its own condition.

Why stop goes last

Because AND NOT Stop multiplies everything before it: press stop and the result is FALSE whatever else is true. This is a general safety rule — the stopping condition beats the starting condition.

In the field, stop is wired NC

On a real panel the stop button is wired normally closed: if the wire breaks, the signal drops, the PLC reads zero and the pump stops. The failure goes to the safe side. The line then becomes:

Pump := (Start OR Pump) AND StopNC;

because StopNC is TRUE when all is well. The lab simulates the button as NO to keep the experiment simple.

Add the interlock

Buttons are not enough. Add the conditions that prevent damage:

Pump := (Start OR Pump) AND NOT Stop AND Level < 95.0;

Now the pump stops by itself before the tank overflows. That is an interlock, and it is what separates a hobby program from an industrial one.

And the drain

Valve := NOT Pump AND Level > 60.0;

The drain valve opens above 60% only while the pump is off, so the two never fight each other.

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

Press START for a moment and let go: the pump keeps running.

Press STOP: it stops at once.

Run again and let the level rise above 95%: it stops by itself, thanks to the interlock.

Delete "AND NOT Stop" and try: the pump will never stop.

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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