The CASE statement and your first state machine

When CASE beats IF

When you test one variable several times. An ELSIF ladder repeats the variable name in every branch; CASE names it once:

CASE State OF
  0: Pump := FALSE;
  1: Pump := TRUE;
  2: Pump := FALSE;
ELSE
  State := 0;
END_CASE

The selector must be an integer. Not a real, not a boolean.

Label forms

CASE Step OF
  0:         Lamp := FALSE;
  1..3:      Lamp := TRUE;          (* all the filling steps *)
  10, 20:    Lamp := Blink;
ELSE
  Step := 0;                        (* unexpected value? go to a known point *)
END_CASE

Always write the ELSE. If memory is corrupted or an HMI writes a value you did not plan for, ELSE is what puts the machine back into a safe state instead of leaving every branch dead.

The state machine

A state machine is only three things:

1. An integer variable holding the current state.
2. A CASE with one branch per state: what the outputs do here.
3. Inside each branch, a transition condition that writes the next state number.

Our tank sequence:

Rules that save you

• Write the outputs in every branch, not only where they are needed. Outputs hold their value, so a pump started in state 1 keeps running in state 2 unless you switch it off explicitly.
• Put the stop above the CASE, not inside it. One condition before the construct returns the machine to zero from any state.
• Do not cross two states in one scan unless you mean to. Here you cannot, because the selector is read once per scan.
• Numbers or names? On a real PLC you declare an enumerated type (TYPE ... END_TYPE) and write Fill instead of 1. The lab does not support TYPE, so we use numbers and document them in a comment.

A timer inside the sequence

The hold state needs time. Call the timer outside the CASE and feed it from a state comparison:

Hold(IN := State = 2, PT := T#3s);

That way the timer is called every scan as it must be, starts by itself when state 2 is entered, and resets when it is left.