State machines in PLC programs

Why

Sequences built from dozens of set/reset flags become impossible to debug: several flags are TRUE at once and nobody knows the real state. A state machine keeps exactly one current state and explicit transitions.

Pattern in ST

TYPE E_Fill : (Idle, Filling, Settling, Draining, Fault); END_TYPE

VAR
    State     : E_Fill := Idle;
    LastState : E_Fill := Idle;
    StepTime  : TON;
END_VAR

StepTime(IN := (State = LastState), PT := T#1H);   (* time in current state *)
LastState := State;

CASE State OF
    Idle:
        InletValve := FALSE;  OutletValve := FALSE;
        IF StartCmd AND Permissive THEN State := Filling; END_IF;
    Filling:
        InletValve := TRUE;
        IF LevelHigh THEN State := Settling;
        ELSIF StepTime.ET > T#120S THEN State := Fault; END_IF;   (* timeout *)
    Settling:
        InletValve := FALSE;
        IF StepTime.ET > T#30S THEN State := Draining; END_IF;
    Draining:
        OutletValve := TRUE;
        IF LevelLow THEN State := Idle; END_IF;
    Fault:
        InletValve := FALSE;  OutletValve := FALSE;
        IF ResetCmd AND NOT LevelHigh THEN State := Idle; END_IF;
END_CASE;

Rules that pay off

• Outputs are a function of the state (and interlocks), not set in many places.
• Every waiting state has a timeout leading to a fault state.
• A global stop / emergency condition is checked before the CASE and forces a safe state.
• Show the state (number and text) on the HMI — commissioning becomes much faster.
• For complex parallel sequences use SFC (GRAPH in TIA Portal) or split into several cooperating state machines.