(* CONVEYOR SHUTTLE CONTROL - Structured Text *)
(* ACC PLC Simulator - Conveyor Scene *)
(* X1 Start PB (N.O.)  X2 Stop PB (N.C.)  X3 Left sensor  X4 Right sensor *)
(* Y1 Conveyor run     Y2 Reverse                                         *)

VAR
    State      : INT  := 0;      (* 0 = Idle, 1 = Forward, 2 = Reverse *)
    StartLast  : BOOL := FALSE;  (* Start button state, previous scan  *)
    StartPulse : BOOL := FALSE;  (* One-shot on the Start press        *)
END_VAR

(* One-shot the Start button *)
StartPulse := X1 AND NOT StartLast;
StartLast  := X1;

(* Stop has priority over everything *)
IF NOT X2 THEN
    State := 0;
ELSIF StartPulse AND State = 0 THEN
    State := 1;
END_IF;

(* State transitions *)
CASE State OF
    1:  (* Forward - travelling right *)
        IF X4 THEN
            State := 2;
        END_IF;
    2:  (* Reverse - travelling left *)
        IF X3 THEN
            State := 0;
        END_IF;
    ELSE
        State := 0;
END_CASE;

(* Outputs driven from state *)
Y1 := (State = 1) OR (State = 2);
Y2 := (State = 2);