Virtual PLC (vPLC)

    PROGRAM Main()
        VAR_INPUT
            I00, I01, Heartbeat: BOOL;
        END_VAR

        VAR_OUTPUT
            Q00, Q01, Q02, Q03: BOOL;
        END_VAR

        VAR
            S: INT := 1;
            HeartbeatM: BOOL;
        END_VAR

        IF I00 AND NOT HeartbeatM AND Heartbeat THEN
            IF I01 THEN
                S := S - 1;
                IF S = 0 THEN
                    S := 4;
                END_IF
            ELSE
                S := S + 1;
                IF S = 5 THEN
                    S := 1;
                END_IF
            END_IF
        END_IF
        HeartbeatM := Heartbeat;

        IF S = 1 THEN Q00 := TRUE ELSE Q00 := FALSE; END_IF
        IF S = 2 THEN Q01 := TRUE ELSE Q01 := FALSE; END_IF
        IF S = 3 THEN Q02 := TRUE ELSE Q02 := FALSE; END_IF
        IF S = 4 THEN Q03 := TRUE ELSE Q03 := FALSE; END_IF
    END_PROGRAM