多个扫描周期, 同时有很复杂, 都可以用状态机

CASE calculate_step OF

    IDLE:

        IF calculate THEN

            calculate_step := CALCULATE_STEP_1;

        END_IF;

    CALCULATE_STEP_1:

            // first calculation

            calculate_step := CALCULATE_STEP_2;

    CALCULATE_STEP_2:        

            // second calculation

            calculate_step := CALCULATE_STEP_3;

    CALCULATE_STEP_3:

            // third calculation

            calculate_step := DONE;

    DONE:

            calculate_done := TRUE;

            IF not calculate THEN

                calculate_done := FALSE;            

                calculate_step := IDLE;

            END_IF;                

END_CASE;