I have made a small PID controller on a Siemens S7-200. Making the Analog part was no problem. But I have to Control a valve. With step up, neutral, or step down signals. I made a test where i used the controllers output to control a timer and a fixed "step" lenght, but the "hardware" I have seen, used a varible step lenght and i varible pause. Is there somebody who can tell me how to convert Analog output signals to stepping signal the "right" way.