The input unit (2) has a pressure sensitive sensor field (3), digital processing system (4), screen (6) and release button (8). The sensor field detects several position signals (16A,B) at the same time. Five or more position signals input by the user's finger, are sensed. The sensed position signals are fed to the processing system (4) and processed to image signals that are transmitted to the screen and represented as indication elements (18A,B) on screen. Components of a process control system are operated via the screen. In an actuating window (2) for a component of the system, action fields (A,E,F1) are represented, e.g. action field A switch off the pump, E switch on pump , F1 an enabling function. When an action field is activated a process order (P) is transferred from the processing system (4) to the control system (10). Information on the operation state of the components is transferred to the processing system and graphically represented on the screen.
