There is provided a control and data transmission system, comprising at least one control device which, in normal operation, is connected by means of a communication network to at least one process device designed as an input and/or output device, wherein the process device comprises an evaluation unit designed to detect a failure in the control system, an emergency control program which can be parameterized and which is stored in a memory of the process device, and a runtime system designed to execute the emergency control program, and wherein the process device is designed to switch to emergency operation in response to a failure in the control system detected by the evaluation unit, in which emergency operation the process device executes the emergency control program. The invention further provides a process device for use in such a control and data transmission system and a method for redundant process control.