A method for defining a cycle time for a transmission cycle on a system bus of a monitoring and/or control system having at least one communication module and at least one input/output module, which is connected to the communication module via the system bus for transmitting measurement and/or control signals and is intended to input and/or output measurement and/or control signals to field applications, the at least one communication module having a time control unit for controlling a transmission cycle which is constantly repeatedly carried out and has defined communication times for the communication and input/output modules which are connected to the system bus, comprises measuring the signal propagation times on the system bus and defining the cycle time for a transmission cycle on the system bus on the basis of the longest signal propagation time measured.