Circuitry comprising: a set of two or more redundant data handling devices (1342, 1340) configured to perform identical data handling functions; control circuitry (1350, 1360) to control an operating state of a data handling device of the set of two or more redundant data handling devices; the control circuitry being configured to control an operating state of the respective controlled data handling device as a state transition from a current operating state of that data handling device to a target operating state in response to the issue of a respective state change signal; the control circuitry comprising a detector responsive to issue of the state change signal in respect of a first threshold number representing some but not all of the data handling devices, to detect whether the state change signal is issued in respect of a further one or more of the devices so that a second threshold number of data handling devices is reached; wherein the set of two or more redundant data handling devices comprises two or more instances of data processing apparatus to perform identical processing operations in a redundant mode of operation, each instance having respective control circuitry, and in which each control circuitry is configured to receive state change signals issued in respect of all of the instances of data processing apparatus; characterised by the circuitry comprising signal routing circuitry (1300, 1310) configured to provide output signals generated by one instance of data processing apparatus as input signals to control circuitry of that instance of data processing apparatus in the case that another instance of data processing apparatus is currently in an inactive state. a set of two or more redundant data handling devices (1342, 1340) configured to perform identical data handling functions; control circuitry (1350, 1360) to control an operating state of a data handling device of the set of two or more redundant data handling devices; the control circuitry being configured to control an operating state of the respective controlled data handling device as a state transition from a current operating state of that data handling device to a target operating state in response to the issue of a respective state change signal; the control circuitry comprising a detector responsive to issue of the state change signal in respect of a first threshold number representing some but not all of the data handling devices, to detect whether the state change signal is issued in respect of a further one or more of the devices so that a second threshold number of data handling devices is reached; wherein the set of two or more redundant data handling devices comprises two or more instances of data processing apparatus to perform identical processing operations in a redundant mode of operation, each instance having respective control circuitry, and in which each control circuitry is configured to receive state change signals issued in respect of all of the instances of data processing apparatus; characterised by the circuitry comprising signal routing circuitry (1300, 1310) configured to provide output signals generated by one instance of data processing apparatus as input signals to control circuitry of that instance of data processing apparatus in the case that another instance of data processing apparatus is currently in an inactive state.