A method of using an interface device to interface to a processing unit is disclosed. The interface device and the processing unit are part of a station of a plurality of stations in a sheet processing line. The method involves using the interface device to receive a control signal from an upstream station, and comparing a selection part of the control signal with a reference, the selection part being part of an addressing part of the control signal. If the comparison result is negative, a further control signal is determined from the control signal, which is derived from and differing from the addressing part of the control signal and the interface device outputs the further control signal to a different interface device corresponding to a processing unit of a downstream station. If the comparison result is positive, an instruction signal is sent from the interface device to the processing unit.