transponders are arranged at control locations associated with sections of a track or travel path . each transponder is in wireless communication through an inductive transmission path and an optical transmission path with a conveyor present in the operable range of the transponder . each transponder is in communication with a central system control via an associated group control unit by means of two - wire lines or conductors . the operational mode of each transponder is controllable by means of a multiplexer such that the transponder operates either as a responder or answering device with simultaneous unidirectional signal transfer and which responder is to be interrogated by a conveyor or as a bidirectional signal transmitter . the apparatus selectively functions as a location or site code transmitter and presence indicator for the individual conveyors or as bidirectional data transmitter with isolated directions between the conveyors and the central system control . due to such dual utilization only one type of layout element is required , resulting in a high reduction in the installation expense and its maintenance . the apparatus is intended for the modular erection of conveyor installations of different sizes and output and is equally well suited for partially autonomous as well as for central control of the conveyors .