The invention relates to a rotary transmitter (2) for machine tools, having an inductive energy transmission section (31), which is arranged between a stator part (4) fixed to the machine and a rotor part (6) fixed to the tool, and a contactless bidirectional data transmission section (35). A special feature of the invention consists in that, in order to make maximum use of the capacity of the energy transmission section (31), precautions are taken with which the optimal operating frequency (fopt) of the energy transmission operating according to the transformer principle is determined at every system start in a test run with a connected test resister (51) and a variable frequency (fp). Furthermore, for the purpose of interference-free data transmission, buffer storage of the data to be transmitted via the data transmission section (35) is proposed, which data are synchronized in predefined time windows with interference-free periods of the energy transmission.