Compounds selected from the group according to claim 1 are inhibitors of tyrosine kinases, in particular of Met kinase, and can be employed, inter alia, for the treatment of tumours. A diode is connected in parallel to the current detection resistor to reduce heat generated in the switching unit by back electromotive force generated by an inductance component of the current detection resistor. Buffering the data packets provides a period of time for digital signal processing techniques to reliably determine whether the characteristics of the detected signal are sufficient to characterize it as a DTMF signal.