Abstract: A data transmission system uses a malfunction detection circuit for detecting malfunction conditions between a data transmitting station and a data receiving station connected with first and second data transmission lines. The malfunction circuit senses a data signal of a first polarity for a preselected duration and provides an output signal when the data signal exceeds the preselected duration. The circuit also supplies a detection signal of a second preselected duration in a polarity opposite the data signal and detects the presence of or the absence of the detection signal. The circuit thereafter provides an output signal indicative of the malfunction condition when the absence of the detection signal exceeds the second preselected duration.