Abstract: A line interface for a two wire bus for detecting defects of the bus. The interface includes a shaping circuit having three comparators. The first comparator having two inputs connected to the two wires of the bus, the second comparator having a first input connected to one of the bus wires and another input connected to a reference signal, and the third comparator having a first input connected to the other wire of the bus and the second input connected to a reference signal. A first counter is connected to the output of the first comparator and counts a number of transitions which occur on the bus during a predetermined period of time. If the number of transitions detected by the first comparator is counted to be less than a predetermined number of transitions, the bus is defective. The second and third counters are respectively connected to the second and third comparators.