Abstract: A method for monitoring symmetrical two-wire bus lines and two-wire bus interfaces and a device for carrying out the method provides pulse weighting of low to high transitions or high to low transitions of the two wires operated in phase opposition of a two-wire bus line. The pulse chains thus obtained are used for step sequencing. In each case, one multistep shift function which is assigned to the first bus wire, is supplied with a first, constant logic read in state and can be reset to a second logic state in all-step fashion and, for all-step resetting of a similar second multistep shift function assigned to the second bus while, and vice versa, the step last achieved for each multistep shift function characterizing the respective fault state of the other bus wire. The pulse weighting is achieved by differentiation or high-pass filtering or by pulse generation controlled by state transition.