An apparatus can be coupled to at least two lines for transmitting a differential signal, which is substantially symmetrical to a predefined reference potential, and/or for receiving such a differential signal. The apparatus includes a balancing unit, which can be coupled to the at least two lines and which includes a center tap. A monitoring unit is coupled to the center tap on the input side and is configured such that it monitors a center tap voltage between the center tap and the predefined reference potential and detects errors if the center tap voltage is greater than a predefined threshold value.