Token ring fault detection

A user terminal (12) is coupled to a token ring (11) via a cross-point circuit (15) and a trunk coupling unit (14). The signal paths (13a, 13b) between the terminal and the cross-point circuit and between the cross-point circuit and the coupling unit are supplied each with a phantom current, there being means for isolating the cross-point circuit from the voltages driving the phantom currents. The phantom currents are used for fault detection, cessation of a phantom current causing the user link to be decoupled from the token ring. Circuit (15) is a high speed silicon cross-point integrated circuit.