Two time division multiplex switches connected to the ends of a time-division multiplex bus are controlled so that they are closed during a periodically repeated time interval. Connected to the terminals of each two-wire line circuit is a capacitor and a switch, which is synchronously controlled with the time-division multiplex switches so that during each time interval the capacitor is charged to a voltage present on the time-division multiplex bus and that during the successive time intervals the discharge time constant of the capacitor is so long that the voltage of the electrodes of the capacitor remains substantially constant. The voltage is connected across the impedance of the corresponding two-wire line circuit.