Abstract: The main features of the present invention reside in the fact that one or more remote groups of subordinated terminal units are serially interconnected and arranged in a closed loop which is terminated at the output and input of a master unit. Each of said groups and said master unit is connected to the transmission line via a modulator and a demodulator. In order to selectively connect a pulse equipment associated with each subordinated terminal unit, and adapted for transmission of data, with the transmission line, each of said subordinated terminal units includes a switching unit, which either connects or disconnects the associated pulse equipment from the transmission line.