Abstract: A system for quality checking of a digital connection circuit using a telephone line. Its principle resides in alternately and repetitively transmitting a check message followed by a quality character from one to the other of two similar equipments respectively located at the ends of the circuit. The quality character takes account of the results of the checking effected by the corresponding equipment when it receives the last previously transmitted check message. The latter message further includes a n times repeated synchronization character and a pseudo-random series of p binary signals and is immediately followed by a quality character which is stored and displayed by the receiving equipment which operates at the considered time.