Abstract: The instant invention relates to the phasing of digital trains containing identical data signals and clock signals of the same frequency. A data testing device compares the coincidence of the bits by packets of bits contained in the signals for emitting a signal (NoCo), if there is at least one non-coincidence in a packet. The testing device delivers a poor base relation signal (ABSCORR) after q non-coincidences. If the testing device has not delivered any non-coincidence pulse for a time t.sub.1, a phase discriminator delivers a phase non-matching signal (ABSPHA). A phase change demand signal (PAP) is delivered by the testing device either upon appearance of the signal (ABSCORR) or the signal (ABSPHA).The invention is appropriate for switching purposes in microwave link transmission.