Abstract: A method and apparatus for locating a discontinuity in a data transmission ring which has a plurality of stations which randomly receive and transmit data along the ring, in which the occurrence of valid and invalid data is sensed and compared and an indication given of the invalid data exceeds valid data by a predetermined amount, transmitting valid data responsive to the indication so that the indication can be removed from stations which receive valid data and transmitting data including station identification from any station showing the identification, which data is received at a separate station located between the discontinuity and the transmitter whereby the locality of the discontinuity can be identified.