Abstract: In this system for detecting errors in data handled on a bus in a biphase format in accordance with a certain bus protocol, a pair of signals are established. The digitally measured time durations of these signals accurately represent the respective positive and negative excursion times of the bus data. Certain analyzers detect errors digitally in response to these measured time durations. Each analyzer means includes a half sync detection means for ascertaining that the time duration of each of these signals corresponds to the expected time duration of half of an initial sync signal transmitted in conformance with the bus protocol. A valid sync detector ascertains the consecutive occurrence of valid half sync signals from each analyzer, and provides a valid sync detected signal, and indicates sync type.