Abstract: In a loop-structured communication system interconnecting a plurality of data processors without central controller, stations comprise modems which can start loop operation and achieve synchronous frame transmission around the loop through all participating stations.In an initialization procedure after system start or after loss of frame synchronization on the loop, modems select a temporary master station.The temporary master will adapt total loop delay to be an integer multiple of the basic frame period, and will transmit available frames. All active modems adapt their timing to the circulating stream of frames until the whole loop and all participating stations are synchronized.Synchronization conditions are continuously monitored in all stations to start resynchronization or initialization procedures if necessary. In a specific arrangement, the location of a loop failure which prevents establishment of synchronization can be determined.