A very useful data transmission system having a plurality of transmission modules connected to a single cable in a multidrop manner is disclosed herein. The system features its ability to check the transmission state at the level of individual modules through use of a proper code notation (typically, Manchester code). The system detects a specific module with proper signals lasting at the same level for more than a given time, except cable outputs, and treats it as involving a faulty condition and compels the transmission state to a halt, thus preventing only the faulty module or modules from transmitting signals thereby avoiding breakdown of the whole system.