in a data communication system in which data communication is carried out between two or more data processing units and at least one device to be controlled thereby , each coupled via data station to a common data transmission path connected in a loop fashion , a conflict preventing circuit is provided between said device to be controlled and the associated data station , which conflict preventing circuit comprises a flip - flop to be set in response to its own device identifying data received from a particular communicating unit for keeping its own device enabled , an address register for storing a unit identifying data of said communicating unit , and an and gate responsive to the device identifying data input and a busy signal from its own device for latching said flip - flop and said address register while the device is busy or communicating with said particular unit , thereby preventing said flip - flop and said address register from being affected from a data transmitted from a different unit while the device is communicating with said unit and thus avoiding a problem of conflict .