Abstract: A system for controlling access to a data transmission network of a bus configuration. After a communication unit has completed transmission of a message block, the communication unit transmits a synchronization signal over the bus. Each of the communication units of the network which has a message block for transmission responds to that synchronization signal by applying to the data bus the logical ones of the address of the associated data station, or of a virtual address resulting from an algorithm performed on that station address, the logical ones being applied in sequence during respective bit intervals. During bit intervals of the logical zeros of the station address, or of the virtual address, the communication unit monitors the bus. If during such monitoring the communication unit detects a logical one on the data bus, the communication unit terminates transmission of its logical ones and awaits the next synchronization signal.