Data bus coupler and method of operation

A data bus coupler having a primary bus connection for connection to a primary bus and a plurality of secondary bus connections each for connection to a respective terminal, said data bus coupler comprising:

• a slave unit connected to the primary bus connection;
• a master unit connectable to the secondary bus connections and configured to send telegrams to and receive telegrams from the secondary bus connections;
• a control unit;
• and
• a plurality of communication switching means each operatively connecting a respective one of the secondary bus connections to the master unit;
• wherein the control unit is configured to control the plural communication switching means to selectively connect and disconnect each secondary bus connection from the master unit;
• wherein the control unit further comprises:
• a command output, the control unit being configured for informing to inform the master unit, via the command output, of a planned query of a particular one of the secondary bus connections;
• and
• a telegram evaluation input connected to the master unit, the control unit being further configured to read the telegrams sent by the master unit and to examine the telegrams for the planned query, and to control, when the planned query is sent by the master unit, the plural communication switching means;
• and
• wherein the control unit is further configured to control a communication switching means associated with the queried particular secondary bus connection to connect the particular secondary bus connection determined to the master unit and to control others of the communication switching means to disconnect the master unit from the respective secondary bus connections.