Data bus coupler having a primary bus connection for a primary bus and a plurality of secondary bus connections (S1, S2,.., S N) for the connection of each one terminal (F1, F2,.., F N) comprising - a slave unit which is connected to the primary bus connection, and - a master unit which can be connected to the secondary bus connections (S1, S2,.., S N) and which is embodied to send telegrams to the secondary bus connections (S1, S2,.., S N) or receive the same therefrom, characterised by - a control unit, - a communication switching means (K1, K2,.., K N) per secondary bus connection (S1, S2,.., S N) which connects or separates the respective secondary bus connection (S1, S2,.., S N) to or from the master unit, wherein the control unit is embodied to control the communication switching means (K1, K2,.., K N) in respect of the connection or separation, wherein the control unit - has a command output and is embodied to inform a determinable secondary bus connection (S1, S2,.., S N) of the planned query of the secondary unit by way of the command output of the master unit, wherein the control unit also - has a telegram evaluation input which is connected to the master unit and the control unit is embodied to read the telegrams sent by the master unit and to examine the telegrams for the planned query, furthermore the control unit is embodied to control the communication switching means (K1, K2,.., K N) at the point in time of sending the query by the master unit,in such a case the control unit is embodied such that the communication switching means (K1, K2,.., K N) associated with the queried secondary bus connection (S1, S2,.., S N) is controlled such that it connects the associated secondary bus connection (S1, S2,.., S N) determined in the query to the master unit and controls the remaining communication switching means (K1, K2,.., K N) such that the master unit is separated from the secondary bus connections (S1, S2,.., S N) not determined in the query. - a slave unit which is connected to the primary bus connection, and - a master unit which can be connected to the secondary bus connections (S1, S2,.., S N) and which is embodied to send telegrams to the secondary bus connections (S1, S2,.., S N) or receive the same therefrom, characterised by - a control unit, - a communication switching means (K1, K2,.., K N) per secondary bus connection (S1, S2,.., S N) which connects or separates the respective secondary bus connection (S1, S2,.., S N) to or from the master unit, - has a command output and is embodied to inform a determinable secondary bus connection (S1, S2,.., S N) of the planned query of the secondary unit by way of the command output of the master unit, wherein the control unit also - has a telegram evaluation input which is connected to the master unit and the control unit is embodied to read the telegrams sent by the master unit and to examine the telegrams for the planned query, furthermore the control unit is embodied to control the communication switching means (K1, K2,.., K N) at the point in time of sending the query by the master unit,in such a case the control unit is embodied such that the communication switching means (K1, K2,.., K N) associated with the queried secondary bus connection (S1, S2,.., S N) is controlled such that it connects the associated secondary bus connection (S1, S2,.., S N) determined in the query to the master unit and controls the remaining communication switching means (K1, K2,.., K N) such that the master unit is separated from the secondary bus connections (S1, S2,.., S N) not determined in the query.