Bus coupler for coupling field devices (11,..,n) to a superordinate bus system, comprising - a connection for the superordinate bus system, - in each case a connection (11a,12,..,na) for, in each case, one field device (11,..,n), - a main transmit-receive unit configured to transmit and receive telegrams on the superordinate bus system, - in each case a transmit-receive unit (1,..,n), configured to communicate with each field device (11,..n), wherein the respective transmit-receive unit (1,...n) - has a signal input for telegrams of the superordinate bus system, which is connected to the main transmit-receive unit, - a field device output for the connection of the field devices (11,..,n), by way of which the telegrams are sent from the main transmit-receive unit to the field devices (11,..n), - a receive signal connection, by way of which the telegrams are forwarded from the field devices (11,..,n) to the main transmit-receive unit, and - a control input and characterised in that it is configured as follows the signal input is connected to a first input (-) of an impedance converter, a second input (+) of the impedance converter is connected to the controllable reference means, an output of the impedance converter is connected to a terminating resistor (Ra) in a series circuit at the field device output, wherein a measuring means for determining a voltage drop is connected to the terminating resistor (Ra) and the measuring means is connected to the reference means, wherein the impedance converter is designed according to the structure of an operational amplifier, further having, - a control facility (μC), which is configured to adjust the reference means in respect of a reference voltage for the second input (+) by way of the control input in order to reduce a current flow through the terminating resistor (Ra) and thus to realise a current limitation for the field device output. - a connection for the superordinate bus system, - in each case a connection (11a,12,..,na) for, in each case, one field device (11,..,n), - a main transmit-receive unit configured to transmit and receive telegrams on the superordinate bus system, - in each case a transmit-receive unit (1,..,n), configured to communicate with each field device (11,..n), wherein the respective transmit-receive unit (1,...n) - has a signal input for telegrams of the superordinate bus system, which is connected to the main transmit-receive unit, - a field device output for the connection of the field devices (11,..,n), by way of which the telegrams are sent from the main transmit-receive unit to the field devices (11,..n), - a receive signal connection, by way of which the telegrams are forwarded from the field devices (11,..,n) to the main transmit-receive unit, and - a control input and characterised in that it is configured as follows the signal input is connected to a first input (-) of an impedance converter, a second input (+) of the impedance converter is connected to the controllable reference means, an output of the impedance converter is connected to a terminating resistor (Ra) in a series circuit at the field device output, wherein a measuring means for determining a voltage drop is connected to the terminating resistor (Ra) and the measuring means is connected to the reference means, wherein the impedance converter is designed according to the structure of an operational amplifier, further having, - has a signal input for telegrams of the superordinate bus system, which is connected to the main transmit-receive unit, - a field device output for the connection of the field devices (11,..,n), by way of which the telegrams are sent from the main transmit-receive unit to the field devices (11,..n), - a receive signal connection, by way of which the telegrams are forwarded from the field devices (11,..,n) to the main transmit-receive unit, and - a control input - a control facility (μC), which is configured to adjust the reference means in respect of a reference voltage for the second input (+) by way of the control input in order to reduce a current flow through the terminating resistor (Ra) and thus to realise a current limitation for the field device output.