A bus coupler for coupling field devices to a superordinate bus system includes an impedance converter arranged in a series circuit with a terminating resistor between a signal input and a field device output to enable a comprehensive diagnosis option separately for each transmission line and the connected field device, wherein the terminating resistor assumes the function of a line terminating resistor and a current limitation, and where a controllable reference voltage source enables the maximum output current to be defined and the output shut off.