To detect an established or unestablished connection (transmission link) and to display this detected result by discriminating the transmission state of a data signal based on both results of a frame type discriminating circuit and an address discriminating circuit.

The data sent from an FB(field bus) transmission line are received at a receiving part 12. A frame code included in the received data is sent to a frame discriminating circuit 21, and the address data are sent to an address discriminating circuit 22. The ANDs are secured by the AND elements 23 and 24 between the detection signals PN and PT of PN and PT frames detected by the circuit 21 and the detection signal of its own station address AD detected by the circuit 22 respectively. Thus, the PN and PT detection signals which are addressed to its own station are generated. The output signal of a flip-flop 25 serves as a signal that shows the established or cut-off state of the link of its own station via the FB.