In a communication system wherein a station transceiver and a plurality of nodes communicate in a TDM system through a signal transmission line, at least a part of the line is shared, a method to judge nodes performing normal operation from the station transceiver comprises a request step to request a return of a test pattern by transmitting a trigger signal for a designated node in the plurality of nodes, a correlation process step to process correlation between a received signal in a timeslot assigned to the designated node and a reference pattern corresponding to the designated node, and a judging step to judge whether the designated node is a normal node according to the correlation process result.