PLC communication system

A PLC communication system comprising:

• a programmable logic controller (PLC);
• a plurality of processing devices;
• at least three communication processing nodes provided at the PLC and the plurality of processing devices respectively to establish communication among the PLC and the plurality of processing devices;
• and
• an interconnecting device and a plurality of communication lines for connecting the communication processing nodes to form a communication route among the communication processing nodes, wherein
• the PLC detects a communication failure state in the communication route formed by the communication processing nodes when a first communication processing node reissues data to a third communication processing node after failing to receive expected data from a second communication processing node, a content of the reissued data including a content of the expected data, and
• a display device displays an abnormal portion in the communication route based on a number of occurrences of the detected communication failure state in the interconnecting device and the plurality of communication lines.