When the nodes are connected to the DCS communication network, usually there is one network interface that the controller sends the data to. The man-machine interface reads the data from network interface. Reading data shall comply with the network communication protocol. The network physical structures are two kinds including ring and bus, the bus network logically is circular. Star network is only used for small systems (within 100 I/O points).The commonly used communication protocol is broadcast-style. As for the node on the network, as long as the data is sent to the Internet, it will broadcast data continuously, the node needing data receives data. In addition to this mode, the network of broadcast-style protocol has the other mode: A node enquires data from other nodes on the network, but other nodes have not the data, it will enquiries the data repeatedly until it reads the data. So the data is not available on the network, can cause network congestion.