Chapter 20 Network Configuration and Data Exchange the PLCs which forward the data along the paths, every PLC has its own routing table. The forwarding of the data is directed on the basis of the routing tables produced. Station address This is to identify RS-485 stations on the internet. The station address is configured base on communication port. Basically, a communication port represents a station, and the workstation address that has the same internet no. cannot be repeated. Therefore, when a node contains several COM ports and is connected, a station address has to be assigned. IP address and DHCP mode IP address is the address for communication ports on the internet.
Basically, a communication port represents a internet point. Therefore, when several communication ports are included and connected to the internet, an IP address has to be assigned and cannot be repeated. DHCP is the mechanism for dynamic allocation of IP addresses. A host allocated by the DHCP has to exist in the internet. When a communication port adopts DHCP mode, the port automatically obtains an IP address from DHCP host. Subnet mask A subnet mask is used to differentiate the subnework parameter values. PLC Link A network mechanism for data exchange performed through an RS-485 cable.
If there are several nodes in an RS-485 network, users can create a mechanism for data exchange in the network. If the parameters which are set are downloaded to the PLC which functions as a master station, the system of the PLC will perform data exchange through special relays and special registers when the PLC runs. A PLC Link is a master/slave model. There is only one master station in an RS-485 network, and the other stations which are slave stations passively receive reading/writing commands from the master station. The Ether Link slave stations cannot exchange data. They have to exchange data through the master station.
A network mechanism for data exchange performed through an Ethernet connection. If there are several nodes in an Ethernet network, users can create a mechanism for data exchange in the network, and select a start mode. If the parameters which are set are downloaded to the PLCs in the network, the systems of the PLCs perform data exchange according to the start mode selected when the PLCs run. An Ether Link is not a master/slave model. It allows a node to send reading commands which ask for data to other nodes. The nodes will send the data to the node after they receive the reading commands.
Owing to the fact that a node cannot send writing commands to other nodes, the use of an Ether Link is safer than the use of a PLC Link. Besides, the system automatically manages the transmission of packets through TCP/IP. Compared with a PLC Link, an Ether Link is more efficient. 20-5