Monday, May 16, 2011

Network Application Architecture in PLC





According to the different network application requirements this module offers two operating modes, they are Server Mode and Client Mode. When runs at server mode, the module of Ethernet will remain for the message receive from the network. Subsequent to decode the received message it will transmit the message through serial port to the main unit of PLC. The respond message from the PLC will interrupt by this module and filled into data packet then send out to the network therefore complete a transaction of server mode.

When runs at client mode, the module of Ethernet will await the message receiving from the serial port. If the accepted message is for the located of PLC at the remote site associated by network, the module of Ethernet will set the message into data packet for transmission network and transmit it to network. Once transmit the message to network the module of Ethernet will await the respond message coming from network, when it receives the respond message it will through the message to serial port for PLC therefore complete a transaction of client mode.

Server Mode
Once runs at server mode, the direct linked single PLC or the stations associated by RS485 are all operate at slave mode, which will stay for instruction message passively and respond the instruction. The A and B of work stations are master that can transmit the command message to FBs-PLC actively, ahead the command message receiving, the module of Ethernet will transmit the message via the port4 to FBs-PLC. When it has more than one message planned to transmit to FBs-PLC, the module of Ethernet will keep the added messages into the message queue then transmit it to FBs-PLC, consequently there will no inconsistency. When run at this mode, there is no required to write any program operation in PLC. In this architecture application, The PLCs link the module of Ethernet with the port4 of RS485 interface.

PLC Client Mode of Ethernet Module
When operate at client mode, the module of Ethernet will await for the port4command message. When it discovers the message is for the station of PLC that located at remote site then it will set the message base on the port mapping table content and transmit it to the network. Subsequently, the module of Ethernet will save an eye on network for the respond message. Ahead receiving the respond message, the module of Ethernet will decode the message then return back to PLC via serial port therefore complete a transaction of client mode. When operate at this mode, the direct associated single PLC or the PLC LINK master station linked by interface of RS485 are all run at master mode, which means it use instruction mode0 of LINK to transmit the command actively. The client mode can be more separated into virtual server mode and standard mode.

Standard Client Mode
The master PLC linked with the module of Ethernet can use LINK instruction mode0 transmit the instruction message to other PLC. The destination PLC that master PLC propose to command can be a local PLC slave associated by the interface of RS485 to master PLC with sever mode Ethernet connection. There is a “Station to translation table of network address” in the module of Ethernet when operate at client mode. This table contains the information about the remote station and the local station mapping, the operator should set this table based on the actual application deployment.

The module of Ethernet will check constantly the received message, if the number of station in the message can be established in the table of translation that means the message will route to network, the module of Ethernet will first change the number of station in message based on the table of translation then update and re-calculate the translated message check sum and summarize it in network data packet and lastly transmit it to the network.



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