LXM32M | 3 Basics |
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3.3Modbus TCP communication
3.3.1Connection management
Establishing of a connection The Modbus TCP server allows for TCP connections via the default port 502. A client can establish a new connection via this port. If the client is to exchange data with a remote server, a new client connec- tion via remote port 502 must be established.
Modbus data transfer A Modbus request is sent via a suitable, open connection. This TCP connection is found using the IP address of the remote device. The connection remains open for all Modbus communication. Up to 8 simultaneous connections are possible.
As described in the next chapter, a Modbus client can initialize several Modbus transactions without having to wait for the previous transac- tion to be finished.
Closing a connection After the Modbus communication between the client and a server is finished, the client causes the connection used to be closed.
The server does not close the connection under normal circumstan- ces.
However, when errors occur and in special cases, the server closes the connection, for example:
•Communication errors
•Communication inactivity
•Maximum number of connections reached
The product can manage up to 8 TCP connections. If an attempt is made to establish a further connection beyond this maximum, the old- est unused connection is closed. If it is impossible to close the oldest unused connection, the new connection is refused.
0198441113843, V1.01, 01.2012
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