Filter
Top Products
3.3 Modbus TCP communication
3.3.1 Connection 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.
LXM32M
3 Basics
Modbus-TCP module 19
0198441113843, V1.01, 01.2012