User manual

IEC61131 User and Reference Manual
April 22, 2008
83
Modbus/TCP
Modbus/TCP is an extension of serial Modbus, which defines how Modbus messages are
encoded within and transported over TCP/IP-based networks. The Modbus/TCP protocol
uses a custom Modbus protocol layer on top of the TCP protocol. Its request and response
messages are prefixed by six bytes. These six bytes consist of three fields: transaction ID
field, protocol ID field and length field. The encapsulated Modbus message has exactly the
same layout and meaning, from the function code to the end of the data portion, as other
Modbus messages. The Modbus „CRC-16‟ or „LRC‟ check fields are not used in
Modbus/TCP. The TCP/IP and link layer (e.g. Ethernet) checksum mechanisms instead are
used to verify accurate delivery of the packet.
 To configure a Modbus/TCP protocol connection, highlight Modbus/TCP in the
Communication Protocols window and click the Configure button. The Modbus/TCP
Configuration window is displayed.
 To select a configured Modbus/TCP protocol connection, highlight Modbus/TCP in the
Communication Protocols window and click the OK button.
 To close the dialog, without making a selection click the Cancel button.
General Parameters
When Modbus/TCP is selected for configuration the Modbus/TCP Configuration dialog is
opened with the General tab selected as shown below.
Figure 32: Modbus/TCP Configuration (General) Dialog Box