User manual

IEC61131 User and Reference Manual
April 22, 2008
75
Modbus RTU in TCP
Modbus RTU in TCP message format is exactly same as that of the Modbus RTU protocol.
The main difference is that Modbus RTU in TCP protocol communicates with a controller
through the Internet and Modbus RTU protocol through the serial port. The Modbus RTU in
TCP protocol does not include a six-byte header prefix, as with the Modbus\TCP, but does
include the Modbus „CRC-16‟ or „LRC‟ check fields. The Modbus RTU in TCP message
format supports Modbus RTU message format.
 To configure a Modbus RTU in TCP protocol connection, highlight Modbus RTU in
TCP in the Communication Protocols window and click the Configure button. The
Modbus RTU in TCP Configuration window is displayed.
 To select a configured Modbus RTU in TCP protocol connection, highlight Modbus
RTU in 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 RTU in TCP is selected for configuration the Modbus RTU in TCP
Configuration dialog is opened with the General tab selected as shown below.
Figure 26: Modbus RTU in TCP Configuration (General) Dialog Box
The Modbus Communication Settings grouping contains Modbus specific communication
settings including the addressing mode, the station address, the timeout interval as well as
the number of attempts.
The Addressing parameter selects standard or extended Modbus addressing. Standard
addressing allows 255 stations and is compatible with standard Modbus devices. Extended