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Simple example of READ_VAR Modbus TCP/IP communication for Premium

Published date: 25 May 2018

Causes and Fixes

The TCP/IP communications can be difficult to program using Unity, since they vary from PAC family to family. This is one of a series of example applications that presents each function block in a working version for the user to build on.

Additional Information

The attached application readvar.zef will send a read register Modbus TCP message to another device on the local Ethernet network. The application consists of two sections. The first is an ST section in which variables are initialized and the switch that powers the block is controlled. The second section, which is the example proper, is a LD section which uses a READ_VAR function block to send the message. An ADDR function block is also needed to format the address correctly. Please Note: The ADDR block uses XWay addressing, ie (network, station). This is setup in the Messaging Tab of the Ethernet network. The station number MUST be between 100 and 163 for Modbus messages. If it is not no error message will be received and the Write_Var will appear to be working although nothing will be sent.
The application is annotated and should provide a start for the user of the READ_VAR block. It is up to the user to change the PAC type to meet his or her own requirements. There is both a .zef and .xef version in the zip file.

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