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A simple example of a READ_VAR application for an M340 or M580 PLC

Published date: 24 July 2019

Goals and Symptoms

Simple example of READ_VAR Ethernet communication for M340 or M580 PLC
Causes and Fixes

The Ethernet 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 read_var.zef will write ten registers to another device. (The code for a server M340 is also attached. The M580 setup is similiar). The application consists of two sections, The first is an ST section in which variables are initialized. The second section, which is the example proper is a LD section which simply uses an ADDM block to feed the address to the READ_VAR function block. The main area of difficulty is the formatting of the address. See the help in Unity Pro. In this example the format ‘NETWORK ’ is used; where NETWORK is the name given to the network associated with the client device (sender), and SERVERIP is the IP address of the server (target). In the example the IP address of the client is, The IP of the server is These should be changed to suit your own addressing scheme. The application is annotated and should provide a start for the user of the READ_VAR block.

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