National Instruments advertises a free native Modbus TCP/IP library that has been used successfully with our Modbus TCP compatable devices. Some of its features include
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Works with standard serial and Ethernet ports
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Facilitates Modbus TCP/IP and serial communication
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Compatible with NI LabVIEW Real-Time Module
Sample Screen Shots
Files
The File Attachment (.zip file) below includes a version of their library and some example VI's that read/write the T7 or T4's test registers.
LabJack_nimodbus_examples_2013_12_21.zip