Noel,
I have used a C library on Github that provides support for Modbus TCP and EtherNet/IP. I have not used the Modbus TCP portion but the EtherNet/IP works very well with Rockwell ControlLogix PLCs. The Github URL is:
https://github.com/libplctag/libplctag
I’ve written a Xojo wrapper module for the library. You can get it at:
libplctag
The wrapper is written for version 2.3 but it should work for the current 2.4 version. I usually put the dll file in the same folder as the executable.
Dennis