Hi!
I can give you a reference to a project as a quick starting point. Go to https://mongoose.ws/wizard/#/output?board=h755&ide=CubeIDE&rtos=baremetal and navigate to the 'Settings' panel. In the "Modbus-TCP - PLC support" section, make sure to enable TCP modbus. This will generate a full CubeIDE project for your H755, which uses the Mongoose Networking library to implement a mock Modbus server on port 502, which you can customize later on according to your needs (search for "modbus" in this documentation page for a better understanding).
To download the project locally, in the same 'Settings' panel, under 'Source Code Generator', select the directory where you want the project to be downloaded and then click the blue 'Generate C/C++ code' on the upper-right corner of the page.
The project should be downloaded to your station. You can then open it with Cube, build it and flash the firmware to your board.
You can also check this video tutorial and this example from the Mongoose repository to quickly get up to speed on creating your own custom Modbus server with Mongoose
Hope this helps you get started!
