ModRSSim2 is a simulator of Modbus RTU and Modbus TCP devices. Using this utility, you can check operation of the OPC server without a real device. The official site of ModRSSim2 is
https://sourceforge.net/projects/modrssim2/
The previous variant of ModRSSim2 is ModRSSim (ModRSSim is no longer advancing) -
The utility does not require an installation. To launch the utility use the shortcut Run ModRSSim2 (see Description of Shortcuts).
If the utility is launched successfully, its main window is displayed:
The detail description of ModRSSim is contained in the separate help section