MODBUS Send window uses Terminal Session to connect to the serial device. A terminal session can be created in Terminal tool window or by the running script. It will then appear immediately in the Session list in the MODBUS Send window. When the terminal session is closed, it is automatically removed from the list.
You cannot use MODBUS Send window if you have no serial devices installed on your computer.
The MODBUS Send window controls stay disabled until you configure and select a terminal session.