By default, TwinCAT handles all variable addressing for you, you do not need to worry about one variable "stepping on" another. TwinCAT does allow you to specify a particular memory location, like this... sHMIOperatorID AT %MW0: WORD; Once you have begun to address variables by memory locations, you must excercise caution, as the compiler will give you no warnings. For details about the Struct, see the previous reply