in fig1 or some other time delay as required by the specific safety system such variable names may be either typed in or selected from the drop down list in a particular cell where prior names of such variables in the vdt are shown or the existing name may be simply typed over with a resulting change in the variable detail table and the function block diagram network as pointed out above there are a number of internal variables which are set by default by the system each internal variable is a variable type of local and begins with an underscore for example _co 1 b as shown in the fbd network pane 29 of fig1 a is not directly accessible but other variables may be declared which incorporate the value of the internal cause state and effect state variables and may be used elsewhere in the logic of the matrix diagram the entry of such shadow variables may be set forth in the blank cells of variable detail table pane 28 and the properties modified using the properties dialog box accessible from the fbd network pane as described hereafter fig1 shows a typical properties dialog box 43 which allows.