A combinational subcircuit can be specified via a truth table instead of gates and wires. The truth table editor allows you to add, rename and remove input and output variables, specify output values for all input assignments, use input and output don't-cares, and re-arrange the appearance of the truth table.
To add a new input variable, type the name the "New Inputs" field and hit return. The new variable will be added and input value assignments generated. Input variables can be renamed by clicking on the variable, picking the rename option, and typing the new name in the dialog box that appears.
To add a new output variable, type the name in the "New Outputs" field and hit return. The new variable will be added with all assignments set to don't-cares. Output variables can be renamed using the same process as renaming input variables.
An output value can be changed by clicking on the value. Each click will toggle the value from don't-care to 0, to 1, and then back to don't-care.
An input value assignment can be changed to a don't-care by clicking on it. However this can't be done if the two rows of the truth table that will collapse into one to account for the don't-care have different values for any of the output variables. For example, in the truth table below, the input value assignment shown in blue cannot be made into a don't care because the two values for the output "f" are different.
a b | f ------+--- 0 0 | 0 0 1 | 1 1 0 | 0 1 1 | 0
Input variables can be removed from the table, but only if there are no resulting conficts among any of the output values. To remove an input variable, click on it and pick delete.
Ouput variables can be removed from the table by simply clicking on the variable and picking delete.
The position of an input or output variable in the truth table can be adjusted by clicking on the variable and picking move left or move right. The entire column will be moved left or right as appropriate. When an input variable is moved, the rows of the truth table will be re-ordered to preserve the property that rows increase in binary value as you go down the table.