First we'll construct a half adder circuit. This is simply an XOR gate for the sum and an AND gate for the carry, both connected to the same two inputs, and looks like this:

Since this is a subcircuit, we will eventually need a way to refer to the inputs and outputs of this subcircuit. Thats what the symbols labelled "A", "B", "SUM" and "Carry" are. These are input and output "pins". Find the symbols in the toolbar and create them by filling in the names in the dialog boxes when they appear. Then create the XOR and AND gates and connect the outputs of the gates to the inputs of the "SUM" and "Carry" output pins.

The wiring between the input pins A and B and the gates is slightly more complex this time because we have to connect one output (e.g., A) to two inputs (the XOR gate and the AND gate). Its easy to do. First connect the output of A to the input of the XOR by drawing a wire.

Next let's draw a wire starting at the middle of the A-to-XOR wire to the top input of the AND gate. First start a wire (via menu or hot key), then drag the wire start circle so that it touches the existing wire somewhere along its length. The wire start circle and the wire will become green when a connection is possible. Left click to connect, then finish the wire to the input of the AND gate. Use the same process to finish connecting B.

This completes the subcircuit. Now let's create another circuit, a full adder, that includes to half adder subcircuits. Without closing the "ha" circuit create a new circuit called "fa". Note the tabs showing that both circuits are avialable to be editted simply by clicking on the associated tag.

The circuit we will construct looks like this:

The two large squares represent "imported" copies of the half adder circuit we just created. These are not links to the half adder, but two distinct copies. We could even change one of the half adder copies and leave the other alone if we wanted to. Also note that a subsequent change to the original half adder circuit will NOT be reflected in the imported copies.

Lets first import the two half adders, then you can hook things up. Click on the Import pull-down list at the right end of the toolbar. The circuit "ha" will appear. Select it. Type in the name "upper" in the dialog box and place the resulting element somewhere in the drawing area. Now do the same thing again, this time giving it the name "lower". "upper" and "lower" are the local names of the subcircuits in the larger "fa" circuit. Now add the OR gate and hook things up.

Just for fun let's modify one of the imported half adders by replacing the XOR gate with the AND/OR/NOT equivalent. Right click on the "upper" element and select modify. Note that the imported circuit appears, and that there is now a tab with the label "upper in fa". This clearly identifies the circuit you are editting as the one called "upper", which is a subcircuit of "fa". There is no longer any association of the subcircuit with the original "ha" circuit.

Now delete the XOR gate by right clicking on it and selecting delete. Then add the equivalent AND, OR and NOT gates. You'll probably have to move things around to make room. When you are done you must close the subcircuit circuit in order to make any changes to the main circuit that contains it.

Now test your full adder circuit by attaching constant inputs and putting probes on the outputs. It should be interesting to see the resulting output signal traces with all the various gate propagation delays.

If you have time, try creating an N-bit adder circuit using full-adder subcircuits. Subcircuits can be arbitrarily nested!