The next structure required to build a system is a sequential circuit. Figure 25 shows a sequential circuit constructed with asynchronous registers and a combinational circuit.

Figure 25. NULL Convention Logic sequential circuit.

The top four gates of the input register hold the inputs from the previous circuit. The top four gates of the output register receive the result values for the next register. The bottom four gates of the output register receive the internal state of the circuit and direct it back to the bottom four gates of the input register through the intermediate register at the bottom of the circuit which buffers successive DATA and NULL wave fronts from the output to the input of the circuit. The next DATA set arriving at the top four gates is synchronized with the previous DATA set's output arriving at the bottom four input gates from the feedback register.

The sequential circuit is symbolically complete. The behavior of the circuit is purely symbolically determined, the behavior of the asynchronous registers are purely symbolically determined and their combination into a system structure is purely symbolically determined. There is no external control authority. The circuit's behavior is driven completely by the DATA-NULL input wave fronts and acknowledge signal interaction with its predecessor circuit and with its successor circuit.

The basic elements to build a system (combinational circuits, registers and sequential circuits) have been demonstrated.