To build a system out of NULL Convention combinational logic circuits we must be able to manage the communication and interaction among its component circuits. There must be an asynchronous register, as shown in Figure 19, that monitors completeness of resolution as well as readiness to accept new input data sets and stores the complete set of data values and the all NULL values between circuits. Each combinational circuit will have an asynchronous register at its input.

Figure 19. The asynchronous register.

The asynchronous register shown in Figure 20 is simply a rank of NULL Convention Logic threshold gates with feedback hysteresis (Intermediate value gates won't work here) and a single gate that watches for complete data sets and all NULL states. It will manage the interaction among combinational circuits and is itself a very simple NULL Convention Logic circuit.

Figure 20. The NULL Convention Logic asynchronous register.

Each register gate (the threshold 2 gates) receives one wire with a data meaning and one wire with a control meaning. The threshold 4 gate is the watcher gate. When it sees a complete set of DATA values it will assert DATA and when it sees all NULL values it transitions to asserting NULL. When the watcher gate sees a complete data set, which in this case is one DATA value each for A, B, C and D input groups, it means that a complete data set has been received and stored by the register gates and it will transition its result value to DATA. The watcher gate will continue to assert the DATA value until all of its input values are NULL, which means that the register gates have received and stored all NULL values.