Abstract: A network access device (NAD) is comprised of a data set for coupling to a data communication trunk, a trunk control unit (TCU) connected with the data set, a trunk control interface (TCI) connecting the TCU to an internal data bus, an internal memory, a microprocessor control and a device interface for connection to a computer or some computer peripheral device. The data set provides for two-way communication from the data trunk to the TCU. Various network access devices connected to the data trunk communicate with one another. Every normal communication between NADs includes a concurrent, predictable state change in both NADs. The response to a message from one NAD to another includes information concerning what occurred upon receipt of the message. Various latches, sequencers, microprocessor logic and registers provide for functions according to various input and output states of response.