The USNAP (ANSI/CEA-2045) standard defines the hardware interface, physical dimensions, data transfer, message contents and protocol specifics for interconnecting Home Automation Network (HAN) and Home Energy Management System (HEMS) devices to multiple different communications platforms.

Internally, the DC format of the Standard relies on the Serial Peripheral Interface (SPI) port found on most communication chips as the transport layer.  The AC format uses the RS-485 serial interface

You can obtain the ANSI/CEA-2045 standard through the Consumer Electronics Association (CEA) here.