Messaging between an ultra-tag and external microcontroller. In an embodiment, a transmitting device is communicatively connected to a receiving device by a clear-to-communicate line and request line. When data to be transmitted has normal priority, the transmitting device detects whether the clear-to-communicate line indicates that the receiving device is available, indicates a request to transmit on the request line if so, waits until the receiving device is available and then indicates a request to transmit on the request line if not, after indicating a request to transmit, transmits the data when the clear-to-communicate line indicates that the receiving device is unavailable, and, after transmitting the data, releases the indication of the request to transmit on the request line. On the other hand, when the data has high priority, the transmitting device indicates a request to transmit on the request line, regardless of an indication on the clear-to-communicate line.