A character may be prefixed by a parity bit for error detection purposes. Errors occur when the receiver cannot make sense out of the signals it receives. A framing error occurs when a receiver's samples of a pulse contain both 1's and 0's. Character overrun errors occur when the CPU does not extract a character from a receiver before another is received. Receivers report break errors as framing error.
Some systems use the break condition to trigger special handling.
The Xinu downloader uses this mechanism to gain control of the system. The higher bytes of all registers except RBUF are unused. The XBUF register is used to output data to a terminal. Writing a character to the XBUF register address causes the SLU to capture the character and start transmitting it on the serial output line. The RBUF register is used for receiving input data.