occur, and incoming characters will be lost. Underrun error An underrun error occurs when the UART transmitter has completed sending a character and the transmit buffer is empty. In asynchronous modes this is treated as an indication that no data remains to be transmitted, rather than an error, since additional stop bits can be appended. This error indication is commonly found in USARTs, since an underrun is more serious in synchronous systems. Framing error A UART will detect a framing error when it does not see a "stop" bit at the expected "stop" bit time. As the "start" bit is used to identify the beginning of an incoming character, its timing is a reference for the remaining bits. If the data line is not in the expected state (high) when the "stop" bit is expected (according to the number of data and parity bits for which the UART is set), the UART will signal a framing error. A "break" condition on the line is also signaled as a framing error. Parity error A parity error occurs when the parity of the number of one-bits disagrees with that specified by the parity bit. Parity checking is often used for the detection of transmission errors. Use of a parity bit is optional, so this error will only occur if parity-checking has been enabled. Break condition A break condition occurs when the receiver input is at the "space" (logic low, i.e., '0') level for longer than some duration of time, typically, for more than a character time. This is not necessarily an error, but appears to the receiver as a character of all zero-bits with a framing error. The term "break" derives from current loop signaling, which was the traditional signaling used for teletypewriters. The "spacing" condition of a current loop line is indicated by no current flowing, and a very long period of no current flowing is often caused by a break or other fault in the line. Some equipment will deliberately transmit the "space" level for longer than a character as an attention signal. When signaling rates are mismatched, no meaningful characters can be sent, but a long "break" signal can be a useful way to get the attention of a mismatched receiver to do something (such as resetting itself). Computer systems can use the long "break" level as a request to change the signaling rate, to support dial-in access at