Page "Universal asynchronous receiver/transmitter" Paragraph 37

Some equipment will deliberately transmit the " break " level for longer than a character as an out-of-band 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 ).

Unix-like systems can use the long " break " level as a request to change the signaling rate, to support dial-in access at multiple signaling rates.