A Serial Break condition is defined in the context of asynchronous serial data transmission as a series of null bits longer than a single character, including the start and stop bits. In other words, the UART expects to see a stop byte. Many UARTs signal a BREAK condition as being held for a duration of two characters, but this duration could be as long as 250ms.
Serial break conditions can be used in a variety of ways. It can be used to pass another I/O line to control a dry contact such as a relay. Another way it can be used is to mark the event to change to a different data format. It was also used for one device to signal another to reset, commonly used for PCs to reset any attached modems. Some manufacturers will also use this scheme in their proprietary protocols, thus allowing shifting to different data formats such as a baud rate to be manifest.
The Digi XStream radio can pass the serial break condition on the DIN pin to the DOUT pin of the receiving modem. The XStream module will need to be programmed with firmware v5000 or higher. To enable a serial break, the BK (Serial Break Passing) command will need to be set to 1 [enable serial break function]. To timeout a serial break the BO (Serial Break Timeout) parameter will need to be set (0-0xFFFF seconds). The IU (Update Timer) parameter is used to send periodic updates of the I/O lines.