Break key

The Break key of a computer keyboard refers to breaking a telegraph circuit, and originated with 19th century practice. In modern usage, the key has no well-defined purpose, but while this is the case it can be used by software for miscellaneous tasks, such as to switch between multiple login sessions, to terminate a program, or to interrupt a modem connection.

Because the break function is usually combined with the pause function on one key since the introduction of the IBM Model M 101-key keyboard in 1985, the Break key is also called the Pause key. It can be used to pause some computer games.

A standard telegraph circuit connects all the keys, sounders and batteries in a single series loop. Thus the sounders actuate only when both keys are down (closed, also known as "marking" — after the ink marks made on paper tape by early printing telegraphs). So the receiving operator has to hold his key down, or close a built-in shorting switch, in order to let the other operator send. As a consequence the receiving operator could interrupt the sending operator by opening his key, breaking the circuit and forcing it into a "spacing" condition. Both sounders stop responding to the sender's keying, alerting the sender. (A physical break in the telegraph line would have the same effect.)

The teleprinter operated in a very similar fashion except that the sending station kept the loop closed (logic 1 or "marking") even during short pauses between characters. Holding down a special "break" key opened the loop, forcing it into a continuous logic 0 or "spacing" condition. When this occurred, the teleprinter mechanisms continually actuated without printing anything as the all-0's character is the non-printing NUL in both Baudot and ASCII. The resulting noise got the sending operator's attention.

This practice carried over to teleprinter use on time-sharing computers. A continuous spacing (logical 0) condition violates the rule that every valid character has to end with one or more logic 1 (marking) "stop" bits. The computer (specifically the UART) recognized this as a special "break" condition and generated an interrupt that typically stopped a running program or forced the operating system to prompt for a login. Although asynchronous serial telegraphy is now rare, the Break key once used with terminal emulators can still be used by software for similar purposes.

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