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. This practice carried over to teleprinter use on time-sharing computers. The resulting noise got the sending operator's attention. When this occurred, the teleprinter mechanisms continually actuated without printing anything, as the all-0s character is the non-printing NUL in both Baudot and ASCII. Holding down a special 'break' key opened the loop, forcing it into a continuous logic 0, or 'spacing', condition. 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. (A physical break in the telegraph line would have the same effect.)

Both sounders stop responding to the sender's keying, alerting the sender. As a consequence, the receiving operator could interrupt the sending operator by opening their key, breaking the circuit and forcing it into a ' spacing' condition. So the receiving operator has to hold their key down or close a built-in shorting switch in order to let the other operator send. 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). 6 Usage for breaking the program's executionĪ standard telegraph circuit connects all the keys, sounders and batteries in a single series loop.

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