Editing Break key (section)

==History==
A standard telegraph circuit connects all the keys, [[Telegraph sounder|sounders]] and batteries in a single series loop. Thus the sounders actuate only when both keys are down (closed, also known as "[[mark and space|marking]]" &mdash; after the ink marks made on paper tape by early printing telegraphs). 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. As a consequence, the receiving operator could interrupt the sending operator by opening their key, breaking the circuit and forcing it into a "[[mark and space|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).

[[File:ASR-33 Teletypewriter REPT, BREAK keys.jpg|thumb|Break key on [[Teletype Model 33]]]]
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.<ref>{{Cite_book |url=https://deramp.com/downloads/teletype/Model%2033/TTY-33%20Theory%20of%20Operation.pdf#page=7 |title=33 Keyboard: General Description and Principles of Operation |date=1971 |publisher=[[Teletype Corporation]] |year= |location=Skokie, Illinois |page=7 |language=en |chapter=Line Break}}</ref><ref name=":1">{{Cite_book |url=https://www.navy-radio.com/manuals/tty/tgc14-mite-course-7207.pdf#page=19 |title=AN/TGC-14(V) Teletypewriter I & O Maintenance. Study guide |date=1972 |publisher=Department of Communications and Missile Training. [[Sheppard Air Force Base]] |location=Texas |page=3-5 |language=en |chapter=Line Break}}</ref> When this occurred, the teleprinter mechanisms continually actuated without printing anything,<ref name=":1" /> as the all-0s character is the non-printing ''NUL'' in both [[Baudot code|Baudot]] and [[ASCII]]. The resulting noise got the sending operator's attention.

This practice carried over to teleprinter use as a [[Computer terminal|terminal]] 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<ref>{{Cite book |last=Schur |first=Lee David |url=https://archive.org/details/timesharedcomput0000schu/ |title=Time-shared computer languages: an introduction to conversational computing |date=1973 |publisher=Reading, Mass., Addison-Wesley Pub. Co |others=Internet Archive |isbn=978-0-201-06761-3 |pages=270 |chapter=PL/1 (RUSH). Using the terminal}}</ref> or forced the operating system to prompt for a login. Similarly to teleprinters, [[Video terminal|video terminals]] kept the {{keypress|Break}} key<ref>{{Cite_book |title=[[Tektronix 4010]] and 4010-1 Users Manual |date=1972 |publisher=[[Tektronix, Inc.]] |year= |page=2-3 |chapter=Section 2 Familiarization and Checkout |chapter-url=https://w140.com/tekwiki/images/c/c2/070-1225-00.pdf#page=15}}</ref><ref>{{Cite_book |title=Lear Siegler [[ADM-3A]] Operators Manual |publisher=[[Lear Siegler]] |page=3-8 |chapter=3.2.2 Special Function Keys |quote=BREAK. This key activates the standard teletypewriter BREAK function, normally used to interrupt an incoming message. |chapter-url=https://deramp.com/downloads/mfe_archive/030-Terminals/LearSiegler/ADM-3A/Lear%20Siegler%20ADM3A%20operators%20manual.pdf#page=41}}</ref> which put transmission line to zero state.<ref>{{Cite_book |url=https://vt100.net/docs/vt100-ug/ |title=VT100 User Guide |date=1984 |publisher=[[Digital Equipment Corporation]] |edition=3rd |chapter=Chapter 3 Programmer Information |quote=Typing the BREAK key causes the transmission line to be forced to its zero state |chapter-url=https://vt100.net/docs/vt100-ug/chapter3.html}}</ref>

With its function of interrupting terminal communication, the {{keypress|Break}} key was adopted by software terminal emulators and in applications which use serial protocols such as [[RS-232]]. This concept of interruption also extends to the boot process of computers, network devices and to halting operations in computer applications like command-line interfaces, debuggers and spreadsheets.<ref>{{Cite web |title=What Does the Break Key Do in Windows & Virtual Machines? |url=https://www.lenovo.com/hk/en/glossary/break-key/ |accessdate=2025-05-17 |website= |publisher=[[Lenovo]] HK}}</ref>