Bell character

A bell code (sometimes bell character) is a device control code originally sent to ring a small electromechanical bell on tickers and other teleprinters (remote printers) and teletypewriters (such as Teletypes, abbreviated TTYs) to alert operators at the other end of the line, often of an incoming message. Though tickers punched the bell codes into their tapes,^[1] printers generally do not print a character when the bell code is received. Bell codes are usually represented by the label "BEL" and have been used since 1870^{[citation needed]} for instance in Baudot code.

To maintain backward compatibility, video display terminals (VDTs) that replaced teletypewriters included speakers or buzzers to perform the same function, as did the personal computers that followed. Modern terminal emulators often integrate the warnings to the desktop environment (for example, the Mac OS X Terminal will play the system warning sound) and also often offer a silent visual bell feature that flashes the terminal window briefly.

Representations

In ASCII and Unicode the character with the value 7 is BEL. It can be referred to as control-G or ^G in caret notation. Unicode also includes a character for the visual representation of the bell code, "symbol for bell" (␇) at U+2407.

In the 5-bit Baudot codes (created by Émile Baudot in 1870, patented in 1874, modified by Donald Murray in 1901, and standardized by CCITT as International Telegraph Alphabet No. 2 (ITA2) in 1930), BEL is represented by the number 11 (0x0B) when in "figures" mode.

In the C programming language (created in 1972), the bell character can be placed in a string or character constant with \a ('a' stands for "alert" or "audible" and was chosen because \b was already used for backspace).

Usage

On Unix-like systems, MS-DOS or Windows, you can cause the equivalent of ringing the bell to happen by typing at the command prompt the command:

echo ^G

where the ^G is produced by holding down Ctrl and typing G. On Unix you may need to type Ctrl+V first to "quote" the ^G.

A program can get the same result by printing the BEL character to a terminal.

On modern systems this may not make a noise, it may instead make a visual indication such as flashing the screen, or do nothing at all.

See also

• C0 and C1 control codes (ISO 646)

References

1. ^ Baudot. [1]. Accessed Feb. 1, 2009.