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.^[2]

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 offer a silent visual bell feature that flashes the terminal window briefly to show the user when (and sometimes where) the alert occurred.

Contents 1 Representations 2 Usage 2.1 Terminals and terminal emulators 2.2 Command processors 2.3 Other 3 See also 4 References

[edit] Representations

When using 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), the bell code is labeled BEL and represented by the number 11 (0x0B in hexadecimal notation) when in "figures" mode.

When using IBM's Extended Binary Coded Decimal Interchange Code (an 8-bit encoding scheme developed in 1963, usually abbreviated EBCDIC), the bell code is labeled BEL and represented by the number 47 (0x2F in hexadecimal notation).

When using the 7-bit American Standard Code for Information Interchange (established in 1963, usually abbreviated ASCII), the bell code is labeled BEL and represented by the number 7 (0x07 in hexadecimal notation), which is often represented as the control character control-G or ^G.

In the 1972 standard ISO 646, the bell code is labeled and represented as it is in ASCII.

In the C Programming Language (created in 1972), the bell character is represented as '\a' ("alert" or "audible").

Unicode (originated in 1987) also uses the number 7 to represent the bell code, but the Unicode representation is written as U+0007. Unicode also includes a character for the visual representation of the bell code, "symbol for bell", U+2407 (␇).

[edit] Usage

[edit] Terminals and terminal emulators

Many terminals and terminal emulation programs use the control-G key combination to send the bell code.

[edit] Command processors

In the Windows command prompt, as with the earlier PC-DOS or MS-DOS command prompts on which it is based, and in the command prompt on Unix-like systems, the user can type the word "echo" followed by a control-G key combination, which will appear as

echo ^G

and when the user presses enter, the computer will emit a beep sound.

The bell can be disabled on Windows systems either through a registry or configuration file change, or through an application such as Microsoft PowerToys.

[edit] Other

In Microsoft Word (1997-2003) documents, the bell character may be used to indicate a change in justification, such as a separator between left-justified and right-justified text.

[edit] See also

• character encoding

[edit] References