# Serial Programming/Modems and AT Commands/Special Commands and Character Sequences

Serial Programming: Introduction and OSI Network Model -- RS-232 Wiring and Connections -- Typical RS232 Hardware Configuration -- 8250 UART -- DOS -- MAX232 Driver/Receiver Family -- TAPI Communications In Windows -- Linux and Unix -- Java -- Hayes-compatible Modems and AT Commands -- Universal Serial Bus (USB) -- Forming Data Packets -- Error Correction Methods -- Two Way Communication -- Packet Recovery Methods -- Serial Data Networks -- Practical Application Development -- IP Over Serial Connections
* Hayes-compatable Modems and AT Commands References:
* -- Special AT Commands and Character Sequences
* -- AT Commands A - M
* -- AT Commands N - Z
* -- AT& Commands

## Special Commands and Character Sequences

This module explains the special commands and character sequences as used by a "generic" Hayes-compatible modem. Different modems use slightly different commands. However, this list is supposed to be as "generic" as possible, and should not be extended with modem-specific commands. Instead it is recommended to provide such command lists in an Appendix.

#### AT: Command Prefix

Syntax:
Description:
Almost every line with commands start with the AT prefix, followed by one or more commands, terminated with a Carriage Return character.
See the #AT Command Format section for details.
Related Commands and Registers:
* #<CR>: End-of-line Character
* #A/: Repeat Last Command

#### +++: Escape Sequence

Syntax:
Description:
See: #On-line State to Command State
Result Codes:
Result Codes :
| Code | Description |
| OK | Escape into command mode was successful |
Related Commands and Registers:
* #O: On-Line Command
* #S2: Escape Sequence Character -- Register to change the character
* #S12: Escape Sequence Guard Time -- register to change the guard time

#### <CR>: End-of-line Character

Syntax:
Description:
In command mode the end of line character (default ASCII 13, alias ASCII 0dH, alias <CR>, alias <Ctrl-M>, alias carriage return) marks the end of a command line. The modem starts to execute the command line after reception of the end-of-line character.
Result Codes:
Result Codes :
| Code | Description |
| OK | All commands in the command line were successfully executed. |
| ERROR | One command in the command line failed. |
Related Commands and Registers:
* #AT Command Format
* #<BS>: Backspace Character
* #S3: Carriage Return Character -- Register to change the character.

#### <BS>: Backspace Character

Syntax:
Description:
In command mode the backspace character (default ASCII 8, alias ASCII 08H, alias <BS>, alias <Ctrl-H>, alias backspace) can be used to edit the command line.
Result Codes:
Result Codes :
| Code | Description |
| <none> | Does not generate a result code. |
Related Commands and Registers:
* #AT Command Format
* #<CR>: End-of-line Character
* #S5: Backspace Character -- Register to change the character.

#### <CAN>: Cancel Character

Syntax:
Description:
In command mode the cancel character (default ASCII 24, aka ASCII 18H, aka <CAN>, aka <Ctrl-X>) cancels a command line as long as the command line has not been terminated with the #<CR>: End-of-line Character.
Result Codes:
Result Codes :
| Code | Description |
| <none> | No result code is returned. |
Related Commands and Registers:
* #AT Command Format
* #<BS>: Backspace Character

#### A/: Repeat Last Command

Syntax:
Description:
The command repeats the last command. It differs from other commands in two ways:
* It is not prefixed with the AT command.
* It should not be followed by the usual <CR>:
The most typical usage is to repeat a previous dialing command that failed because of a BUSY line.
Related Commands and Registers:
* #D: Dial Command
Serial Programming: Introduction and OSI Network Model -- RS-232 Wiring and Connections -- Typical RS232 Hardware Configuration -- 8250 UART -- DOS -- MAX232 Driver/Receiver Family -- TAPI Communications In Windows -- Linux and Unix -- Java -- Hayes-compatible Modems and AT Commands -- Universal Serial Bus (USB) -- Forming Data Packets -- Error Correction Methods -- Two Way Communication -- Packet Recovery Methods -- Serial Data Networks -- Practical Application Development -- IP Over Serial Connections
* Hayes-compatable Modems and AT Commands References:
* -- Special AT Commands and Character Sequences
* -- AT Commands A - M
* -- AT Commands N - Z
* -- AT& Commands