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Using AT Commands and Result Codes







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[Table of Contents]
[Introduction]
[Communication Standards]
[Dialing]
[Answering]
[High Speed Communication]
[Fax Features]
[Solving Problems]
[If You Need Help]
[AT Commands]
[S Registers]
[Result Codes]
[Glossary]


This chapter discusses the following topics:

Issuing AT Commands
Command State and Online State
Escape Code (+++)
Command Line Syntax
Command Line Buffer
Result Codes


When you use your telecommunications software to perform a task (such as dialing a phone number), it uses your modem's AT commands to perform the task. It is usually easier to let your telecommunications software issue the commands for you than it is to issue the AT commands yourself.

However, when your telecommunications software can not perform a task (or you don't want it to), you can issue the AT command yourself. AT commands can be used to perform various tasks.

The Dialing and Answering sections in this manual explain how to use specific AT commands. This section explains how the AT commands work, and the AT Commands section lists them.

Issuing AT Commands
To issue AT commands, you must be in your software's terminal mode -- also called "local mode" or "command mode." In this mode, the commands you type go to the modem rather than to your software or the remote computer.

Some software uses terminal mode as soon as you start the program. To determine if you are in terminal mode, type:

AT[CR] ([CR] represents a carriage return.)

If the modem responds with OK, it means that the modem has received and understood the command. If you are not in terminal mode, refer to your software manual to determine how to get there.

Command State and Online State
The modem can operate in either command state or online state:

If you're connected to a network, Bulletin Board System, or other remote computer (i.e., you're online) you can transmit data, but you must change to command state to send AT commands.

To change from one state to the other, use one of the following methods:

Escape Code (+++)
The Escape from data mode may be used while you are online to force the modem to the command state without hanging up. It is implemented using T.I.E.S. (Time Independent Escape Sequence).

The escape sequence is a string of three ASCII plus signs (+++). To escape to command mode when online, issue +++ (no time restriction between the +s), then send an AT command (e.g., ATH). Do not press Carriage Return [CR] or Enter when you finish.

The modem responds to these steps with OK (or Ø if in numeric Result Code mode). If it doesn’t, either it received data during the guard time or the escape code was typed too slowly. The time between each plus sign ("+") in the escape code should not exceed the guard time.

The plus signs in the escape code can be changed to another ASCII character by changing the value in Register S2. Appendix D provide information on setting S Registers.

Command Line Syntax
The prefix AT ("ATtention") must start each command line except A/. The modem examines the AT code to determine your computer's rate, parity, word length, and number of stop bits. It then stores your current configuration in its memory.

Most multiple commands can appear on one command line. You can separate these with spaces to improve readability if you like. (The modem ignores the spaces.)

Each command line (except A/) must end with a Carriage Return ([CR] -- ASCII 13). The command is not acted upon until the Carriage Return is pressed. The following are valid examples:

AT DT 555-5055[CR] (Dial a touch tone phone number.)
ATH[CR] (Hang up.)
AT &F &C1 &D2 &W[CR] (Configure the modem to the following and store it to memory.)
A/ (Re-execute last command.)

Commands are executed in order, from left to right. If a command contradicts an earlier command, the later command takes precedence.

Command Forms
The International Telecommunications Union (ITU) has made an effort to standardize the AT command set for modems. Some of the resulting command forms are:

Complex extended commands also have three forms. These forms allow you to:

Command Line Buffer
The modem places each command issued into a command buffer that holds up to 160 characters. The modem does not place the AT, carriage return, or line feed characters into the buffer, so the buffer’s 160 spaces hold only actual command characters (and any spaces between them). If a command line contains more than 160 characters, the modem stops showing what you type on the screen and it ignores additional commands.

Result Codes
After the modem receives a command, it returns a Result Code. The most common Result Code is OK, which means the modem understood your command.
Result Codes can be represented by English words (verbose mode) or with numbers (numeric mode). The Result Code command ATV tells the modem whether to respond with verbose or numeric Result Codes. Words are preceded and followed by carriage return and line feed characters. Numbers are followed by a carriage return.

For more information, refer to the list of Result Codes.

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