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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 doesnt, 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
buffers 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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