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Chapter 1 – Introduction
Multi-Tech Systems, Inc. AT Commands Reference Guide (S000273F) 4
Chapter 1 – Introduction
AT commands are used to control the operation of your modem. They are so called because each command must be
preceded by the characters AT to get the ATtention of the modem.
AT commands can be issued only when the modem is in command mode or online command mode.
• The modem is in command mode whenever it is not connected to another modem.
• The modem is in data mode whenever it is connected to another modem and ready to exchange data.
• Online command mode is a temporary state in which you can issue commands to the modem while
connected to another modem.
To put the modem into online command mode from data mode, you must issue an escape sequence: Type +++ and
wait for the OK response. Then issue the hang-up command by typing ATH<CR>. <CR> indicates that you must
click Enter). To return to data mode from online command mode, type the command ATO.
To send AT commands to the modem you must use a communications program, such as PhoneTools, a
communications program included with your modem, or HyperTerminal in Windows 95, 98, NT 4.0, Me, 2000, XP.
You can issue commands to the modem either directly or by typing them in the terminal window of the
communications program. You issue them indirectly by configuring the operating system or communications program
to send the commands automatically. Fortunately, communications programs make daily operation of modems
effortless by hiding the commands from the user. Most users, therefore, need to use AT commands only when
reconfiguring the modem; e.g., to turn autoanswer on or off.
The format for entering an AT command is ATXn, where X is the command and n is the value for the command,
sometimes called the command parameter. The value is always a number. If the value is zero, you can omit it from
the command. Thus, AT&W is equivalent to AT&W0.
Most commands have a default value, which is the value that is set at the factory. Default values are provided with
each command in the AT Commands section.
You must press Enter to send the command to the modem.
Any time the modem receives a command, it sends a response known as a result code. The most common result
codes are OK, ERROR, and CONNECT. For a table of valid result codes, see the “Result Codes” sections of this
manual.
You can issue several commands in one line, in what is called a command string.
The command string begins with AT and ends when you press Enter. Spaces to separate the commands are
optional; they are ignored by the command interpreter. The most familiar command string is the initialization string,
which is used to configure the modem when it is turned on or reset, or when your communications software calls
another modem.