AT Command Sets

Long ago, Hayes set a standard for modem commands with its Smartmodem 300. Most modem manufacturers adopted this command set in order to call themselves "Hayes compatible." The command set used by the Smartmodem 300, as well as most modems today (with a few advanced commands), is known as the AT command set. AT stands for attention, and precedes all (with the exception of A/) commands directed to the modem. For example, when dialing, it is necessary for either the software or the user to issue an ATDT or ATDP command followed by the number and enter. AT tells the modem that it is about to receive a command. DT tells it to dial by tone, while DP tells it to pulse dial. Finally, the modem dials the number given to it after the command. Different modems do have slightly different command sets, but generally most modems follow the standard set by Hayes.

Modem Commands

The modem supports the standard and extended Hayes* AT command set. The AT prefix (also known as the Attention Code), signals the modem that one or more commands are to follow. These commands are industry standard language used to communicate with the modem. Your modem is always either in the command mode, or the on-line mode. The modem starts up in command mode when it is first switched on. Commands are only accepted by the modem when it is in command mode. Commands input when the modem is on-line, are treated as data, not as commands. Commands may be entered from the terminal mode of most communications software packages.

Settings made via AT commands are automatically reused by the modem until another command is received to change them, or the modem is turned off.

Setting Up the Command Line

All commands except two, must begin with the characters AT. The two exceptions are the escape sequence (+++), and the repeat command (A/). The command line prefix (letters AT) and the command sequences which follow, can be typed in upper case, or lower case, but case must not be mixed. More than one command can be typed on one line, separated by spaces if you wish for easier reading. The spaces are ignored by the modem's command interpreter. The command line buffer accepts up to 39 characters including "A" and "T". Spaces, carriage return, and any line feed characters do not go into the buffer, and don't count against the 39 character limitation. If more than 39 characters are entered, or a syntax error is found anywhere in the command line, the modem returns an ERROR result code, and the command input is ignored.

Command Acknowledgment

The modem will acknowledge most commands almost immediately. As soon as you type in a command ending with the carriage return character your modem will respond with OK.

Basic Commands

With the following basic AT commands, you can make calls directly, select the dialing method (tone or pulse), control the speaker volume, and perform a number of other basic modem operations. IMPORTANT: You must be in the Command mode of your communication software to use the AT commands. Refer to the documentation that came with your communications software for information on entering the Command mode.

• AT -- This prefix begins all but two commands you issue to the modem locally, and tells the modem "ATtention! commands to follow".
• D -- Dial. Use the D command to dial a telephone number from the command line. The format of the command is as follows:
  ATD [string]

  The string parameter can contain up to 45 characters, the phone number and dial modifiers. The dial modifiers instruct a modem how to place the call. Do not use any added characters, such as parentheses or hyphens in the phone number.

• +++ -- Escape Character Sequence. After you have connected to another modem, you may need to return to command mode to adjust the modem configuration, or more commonly, to hang up. To do this leave your keyboard idle (press no keys) for at least one second, then press "+" three times. This is one of the two commands that do not use the AT prefix, or a carriage return to enter. After a moment, the modem will respond with OK indicating you have been returned to Command mode.
• A/ -- Repeat. This command does not use the AT prefix nor does it require a carriage return to enter. Typing this command causes the modem to repeat the last command line entered, and is most useful for redialing telephone numbers which are busy.
• &Fn -- Factory Defaults. This command (in which n=0 or 1) returns all parameters to the selected set of factory defaults.
• H -- Hang Up. This command tells the modem to go "on-hook", or to disconnect the telephone line.
• O -- On-Line. This command returns the modem to the on-line mode and is usually used after the escape sequence (+++) to resume communication.
• Zn -- Reset Modem. This command (in which n=0 or 1) resets the modem to the configuration profile stored in non-volatile memory location 0 or 1.

Dial Command Modifiers

Command modifiers define additional parameters to the modem that instruct the modem to perform certain functions automatically when dialing a phone number. They are only valid when they are contained in a dial string (that follows the D command). The commands that are used to accomplish this task are called dial modifiers, and are placed in the dial string prior to issuing the command.

Syntax: ATD{dial modifier} 1234567 [Enter]

Basic dial modifiers are:

• P -- Pulse dialing. Also known as rotary dialing, this dial modifier follows the D command and precedes the telephone number to tell the modem to dial the number using pulse service.
• T -- Tone dialing. This modifier selects the tone method of dialing using DTMF tones. Note: Tone and pulse dialing can also be combined in a dial command line when both dialing methods are required.
• ; -- Resume command mode after dialing. If you need to dial a number that is too long to be contained in the command buffer (45 characters for the D command), use the (;) modifier to separate the dial string into multiple dial commands. All but the last command must be end with the ; modifier.
• , -- Pause While Dialing. The comma (,) dial modifier causes the modem to pause while dialing. The modem will pause the number of seconds specified in S-Register S8 and then continue dialing. If a pause time longer than the value in S-Register S8, it can be increased by either inserting more than one (,) in the dial command line or changing the value of S-Register S8. In the following example, the command accesses the outside (public) telephone line with the 9 dial modifier. Because the comma (,) dial modifier is present, the modem delays before dialing the telephone number 5551212. Ex: ATD 9, 5551212 [Enter]
• ! -- Using the Hook Flash. The exclamation mark (!) dial modifier causes the modem to go on-hook (hang up) for one-half second and is equivalent to holding down the switch-hook on your telephone for one-half second. This feature is useful when transferring calls.
• W -- Wait for a Subsequent Dial Tone. The W dial modifier causes a modem to wait for an additional dial tone before dialing the numbers that follow the W. The length of time the modem waits depends on the value in S-Register S7. The modem can be instructed to dial through PBXs (Private Branch Exchanges) or long-distance calling services that require delays during dialing. This can be done with the W command to wait for a secondary dial tone or with a comma (,) command to pause for a fixed time and then dial. Ex: ATDT 9 W 1 2155551212 [Enter]

Making a Call

The following examples show how to place a call using several of the dial modifiers.

ATD 1234567

The modem dials the telephone number 1234567 and then waits for a carrier from a distant, or remote modem. If no carrier is detected within a given time (as defined by the initial settings in S-Register 6), then the modem automatically releases the line and sends a NO CARRIER result code. If a carrier is detected, the modem gives a CONNECT result code and goes on-line, allowing communications with the remote modem. The connection between the two modems ends when any of the following occurs causing the modem to hang up, return to command mode, and send the NO CARRIER response:

• The local modem loses the carrier signal from the remote modem.
• The Hang Up command (H) is sent.
• The DTR interface signal is dropped between the local DTE and modem when the &D2 or &D3 command is in effect.

AT &F1DT9,P5551234 [Enter]

This command restores the factory default settings, dials, using tones, a 9 to access an outside line, pauses briefly, then pulse dials the number 555-1234.

Manually Dialling with the Telephone

To manually dial the phone, you should be in your communications software's command mode.

• 1.Lift the receiver of the telephone and dial the number you wish to call.
• 2.Type ATH1 and press [Enter] to connect the modem and then hang up the receiver.
• 3.Type AT0 and press [Enter] to tell the modem to go on line.

Manual Answer

When the automatic answer feature (S-Register S1) is not being used, incoming calls can be answered manually by typing:

ATA [Enter]

When an incoming call is received. The modem will answer the incoming call and [Enter] the on-line mode.