Hayes AT command set

Most dialup modems follow the Hayes Command Set to a large extent. The modem initialization string consists of a series of commands. It prepares the modem for communications, setting such features as dialing mode, waits, detection of the busy signal and many other settings. Newer modem communications programs reset the initializations string for you according to which menu options selected or which features are enabled.

For many years Hayes modems have been the standard. As the field of modem manufactures has grown, most have adhered at least loosely to the Hayes standard. The following is a partial list of the Hayes command set. (also called the AT commands, AT is short for attention).

The Hayes Command Set can be divided into four groups:

• Basic Command Set - A capital character followed by a digit. For example, M1.
• Extended Command Set - An “&” (ampersand) and a capital character followed by a digit. This is an extension of the basic command set. For example, &M1. Note that M1 is different from &M1.
• Proprietary Command Set - Usually started by either a backslash (“\”), or a percent sign (“%”), these commands vary widely among modem manufacturers.
• Register Commands - Sr=n where r is the number of the register to be changed, and n is the new value that is assigned.

A “register” is a specific physical location in memory. Modems have small amounts of memory onboard. This fourth set of commands is used to enter values in a particular register (memory location). The register will be storing a particular variable (alpha-numeric information) which is utilized by the modem and communication software. For example, S7=60 instructs your computer to “Set register #7 to the value 60”.

Although most commands are defined by a letter-number combination (L0, L1 etc.), the user of a zero is optional. In this example, L0 is the same as a plain L. Keep this in mind when reading the table below.

Here are some of the most important characters that may appear in the modem initialization string. These characers normally should not be changed.

• AT - Tells the modem that modem commands follow. This must begin each line of commands.
• Z - Resets the modem to it's default state.
• , (a comma) - Makes the software pause for a second. More than one can be used in a row. For example, ,,,, tells the software to pause four seconds. (The duration of the pause is governed by the setting of register S8.
• ^M - Sends the terminating Carriage Return character to the modem. This is a control code that most communication software translates as a Carriage Return.

When in data mode the escape sequence "+++" is used to return the modem to command mode.

The Basic Hayes Command Set

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Command	Description	Comments
A0 or A	Answer incoming call
A/	Repeat last command	Don't preface with AT. Enter usually aborts.
B0 or B	Call negotiation	V32 Mode/CCITT Answer Seq.
B1	Call negotiation	Bell 212A Answer Seq.
B2	Call negotiation	Verbose/Quiet On Answer
D	Dial	Dial the following number and then handshake in orginate mode. P - Pulse Dial T - Touch Tone Dial W - Wait for the second dial tone , - Pause for the time specified in register S8 (usually 2 seconds) ; - Remain in command mode after dialing. ! - Flash switch-hook (Hang up for a half second, as in transferring a call. L - Dial last number
E0 or E	No Echo	Will not echo commands to the computer
E1	Echo	Will echo commands to the computer (so one can see what one types)
H0	Hook Status	On hook - Hang up
H1	Hook status	Off hook - phone picked up
I0 or I	Inquiry, Information, or Interrogation	This command is very model specific. I0 usually returns a number or code, while higher numbers often provide much more useful information.
L0 or L	Speaker Loudness. Modems with volume control knobs will not have these options.	Off or low volume
L1		Low Volume
L2		Medium Volume
L3		Loud or High Volume
M0 or M	Speaker off	M3 is also common, but different on many brands
M1		Speaker on until remote carrier detected (i.e. until the other modem is heard)
M2		Speaker is always on (data sounds are heard after CONNECT)
N0 or N	Handshake Speed	Handshake only at speed in S37
N1		Handshake at highest speed larger than S37
O0 or O	Return Online	See also X1 as dial tone detection may be active.
O1		Return Online after an equalizer retrain sequence
Q0 or Q1	Quiet Mode	Off - Displays result codes, user sees command responses (e.g. OK)
Q1	Quiet Mode	On - Result codes are suppressed, user does not see responses.
Sn?		Query the contents of S-register n
Sn=r	Store	Store the value of r in S-register n
V0 or V	Verbose	Numeric result codes
V1		English result codes (e.g. CONNECT, BUSY, NO CARRIER etc.)
X0 or X	Smartmodem	Hayes Smartmodem 300 compatible result codes
X1		Usually adds connection speed to basic result codes (e.g. CONNECT 1200)
X2		Usually adds dial tone detection (preventing blind dial, and sometimes preventing AT0)
X3		Usually adds busy signal detection
X4		Usually adds both busy signal and dial tone detection
Z0 or Z	Reset	Reset modem to stored configuration. Use Z0, Z1etc. for multiple profiles. This is the same as &F for factory default on modems without NVRAM (non voltaile memory)

External Link

• http://docs.kde.org/en/3.1/kdenetwork/kppp/appendix-hayes-commands.html