Appendix B. The Hayes Modem Command Set

Here is a description of the Hayes Command Set. Most modems follow this command set to large extent. If you lost your modem manual or never had one in the first place, this reference might come in handy. I for instance finally found out how to turn my modems speaker off: ATM0 -- Finally: Silence!

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 you select, which features you enable, etc.

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. (called the AT commands). 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. For that reason, only a few of these commands are listed below.

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 computerese for 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 use 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 characters normally should not be changed.


Tells the modem that modem commands follow. This must begin each line of commands.


Resets the modem to its default state

, (a comma)

makes your software pause for a second. You can use more than one , 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.)


Sends the terminating Carriage Return character to the modem. This is a control code that most communication software translates as Carriage Return

The Basic Hayes Command Set

In alphabetical order:

Table B.1. Basic Hayes Command Set

A0 or AAnswer incoming call 
A/Repeat last commandDon't preface with AT. Enter usually aborts.
B0 or BCall negotiationV32 Mode/CCITT Answer Seq.
B1Call negotiationBell 212A Answer Seq.
B2Call negotiationVerbose/Quiet On Answer

Dial the following number and then handshake in originate mode.


Pulse Dial


Touch Tone Dial


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.)


Dial last number

E0 or ENo EchoWill not echo commands to the computer
E1EchoWill echo commands to the computer (so one can see what one types)
H0Hook StatusOn hook - Hang up
H1Hook statusOff hook - phone picked up
I0 or IInquiry, Information, or InterrogationThis command is very model specific. I0 usually returns a number or code, while higher numbers often provide much more useful information.
L0 or LSpeaker 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 MSpeaker offM3 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 NHandshake SpeedHandshake only at speed in S37
N1 Handshake at highest speed larger than S37
O0 or OReturn OnlineSee also X1 as dial tone detection may be active.
O1 Return Online after an equalizer retrain sequence
Q0 or Q1Quiet ModeOff - Displays result codes, user sees command responses (e.g. OK)
Q1Quiet ModeOn - Result codes are suppressed, user does not see responses.
Sn? Query the contents of S-register n
Sn=rStoreStore the value of r in S-register n
V0 or VVerboseNumeric result codes
V1 English result codes (e.g. CONNECT, BUSY, NO CARRIER etc.)
X0 or XSmartmodemHayes 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 ZResetReset modem to stored configuration. Use Z0, Z1 etc. for multiple profiles. This is the same as &F for factory default on modems without NVRAM (non-volatile memory)