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, .
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 ), 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 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.
^M 	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:

Basic Hayes Command Set
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 ( 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 (⪚ 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 (⪚ CONNECT, BUSY, NO CARRIER )
X0 or X	Smartmodem	Hayes Smartmodem 300 compatible result codes
X1		Usually adds connection speed to basic result codes (⪚ 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, Z1 for multiple profiles. This is the same as &F for factory default on modems without NVRAM (non voltaile memory)

The Extended Hayes Command Set

Ampersand Commands

The Extended Hayes Command Set
Command	Description	Comments
&B0 or &B	Retrain Parameters	Disable auto retrain function
&B1	Retrain Parameters	Enable auto retrain function
&B2	Retrain Parameters	Enable auto retrain, but disconnect if no line improvement over the period dictated by S7
&C0 or &C1	Carrier detect	Signal always on
&C1	Carrier detect	Indicates remote carrier (usual preferred default)
&D0 or &D	Data Terminal Ready (DTR	Signal ignored (This is modem specific, you must see your manual for information on this one!)
&D1	Data Terminal Ready (DTR	If DTR goes from On to Off the modem goes into command mode (Some modems only)
&D2	Data Terminal Ready (DTR	Some modems hang up on DTR On to Off transition (This is the usual preferred default)
&D3	Data Terminal Ready (DTR	Hang up, reset the modem, and return to command mode upon DTR
&F0 or &F	Factory defaults	Generic Hayes-compatible defaults. This is usually a good thing to use in your init string, since the &F1-&F3 settings can vary among modems, and they may actually be the cause of connection problems. (Since you never know exactly what Brand X's &F2 really changes. On the other hand, it pays to try out the other options below; many people's problems can be solved by replacing a complicated init string with a simple &F2 or the like. However, if you're building an init string, it's best to start with a simple &F, and not use the customized form of defaults.
&F1	Factory Defaults	Factory Defaults tailored to an IBM PC compatible user
&F2	Factory Defaults	Factory defaults for a Mac w/software handshaking
&F3	Factory Defaults	Factory defaults for a Mac w/hardware handshaking
&G0 or &G	Guard tones	Disable guard tones
&K0 or &K	Local flow control	Disable local flow control
&K1	Local flow control	Enable RTS/CTS hardware local flow control
&K2	Local flow control	Enable XON/XOFF software local flow control
&K3	Local flow control	Enable RTS/CTS hardware local flow control
&K4	Local flow control	Enable XON/XOFF software local flow control
&L0 or &L	Dial mode	Select dial-up mode
&M0 or &M	Error control mode	Select asynchronous non-EC mode (the same as &Q0)
&P0 or &P	Pulse dialing ratio	U.S./Canada pulse dialing 39% make / 61% break ratio
&P1	Pulse dialing ratio	U.K./Hong Kong pulse dialing 33% make / 67% break ratio
&Q0 or &Q	Error control mode	Asynchronous non-EC more. No data buffering. ASB disabled.
&Q5	Error control mode	Select V.42 EC operation (requires flow control)
&Q6	Error control mode	Asynchronous mode with ASB (requires flow control)
&Q8	Error control mode	Select alternate EC protocol (MNP)
&Q9	Error control mode	Conditional data compression: V.42bis = yes, MNP5 = no.
&S0 or &S	DSR action select	Always on (default)
&S1	DSR action select	Follows EIA specification (Active following carrier tone, and until carrier is lost.)
&T0 or &T	Self test	Model specific self test on some modems
&U0 or &U	Trellis code modulation	Enable V.32 TCM
&U1	Trellis code modulation	Disable V.32 TCM
&V0 or &V1	View active	(and often stored) configuration profile settings (or ATI4
&W0 or &W	Store profile	In NVRAM (&W0, &W1 etc. for multiple profiles) Some settings cannot be stored. These often don't show on &V or ATI4
&Y0 or &Y	Select configuration loaded at power-up	Load profile 0 (default)
&Y1	Select configuration loaded at power-up	Load profile 1
&Zn=x	Soft reset and load stored profile number n	Note that all items after the &Z on the command line are ignored

Backslash and Percent Commands

Backslash and Percent Commands
Command	Description	Comments
\A0 or \A	Character maximum MNP block size	64 character maximum
\A1	Character maximum MNP block size	128 character maximum
\A2	Character maximum MNP block size	192 character maximum
\A3	Character maximum MNP block size	256 character maximum
%C0 or %C	Data Compression Enable/Disable	Disabled
%C1	Data Compression Enable/Disable	MNP5 enabled
%C2	Data Compression Enable/Disable	V.42bis (BTLZ) Enabled
%C3	Data Compression Enable/Disable	MNP5 & V.42bis (BTLZ) Enabled
%D0 or %D	Data compression	512 BLTZ dictionary size
%D1	Data compression	1024 BLTZ dictionary size
%D2	Data compression	2048 BLTZ dictionary size
%D3	Data compression	4096 BLTZ dictionary size
%E0 or %E1	Escape method	ESCAPE DISABLED
%E1	Escape method	+++AT method (default)
%E2	Escape method	Break AT method
%E3	Escape method	BOTH methods enabled
%E4	Escape method	Disable OK to +++
%E5	Escape method	Enable OK to +++
\J0 or \J	DTE Auto Rate Adjustment	Disabled
\J1	DTE Auto Rate Adjustment	DTE rate is adjusted to match carrier rate.
\N0 or \N	Connection type	Normal connection (see below for definitions)
\N1	Connection type	Direction connection
\N2	Connection type	MNP Auto-reliable connection
\N3	Connection type	Auto-reliable connection
\N4	Connection type	V.42bis reliable link with phase detection
\N5	Connection type	V.42bis auto-reliable link with phase detection
\N6	Connection type	V.42 reliable link with phase detection
\N7	Connection type	V.42 auto-reliable link with phase detection
A direct connection is a simple straight-through connection without any error connection or data compression. In this case, the computer-to-modem and modem-to-modem speeds must be identical.
A normal connection uses flow control (either software or hardware) to buffer the data being sent or received, so that the modem can transmit data at a different rate than the computer is actually sending or receiving it. For example, a computer may send actual data at 57kbps, but using compression, the modem only actually sends 28.8kbps. This is the mode use by most modems.
A reliable connection is a type of normal connection; if, for some reason, data compression or error correction cannot be established or maintained, the connection will hang up. (In essence, such a modem ensures that all connections are reliable, for it will hang up if the connection isn't.)
Likewise, an auto-reliable connection is virtually the same, except that the modem will try to renegotiate the connection in order to establish a reliable connection. Again, this is the mode that most modems use.

S-Registers

S Registers
Register	Range	Default	Function
S0	0-255 rings	1-2	Answer on ring number. Don't answer if 0
S1	0-255 rings	0	if S0 is greater than 0 this register counts the incoming rings.
S2	0-127 ASCII	43 +	Escape to command mode character
S2	>127		no ESC
S3	0-127 ASCII	13 CR	Carriage return character
S4	0-127 ASCII	10 LF	Line feed character
S5	0-32, 127 ASCII	8 BS	Backspace character
S6	2-255 seconds	2	Dial tone wait time (blind dialling, see Xn
S7	1-255 seconds	30-60	Wait time for remote carrier
S8	0-255 seconds	2	Comma pause time used in dialing
S9	1-255 1/10ths second	6	Carrier detect time required for recognition
S10	1-255 1/10ths second	7-14	Time between loss of carrier and hangup
S11	50-255 milliseconds	70-95	Duration and spacing of tones when tone dialing
S12	0-255 1/50th seconds	50	Guard time for pause around +++ command sequence
S36	Fallback options when error correction link fails: 0 - Disconnect 1 - Establish Direct Connection 3 - Establish Normal Connection 4 - Establish an MNP connection if possible, else disconnect 5 - Establish an MNP connection if possible, else Direct Connection. 7 - Establish an MNP connection if possible, else Normal connection	7	Negotiation Failure Treatment
S37	1 = 300 bps 5 = 1200 bps 6 = 2400 bps 7 = 1200/75 bps (v.23 mode) 8 = 4800 bps 9 = 9600 bps 10 = 12000 bps 11 = 14400 bps 12 = 7200 bps	0	Negotiation Speed (Intial handshake)
Many modems have dozens, even hundreds, of S registers, but only the first dozen or so are fairly standard. They are changed with a command like ATSn=N, and examined with ATSn? (⪚ AT S10 =70 S1? would tell the modem not to hang up for seven seconds should it not hear the answering modem, and return the number of times the phone last rang.)