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 being assigned.
A "register" is computerese for a specific physical location in memory. Modem 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."
Note: Although most commands are defined by a letter-number combination (L0, L1, etc.),
the use of a zero is optional. In this case, 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.
- AT
- tells the modem that modem commands follow. This must begin each line of
commands.
- Z
- resets the modem to default state
- , (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 translate as a "carriage return."
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 ans Seq
|
B1 |
Bell 212A Answer Seq
|
B2 |
Verbose/Quiet On Answer
|
D |
Dial |
Dial the following number and then
handshake in originate mode.
Dial Modifiers
(These are common but most modems will
have more.)
- P
- Pulse dial
- T
- Touch Tone dial
- W
- Wait for second dial tone
- ,
- Pause for 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 or H |
Hook status |
On hook - hang up
|
H1 |
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 |
(O0 see also X1 as dial tone detection may
be active)
|
O1 |
Return Online after an equalizer retrain
sequence
|
Q0 or Q |
Quiet mode |
Off - Displays result codes, user sees
command responses (e.g. OK)
|
Q1 |
On - Result codes are suppressed, user
does not see responses
|
Sn? |
|
Query the contents of S-register n
|
Sn=r |
Store |
Store the value r in S-register n
|
V0 or V |
Verbal? |
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 ATO)
|
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 (Z0, Z1
etc. for multiple profiles)
(Same as &F (factory default) on modems
with out NVRAM (non volatile memory)
|
Command |
- Description - |
Comments
|
&B0 or &B |
Retrain Parameters |
Disable auto retrain function
|
&B1 |
Enable auto retrain function
|
&B2 |
Enable auto retrain, but disconnect if no line
improvement over period dictated by S7
|
&C0 or &C |
Carrier detect |
signal always on
|
&C1 |
Indicates remote carrier (usual preferred default)
|
&D0 or &D |
Data Terminal Ready
(DTR) |
Signal ignored (See your manual on this one!)
|
&D1 |
If DTR goes from On to Off the modem goes into
command mode (some modems)
|
&D2 |
Some modems hang upon DTR On to Off
transition. (Usual prefered default)
|
&D3 |
Hang up, reset 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 tailored to an IBM-PC compatible
user
|
&F2 |
Factory defaults for a Mac w/software handshaking
|
&F3 |
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 |
Enable RTS/CTS hardware local flow control
|
&K2 |
Enable XON/XOFF software local flow control
|
&K3 |
Enable RTS/CTS hardware local flow control
|
&K4 |
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 (same as
&Q0)
|
&P0 or &P |
Pulse dialing ratio |
U.S./Canada pulse dialing 39% make/ 61% break
ratio
|
&P1 |
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 |
Select V.42 EC operation (requires flow control)
|
&Q6 |
Asynchronous mode with ASB (requires flow
control)
|
&Q8 |
Select alternate EC protocol (MNP)
|
&Q9 |
Conditional data compression: V.42bis = yes,
MNP5 = no.
|
&S0 or &S |
DSR Action Select |
Always on (default)
|
&S1 |
Follows EIA specification (Active following carrier
tone, and until carrier is lost.)
|
&T0 or &T |
Self test |
Model specific self tests on some modems
|
&U0 or &U |
Trellis code modulation |
Enable V.32 TCM
|
&U1 |
Disable V.32 TCM
|
&V0 or &V |
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 |
Load profile 1
|
&Zn=x |
Soft reset and Load
Stored profile #n |
Note: All items after the &Z on the command line
are ignored
|
Command |
- Description - |
Comments
|
\A0 or \A |
Character maximum MNP block size |
64 character maximum
|
\A1 |
128 character maximum
|
\A2 |
192 character maximum
|
\A3 |
256 character maximum
|
%C0 or %C |
Data Compression Enable/Disable |
Disabled
|
%C1 |
MNP5 Enabled
|
%C2 |
V.42bis (BTLZ) Enabled
|
%C3 |
MNP5 & V.42bis (BTLZ) Enabled
|
%D0 or %D |
Data compression |
512 BLTZ dictionary size
|
%D1 |
1024 BLTZ dictionary size
|
%D2 |
2048 BLTZ dictionary size
|
%D3 |
4096 BLTZ dictionary size
|
%E0 or %E |
Escape method |
ESCAPE DISABLED
|
%E1 |
+++AT method (default)
|
%E2 |
<BREAK>AT method
|
%E3 |
BOTH methods enabled
|
%E4 |
Disable "OK" to +++
|
%E5 |
Enable "OK" to +++
|
\J0 or \J |
DTE Auto Rate Adjustment |
Disabled
|
J1 |
DTE rate is adjusted to match carrier
rate.
|
\N0 or \N |
Connection type |
Normal connection (see below for
definitions)
|
\N1 |
Direction connection
|
\N2 |
MNP Auto-reliable connection
|
\N3 |
Auto-reliable connection
|
\N4 |
V.42bis reliable link with phase
detection
|
\N5 |
V.42bis auto-reliable link with phase
detection
|
\N6 |
V.42 reliable link with phase detection
|
\N7 |
V.42 auto-reliable link with phase
detection
|
Note: A direct connection is a simple straight-though connection without any error correction
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.
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>0 this register counts
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 dialing,
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 1/10 sec. |
6 |
Carrier detect time required for
recognition
|
S10 |
1-255 1/10 sec. |
7-14 |
Time between loss of carrier and
hangup
|
S11 |
50-255 millisec. |
70-95 |
Duration and spacing of tones
when tone dialing
|
S12 |
0-255 1/50 sec. |
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 a MNP connection if
possible, else Disconnect
5 - Establish a MNP connection if
possible, else Direct connection
7 - Establish a 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 (Initial
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?
(e.g. "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.) |
