What a Modem Does.
What a Modem Does.
The way calls were made with older telephone systems
illustrates how modems make connections. By talking the
receiver off the hook, then turning a crank, you got an
operator to come on the line. You told the operator the number
you wanted, and waited for the call to be placed for you. Until
the person on the other end picked up the receiver, you stayed
connected to the operator. The operator generally reported the
progress of the call, then gave the line to you to wait for the
person on the other end to answer. Your modem, like the
operator, responds to your commands to dial a number or answer
a call, then gives the line to your computer to begin data
communications when the connection is made.
Why You Need Communications Software.
Like telephones that store the numbers you frequently dial,
communications software automates your connections. This can
range from saving numbers in a "telephone book" so you don't
have to look them up and enter them each time, to handling the
complete process from dialing through file transfer.
You can use what is called "terminal emulation" software to
issue modem commands that set up the modem and dial a number,
but just as you make selections from within your word processor
or spreadsheet software to use a printer, the easiest most
effective way to use your modem is through communications
software. Once you've made the connections to your computer and
the telephone lines, you can almost forget the modem is there,
and access its features through the software. In the same way
you don't have to issue commands directly to the printer, you
can simply reach for the printout, knowing that the software
has taken care of everything. Your new modem will work with
nearly any communications software designed for personal
computers. Regardless of its level of sophistication, software
is required to provide the link between your modem and the
If your software has a problem communicating with the modem,
you'll know when you start the program or when you run the
software's installation procedure. A message like "modem not
responding", means the software is not recognizing the modem.
If you've made the port selection with the software and still
can't make a connection, you'll need to use the software to
manually change some of the settings that control how the modem
Most programs have an option for setting a modem
"initialization" or "command" string to set the modem up to
talk to the computer the way the software expects. The changes
will probably involve the way the modem and the computer use
the signals to talk to each other across the cable that
connects them. Most programs that do not work with the modem's
factory settings will operate properly if you change the way
the modem uses the signal on the DCD and DTR line. For example,
at the command line or window in which you adjust these
settings, simply type AT&C1&D2, or add &D2&C1 to the string. If
the string contains &K3, &F, or &W remove them. (These
ampersand-letternumber combinations are modem AT commands.) The
software will use the AT commands you put in this string each
time it sets up the modem.
For software that requires additional modem setup, refer to the
software manual for specific descriptions of the settings that
frequently need to be adjusted are result code display
(messages the modem send in response to commands), and local
character echo (the way your commands are displayed on the
screen). These and other options are easily specified with an
initialization string, or with the AT Command Set described in
the following pages. If your software requires more settings to
be changed, check your software's user's guide, then refer to
the sections that follow for the particular command option.
With the setup string in place, you will be able to use your
software to place and answer calls to go on-line with another
computer without issuing the commands to setup the modem each
time you start the program.
How to "Talk" to the Modem.
If you find it necessary to communicate directly to the modem
to set it up or to initiate calls, you are probably using
terminal emulation software. This means that the only function
the software provides is linking your keyboard input directly
to the modem. This would be like connecting your printer
directly to your keyboard; what you type would appear on the
screen, and be sent out to the printer.
Unless you have selected the option that some programs call
"local mode" or "go local", communications software does not
operate in this way. For certain uses, like determining whether
there is a problem with your modem, the software, the line, or
the other modem, this is a helpful way of talking to the modem.
The next sections explain in detail how to setup your modem and
initiate and answer calls using AT Commands once your software
has linked your keyboard to the modem.
Issuing AT Commands.
The Hayes Standard AT Command Set for intelligent modems is the
language understood by most modems used with personal
computers. AT commands can be used to set up the modem to suit
the way your software and/or computer wants it to be
configured. They can be used for dialing or answering. However,
AT commands are not used to perform communications activities
like sending a file. Once on-line, you'll be talking with
another computer. The command to break the connection is
probably the only instruction you would issue after going on-
AT Command Format.
Modem commands begin with an AT prefix that gets the modem's
attention (also tells the modem the speed and character format
of the data coming from the computer). The AT prefix is
followed by the command(s). The modem executes the command line
when you press the key on your keyboard that you normally use
to send commands. The command descriptions here show this key
as , although it may be labeled on your keyboard as
carriage return, Enter, or Return. For example, the command
line ATV0 gets the modem's attention and uses the V0
command option to tell the modem to display the responses to
commands (result codes) as numbers rather than as words (the V1
option is the factory setting). The sends the command to
Many commands are used to select between two or more options.
Numeric parameters indicate the form of the command you want to
be in effect. If you issue a command without specifying a
numeric parameter, the modem assumes the 0 command option. For
example, Q tell the modem to respond to commands with result
codes. Issuing Q1 tells the modem not to respond to result
codes (some software programs call this "quiet mode"). Q and Q0
A command line can contain as many as 40 characters (the AT,
spaces, and the are not counted). The command prefix can
be entered in either all upper case or all lower case letters,
but not in a combination (e.g., AT or at, but not At or aT).
If you need to change something in the line before entering the
, use the backspace key. To abort the command line, hold
down the key your computer recognizes as the control key (ctrl)
and press "x". The modem will return OK without executing the
command(s) on the line.
Moving Between Command and On-line States.
Before beginning communications or after "hanging up," the
modem is in command state, ready to accept configuration
commands or to go on-line when you instruct it to dial the
number of a remote modem. You may also place the modem in
command state from the on-line state without breaking the
Escape Sequence -- +++.
Not actually a command, the escape sequence tells the modem to
"escape" or leave the on-line state and enter the command
state. The sequence consists of a single character issued three
times in succession. A one second "guard time" interval
prevents the modem from mistaking a ramdom occurrence of of the
same three characters as the escape sequence. The character
used in the sequence and the duration of the guard time can be
changed by writing values to the modem's S-Registers (special
memory locations described later in this guide).
To issue the escape sequence simply hold down the shift key and
press the plus key three times (+++). Be sure to pause at least
a second before and after issuing the three characters. If
operating properly, the modem will return the OK result code as
an indication it is ready to accept commands.
On-Line Command -- O (letter, not the number 0).
This command returns the modem from command state to on-line
state when a connection has been "escaped" but not broken. If
you have placed the modem in command state and are still
connected to the remote system, issue ATO to go back on-
line. What you type will then be transmitted to the modem
attached to computer on the other end of the line. If the
connection is broken, to go on-line again you must instruct the
modem to re-dial the telephone number.
In the same way voice telephone calls are made, one of the
modems must originate the call, and the other must be set up to
answer the incoming call. Either modem may originate or answer
the call. The following examples illustrate how to use the
modem to place and answer calls from your computer.
Originating a Call.
To originate a call with another system, issue the D command to
"dial" the number. This command instructs the modem to go off
hook, claim the telephone line, and originate a call. When you
issue dialing commands to the modem, the data is communicated
to the modem, but does not pass across the telephone line.
The following dial modifiers specify the number and method the
modem uses to dial.
T Specifies Tone method for subsequent dialing; follows the
D command and precedes telephone number; can be issued as
a command separate from dialing string.
P Specifies Pulse method for subsequent dialings; follows
the D command and precedes telephone number; can be issued
as a command separate from dialing string. (factory
, Pause 2 seconds before continuing to process the
characters in the dialing string.
; Return to command state after dialing and maintain
connection. This modifier is useful in situations like
electronic banking where the modem needs to stay in
command state to receive other connection instructions.
0-9 Numbers and characters for the modem to dial.
A typical dial command such as as ATD,T5552368 gets the
modem's attention (AT), instructs it to go off hook to
originate a call (D), pause 2 seconds (,), and use the tone
method (T) to dial the telephone number (5552368). The
sends the commands to the modem.
Answering a Call.
Incoming calls to your modem can be answered in either of two
ways: you can issue a command to the modem when you hear the
telephone ring, or can also set up the modem to answer
automatically when it detects a number of rings.
Answering Manually - A.
To manually answer a call from a remote system, issue the A
command. ATA instructs the modem to go off hook, take
control of the line, issue an answer tone, and connect with the
incoming call. This command is used when the phone is ringing
and you are expecting a modem call (rather than a voice call).
Answering Automatically - S0=n.
One of the modem's S-Registers provides the capability for
The S0=n command ("0" is the number of the register or memory
location; "n" is the number of rings to wait before answering)
lets you specify the number of rings. Setting S0 to a number
other than its factory setting of 0 configures the modem for
auto-answer. The modem will then operate the same way as a
telephone answering machine, picking up the phone after it
detects the specified number of rings.
For example, ATS0=3 sets up the modem to go off hook and
issue answer tone (no ATA is necessary) when the modem
detects the third ring.
Communicating While On-Line.
When you are connected (on-line), data is passed across the
line between the two computers. Either party can type
information to the other. Characters received by the modem on
the other end of the connection will be displayed to the
screen. The user on the other end can also type characters to
your screen. Sentences and words will mix on the screen,
however, if you both type at the same time.
If you cannot see what you are typing, you will need to change
the local character echo setting (see Configuration Options
later in this guide).
Terminating a Call.
Whenever you are ready to end communications, you must issue
the escape sequence (+++) to place the modem in the command
state. Then use the H command to hang up: ATH. This command
puts the modem on hook and releases control of the line, so it
can be used by a telephone.
Unexpected interruptions to communication occur with modem
connections just as with telephone calls. Line noise, equipment
malfunction, and user error can all contribute to disruptions
in communication. The modem indicates a loss of connection with
the NO CARRIER result code. To re-establish the connection,
simply re-dial the number.
To interrupt call placement during the handshaking process, you
can press any key before the two modems connect.
Monitoring Operation and Call Progress.
When you send a command or issue the escape sequence, the modem
responds with a "result code." If it understands the
instructions, the modem will return the OK result code; if it
does not understand, the modem will return the ERROR result
code. The modem also provides call progress messages to let you
know how your connection attempt is going:
Result Code Explanation.
0 OK Command reconized and accepted.
1 CONNECT Connection established at 0-300
if X1, X2, X3, or X4 is
selected (no speed indicated if
X0 is selected).
2 RING Incoming Call
3 NO CARRIER Carrier signal lost or not
4 ERROR Command not recognized; command
line exceeds 40 characters or
in an improper format; modem
5 CONNECT 1200 Connection established at
1200bps if X1, X2, X3, or X4 is
6 NO DIAL TONE Dial tone not detected when
7 BUSY Number dialed is use.
10 CONNECT 2400 Connection established at
2400bps if X1, X2, X3, or X4
selected (for modems that
The X command allows you to set up the modem for a variety of
call progress monitoring options and corresponding result
codes. Regardless of the option selected, the modem will report
the OK, CONNECT, RING, NO CARRIER, and ERROR result codes.
Other call progress monitoring feature combinations, like
reporting 1200bps and 2400bps connections, detection of busy
signal, and no dial tone can be selected with the various
Result Code... X0 X1 X2 X3 X4.
OK Y Y Y Y Y
CONNECT Y Y Y Y Y
RING Y Y Y Y Y
NO CARRIER Y Y Y Y Y
BUSY Y Y
NO DIAL TONE Y Y
ERROR Y Y Y Y Y
CONNECT 1200 Y Y Y Y
CONNECT 2400 Y Y Y Y
If you want the modem to wait for a dial tone before dialing,
you should select either the X2 or X4 command options. Either
of these configures the modem to wait until it detects a dial
tone before proceeding; if it does not, it returns the NO DIAL
TONE result code. The same is true for configuring the modem's
response to a busy signal; if X1 or X4 is selected, the modem
recognizes a busy signal from the other end and returns the
BUSY result code. The X4 option provides full call progress
monitoring. However, X0 is the factory setting because many
software programs expect 1200bps modems to be configured
initially with the basic call progress capabilities and
corresponding result codes.
Managing Modem Configuration.
When your modem is first installed and powered up, it is
configured with the settings selected at the factory. The
options specified for each command and the value is each of the
modem's S-Registers determine how the modem operates. The
modem's factory settings suit a wide range of user
applications, while taking advantage of the modem's full
Creating a Special Configuration.
If your software or the system with which you are attempting to
connect requires that you set up the modem to operate other
than with factory settings for commands and S-Registers, you
can create a special configuration, or profile, by issuing AT
commands. The configuration you create will be used until you
issue commands to change the settings, or reset the modem
(either with the Z command or by unplugging/replugging it).
Resetting the Modem.
If the modem ever seems to be set in a way that makes it not
communicate as it should, or if you simply want to restore the
factory settings, you can reset it with the Z command. ATZ
performs a "soft reset," replacing values currently in S-
Registers and AT command options to factory settings. The
modem's factory configuration is also recalled when the modem
is unplugged and plugged back in, or if power is lost to the
wall plug. If you are experiencing any difficulty with the
modem, you may wish to issue the Z command before unplugging
the modem or issuing the test commands.
Adjusting Modem Speed.
Modem speed is measured in bits per second(bps). For example,
if Hayes Personal Modem 1200 is operating at its highest rate,
data will be transmitted at 1200bps. This speed is not set by
an AT command; it is the speed at which the software sends the
AT prefix (within the range supported by the modem) that
determines the speed at which the modem attempts to make the
next connection. Some programs offer an option for transmission
speed, often called "baud rate," to specify the speed of the
connection. The software sends the AT prefix at the desired
In addition, the transmission rate of the connection will be
determined by the highest common speed supported by the two
connecting modems. For example, if the modem at the other end
is capable of a maximum speed of 300bps, the connection and
data transmission will be at 300bps, even if your modem can
communicate at 1200bps or 2400bps.
Choosing a Communication Standard.
Communication standards are protocols that determine how the
modems perform their connection handshake and the format and
speed used to transmit data. For example, connections at 300bps
use the domestic 103 standard; connections at 1200bps use the
domestic 212A standard. Because standards are closely linked to
speed, the modem selects the appropriate standard as it adjusts
to the speed.
Testing the Modem.
In addition to the diagnostics the modem automatically performs
each time it is powered up, the I command can be used to gain
information about the modem.
I0 Modem reports Hayes product code.
I1 Modem calculates and reports ROM Checksum (3-
The results of these tests are primarily used by Hayes Customer
Service and certain software programs to determine the
manufacturer/model of the modem you are using, and its feature
Refer to the descriptions in the Personal Modem Installation
Guide for troubleshooting suggestions. If you are able to issue
commands to the modem and it returns an OK result code, the
modem is probably operating correctly. If the modem does not
seem to be responding, try resetting the modem (see above).
Setting Local Character Echo.
Your computer and software combination may require that you
adjust the character echo feature of your modem to provide
display of characters as you type commands, or to prevent
double characters. Two options are available:
E0 Modem does not return (echo) characters from the
keyboards to the screen when in command state.
E1 Modem returns (echoes) characters typed from the
keyboard to the screen when in command state
This command is effective only if you are entering instructions
from a command line. The settings for this command do not
affect the display of characters while you are on-line with
another system. The software or remote system must provide on-
line character echo, if desired.
Adjusting Result Code Display.
The two commands below configure the modem's use of result codes.
Q0 Modem responds to commands with result codes
Q1 Modem does not respond to commands with result
V0 Modem reports result codes as numbers.
V1 Modem reports result codes as words (factory
Some software requires that result codes not be displayed, or
that they be numbered rather than as words.
Configuring Modem Signals.
Some computers or software expects settings for signalling
between modems and between the modem and the computer to be
configured in ways other than that set at the factory. Any
special modem-to-computer signalling requirements should be
described by your software's user's guide or your computer's
The C command controls the modem's response to an incoming
carrier signal over the line from the remote modem.
C0 Modem ignores an incoming carrier signal; the
transmitter carrier is always off.
C1 Modem turns the transmit carrier signal on and
off to reflect the presence of and incoming
carrier signal. When the modem originates a
call, answers a call, or is connected to
another system, the signal is on, and is off
when the modem is not in one of these
situations (factory setting).
The &C command determines the way the modem uses the carrier
signal when controlling the Data Carrier Detect (DCD) line (pin
2 on modem-to-computer cable; pin 8 with 25-pin adapter):
&C0 Modem presumes the presence of the carrier
signal and maintains the DCD signal on at all
&C1 Modem turns the DCD signal on when it detects a
carrier signal from the modem on the other end
of the telephone line, and off when it does not
The &D command selects the modem's response to signal sent from
the computer on the Data Terminal Ready(DTR) signal (pin 1 on
the modem-to-computer cable; pin 20 with 25-pin adapter):
&D0 Modem presumes presence of DTR signal and
ignores the actual status of the signal
&D1 Modem monitors DTR signal. When communications
software is used to cause an on-to-off
transition of DTR signal, the modem enters
command state and ignores AT commands. When the
software causes an off-to-on transition of DTR,
the modem will again respond to AT commands and
can be returned to the on-line state (if the
connection has not been broken) with the
&D2 Modem monitors DTR signal. When communications
software is used to cause an on-to-off
transition of DTR, the modem will again respond
to AT commands.
Writing to the Modem's S-Registers.
S-Registers provide special access to settings that determine
modem operation. The factory-set values stored in these
registers are recalled and used to control the modem each time
it is powered up or reset.
Aside from using S0 to store the number of rings for automatic
answer (described earlier), you will have little need to change
an S-Register value. However, if your use of the modem requires
that one or more of these be changed, you can do so by using
the Sr command:
Sr? Modem reads and responds with the value of
register "r" (r=number of register; "?"
requests the value).
Sr=n Modem sets the value of register "r" to "n"
(n=value within the range of the register).
Writing to the modem's S-Registers is not recommended unless
your use of the modem requires that one or more of these
settings be changed.
For your reference, the modem's S-Registers, their function,
ranges in units, and factory settings are provided in the chart
S-Register Number & Units Range Factory
S0 Ring to answer on Rings 0-255 0
S1 Rings received Rings 0-255 0
S2 Escape sequence ASCII 0-127 43 (+)
S3 Carriage return ASCII 0-127 13 (CR)
S4 Linefeed character ASCII 0-127 10 (LF)
S5 Backspace character ASCII fixed at 8 (BS)
S6 Wait time for dial seconds fixed at 4
S7 Wait time for carrier seconds fixed at 45
S8 Duration of comma seconds fixed at 2
S9 Carrier detect 1/10 sec 1-255 6
S10 Delay between loss 1/10 sec 1-255 7
of carrier signal
and modem hang up
S11 Duration of DTMF msec fixed at 95
tones for dialing
S12 Escape sequence 1/50 sec 20-255 50
S25 Delay between loss 10 mec 1-255 5
of DTR signal and
S30 Delay time from no 10 sec 0-255 60
modem activity until
The table below describes the pin number on the modem's cable,
the direction the signal pass, and the name of the signal used
by the modem.
Pin HOST_Direction_Equip Signal
1 ------------> DTR - data terminal ready
2 <----------- DCD - data carrier detect
3 ------------> TxD - transmit data
4 Not applicable ground - signal ground
5 <----------- RxD - receive data
6 <----------- DSR/CTS - data set ready/clear to send
7 <----------- RI - ring indicator
8 Not applicable ground - signal ground
The housing of the connector provides a shield that reduces noise
The actual pin number on which a signal is carried will vary
depending on the adapter used.
The table below cross-refrences the pin numbers used for signals
on the 8-pin DIN with those for the adapters provided with your
DB-9 DB-25 Signal
1 <--- 8 DCD Data Carrier Detect "Модем: соединен"
2 <--- 3 RxD Receive Data
3 ---> 2 TxD Transmit Data
4 ---> 20 DTR Data Terminal Ready "Host включен"
5 7 signal ground
6 <--- 5 DSR Data Set Ready "Модем включен"
7 ---> 4 RTS Request To Send "Host: я готов"
8 <--- 5 CTS Clear To Send "Модем:я готов"
9 <--- 22 RI Ring Indicator
NOTE: If you construct an adapter for use with the modem, consult
your computer's owner's manual for the signals supported and the
appropriate pin numbering.
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