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OPGUIDPM.TXT
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1990-01-25
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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 computer's
keyboard.
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 operates.
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-letter-
number 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-line.
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 <CR>,
although it may be labeled on your keyboard as carriage return,
Enter, or Return. For example, the command line ATV0<CR>
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 <CR> sends the command to the modem.
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 are
equivalent.
A command line can contain as many as 40 characters (the AT,
spaces, and the <CR> 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
<CR>, 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 connection.
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 "gua