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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 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 easiset 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
chnage 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 amper-sand-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 the requires additional modem setup, refer the 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 doe 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 modems used with most personal computers.
AT commands can be used to set up the modem to suit the way your
softawre and/or computer wants it to be configured. They can be
used for dialing or answering. However AT coomands 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 Key. 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> send 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, Q0 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").
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 mayalso 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 "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--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<CR> to fo back on-line. What you type
wil 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.
Getting On-Line
---------------
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 setting).
, 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<CR> 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 <CR> 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. You 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<CR> 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
automatic answer.
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 numbers 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.
Answering a Call
----------------
Inconing 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. You 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<CR> 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 that a voice call).
Answering Automatically - S0=n
------------------------------
One of the modem's S-Registers provides the capability for
automatic answer.
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<CR> sets up the modem to go off hook and issue
answer tone (no ATA<CR> 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<CR>. This command puts the
modem on hook and releases control of the line, so it can be used
by a telephone.
Unexpected interruptions to communicate 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
detected.
4 Error Command not recognized; command
line exceeds 40 characters or in
sn improper format; modem checksum
incorrect.
5 Connect 1200 Connection established at 1200bps
if X1, X2, X3, or X4 is selected.
6 No Dial Tone Dial tone not detected when
expected.
7 Busy Number dialed is use.
-------------------------------------------------------------------
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
connections, detection of busy signal, and no dial tone can be
selected with the various command oprions.
-------------------------------------------------------------------
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
-------------------------------------------------------------------
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 capabilities.
Creating a Special Configuration
--------------------------------
If your software or the system with which you are attempting to
connect required 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<CR>
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 sprrd, often called
"baud rate," to specify the speed of the connection. The software
sends the AT prefix at the desired speed.
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.
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-digit
number).
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
set.
Refer to the descriptions in the Personal Modem 1200 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
(factory setting).
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 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
(factory setting).
Q1 Modem does not respond to commands with result
codes.
V0 Modem reports result codes as numbers.
V1 Modem reports result codes as words (factory
setting).
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
owner's manual.
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).
&C0 Modem presumes the presence of the carrier signal
and maintains the DCD signal on at all times.
&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
(factory setting).
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).
&D0 Modem presumes presence of DTR signal and ignores
the actual status of the signal (factory setting).
&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 AT0<CR>.
&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 setting 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 refrence, the modem's S-Registers, their function, ranges
in units, and factory settings are provided in the chart below.
-------------------------------------------------------------------
S-Register Number & Units Range Factory Function Setting
-------------------------------------------------------------------
S0 Ring to answer on Rings 0-255 0
S1 Rings received Rings 0-255 0
S2 Escape sequence ASCII 0-127 43
character
S3 Carriage return ASCII 0-127 13
character
S4 Linefeed character ASCII 0-127 10
S5 Backspace character ASCII fixed at 8
S6 Wait time for dial seconds fixed at 4
tone
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
response time
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
guard time
S25 Delay between loss 10 mec 1-255 5
of DTR signal and
modem hangup
S30 Delay time from no 10 sec 0-255 60
modem activity until
auto hangup
-------------------------------------------------------------------
Cable Connections
-----------------
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 Direction Signal
-------------------------------------------------------------------
1 From Computer DTR - data terminal ready
2 To computer DCD - data carrier detect
3 From computer TxD - transmit data
4 Not applicable ground - signal ground
5 To computer RxD - receive data
6 To computer DSR/CTS - data set ready/
clear to send
7 To computer RI - ring indicator
8 Not applicable ground - signal ground
-------------------------------------------------------------------
The housing of the connector provides a shield that reduces noise
and interference.
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
modem.
-------------------------------------------------------------------
Signal DIN-8 DB-9 DB-25
-------------------------------------------------------------------
DTR 1 4 20
DCD 2 1 8
TxD 3 3 2
ground 4 5 7
RxD 5 2 3
DSR/CTS 6 8 5
RI 7 9 22
signal ground 8 5 7
shield shield shield 1
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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