The Online World resources handbook

Getting started http://home.eunet.no/~presno/bok/v2.html
the modem could take the phone off hook and set up a connection with a remote
modem. The modem enabled a connected PC to act as an electronic answering machine.
Hayes compatible modems can report call progress to the local screen using short
numeric codes or words like CONNECT, CONNECT 1200, CONNECT 2400, NO
CARRIER, NO DIALTONE, BUSY, NO ANSWER, RING etc.
There can be small differences between such modems. The message DIALTONE
on one modem may be DIAL TONE on another. Most of the main progress messages,
however, are the same across brands.
The old Smartmodem had switches used to configure the modem. Most modern
Hayes compatible modems come without switches and have more commands than their
ancestor.
Today's Hayes compatible modems have a core of common commands, the "real"
Hayes commands, and several unstandardized additional commands. Here is an
example:
A standard on the move
On the Quattro SB2422 modem, 2400 bps speed without automatic speed detection is
set by the command "AT&I1". The equivalent command on Semafor's UniMod 4161 is
"AT+C0".
Automatic detection of speed is a feature that lets the modem discover the speed of
the remote modem to set its own speed at the same level. (Other modems may use
different commands to set this.)
When I want Windows Terminal to call a bulletin board, it first sends a sequence of
Hayes commands to the Semafor modem. These commands are stored in Terminal's
settings (Click Settings, Modem Commands. Originate to update or change them.)
The purpose of these commands is to "configure" the modem before calling. It
sends the following:
AT S0=0 +C0 S7=40 S9=4 &D2
The cryptic codes have the following meaning:
AT "Attention modem. Commands following.."
S0=0 No automatic answer
+C0 No automatic speed detection (fixed speed)
S7=40 Wait 40 seconds for an answer tone from
the remote modem.
S9=4 Wait 4/10 seconds for detection of carrier
&D2 Go on hook if the DTR signal is being
changed.
If this command is sent to the Quattro modem, it will reply with "ERROR." The code
"+C0" must be replaced with an "&I1". The rest of the commands are the same. (Note:
when a modem responds with "ERROR," it has usually rejected all commands sent to
it!)
This setup is held in the modem's memory when Terminal sends its dialing
command: ATDT4737031378. AT stands for ATtention, as above. DT stands for Dial
Tone. Here, it is used to dial the number 4737031378 using tone signaling (rather than
pulse dialing).
The modem cable
If you have an external modem, you must connect your computer to the modem with a
cable. Some modems are sold without a cable.
This cable may be called a serial cable, a modem cable, a RS232C cable, or
something else. Make sure that you buy the correct cable for your system.
Make sure that the connectors at each end of the cable are correct. If a male
connector (with pins) is required in one end and a female (with holes) in the other, do
not buy a cable with two male connectors.
Some connectors have 9 pins/holes, while others have 25 or 8 pin round plugs
(Apple computers). Use a shielded cable to make sure there is minimal interference with
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