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MultiModemBA User Guide
4.1 Introduction
We’ll assume that yours is the very common application, where you are using a modem to dial up a
remote computer. The modem has been factory preset for originating a call to a compatible baud
modem (also set up for hardware flow control, V.42 error correction, V.42bis data compression and
CTS/RTS operation). If the answering modem is not set up similarly, the modem automatically
adjusts to the appropriate protocol.
4.2 Dialing/On-Line/Answering
There are several basic steps for “originating” and “answering” in data communication mode:
1. Load communication software
2. Dial
3. Establish On-Line connection
4. Terminate Call
A simple way to dial is from the keyboard of your computer or terminal. You enter a command on the
keyboard to tell the modem to dial.
If you use a sophisticated communications software package, the software tells you to enter phone
numbers and other information. If this is the case, the software, and not you, gives the dialing
commands to the modem. All of the commands, option registers and intelligent features of the
modem is taken care of by the software.
If you are giving commands directly to your modem (and not through your software), each command
must begin with AT (
ATtention Characters). AT characters may be entered in upper or lower case.
Entering AT automatically sets the modem’s speed to match the speed of the computer or terminal,
and also sets the modem’s parity.
The AT characters alert the modem that a command follows. The AT Command can also be used to
clear the command buffer, by simply typing AT and hitting RETURN.
The letter D in a command causes the modem to dial the numbers immediately following it (e.g.,
ATD6127853500). You have a choice of either pulse (ATDP) or tone (ATDT) dialing methods.
The modem responds with “CONNECT” on your video display after the number is dialed and a
connection signal is detected. The modem is now in “On-Line” mode, and is ready to communicate
with a host site.
If no connection signal is detected within 45 seconds (this time period can be adjusted by S-Register
S7), the modem goes On Hook/hangs up and returns to Command mode. At this point, your video
displays “NO CARRIER”.
Enter +++ATH to Hang Up On-Line (bring modem on-hook), and terminate the modem's On-Line
mode. At this point, your video displays “OK”.
In addition to the call originating capabilities, the modem can also automatically answer incoming
calls. You need not be present. You can, however, control the situation by configuring the modem to
either answer or not to answer, or to answer after a specified number of rings. This is done by setting
the value of S-Register S0 (modem defaults to automatically answer an incoming call after one ring).
Refer to Chapter 6 for S-Register details.