Black Box MD1641A Manuale Utente

We have shown that the Series II Modem can automatically answer
incoming calls. The Modem can also answer manually, under your
control. The most typical application involves you and another person,
who, after carrying on a voice conversation, want to convert to data
communications between your modems without having to hang up 
and dial again.

The problem here is that both modems are in originate mode. For two
modems to communicate, one of them must be in originate mode and
the other must be in answer mode. The solution here is to force either
one of the two modems to Answer mode, and the other to Originate
mode.

The Modem can be forced into either Answer mode or Originate mode
by using the Voice/Data switch on the front of the modem.

In our example, let's say that you and your friend Bill have just had a
voice conversation. Both of you have your telephones connected to your
Series II Modems, and your modems are connected to computers or
terminals. Set the speed switches to the proper speed on both modems.
To switch to Data mode from Voice mode, you would switch the
Voice/Data switch Down (to the Originate position), and Bill would
switch his up to Answer. This switches your modem to the originate on-
line mode, and Bill's to the Answer on-line mode. You would both then
hang up your telephone handsets, and continue the conversation via
your keyboards.

This section briefly explains what happens between two modems in a
normal call. We are assuming that there are Series II Modems at both
the originating and at the answering end of the telephone line. (If
other brands of modems are used with our modems, they will still
communicate, but each manufacturer tends to use slightly different
delay timings and sequences, and many of the brands vary in their 
LED designations.)