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Software Usage and Examples
LOW-LEVEL MODEM SOFTWARE
The low-level modem software provides the ability to directly control the modem for operations such as
dialing, changing modem operating modes, and setting handshake protocol and parity. All low-level
commands are sent to the modem using the high-level
t'10DOUT
command. This section will describe the
operation of the modem and examples of the use of its commands.
The examples assume that you
program has turned the modem on (see "Turning the Modem On" earlier in this section).
Modem Modes
The modem has four primary operating modes that directly affect its operation. These are described
below:
Mode
MODEM
VOICE
ORIGINATE
ANSWER
Description
MODEM mode is the data communication mode of the modem. In
MODEM mode, the modem is waiting for either a dialing operation or
a ringing signal.
If a dialing operation occurs, the modem is
originating the communication session.
It
will then monitor the
phone line for a ring back from the remote modem, and attempt to
enter ORIGINATE mode. If a ringing signal occurs, a remote modem
is trying to initiate a communication session. The modem will then
attempt to enter ANSWER mode and answer the incoming call.
VOICE mode allows you to use the modem as an auto-dialer for
voice communication. The modem is not monitoring the status of
the phone line for either a ring by a remote phone or for incoming
calls, and performs none of the data communication activity that
occurs in MODEM mode.
The modem is issumg an originate tone or waiting for an answer
tone.
The modem is issuing an answer tone.
The modem can also be off-hook or on - hook, analogous to a telephone. When a phone handset is in use,
it is off the hook-using the phone line. When a phone handset is hung up, it is on the hook-not using
the phone line. Similarly, when the modem is off -hook, it is using the phone line, and when the modem is
on-hook, it is not using the phone line.
When the modem is on-hook, it can be either in MODEM mode, waiting for a dialing command or an
incoming call, or in VOICE mode, not monitoring the phone line. When the modem is off -hook, it can be
in ORIGINATE mode, waiting for a ring back from a remote phone or an answer tone, in ANSWER mode,
waiting for an originate tone, or in VOICE mode, not monitoring the phone line.
Virtually all high-level modem commands are geared toward use with MODEM mode. Detecting carrier
and on-hook/off-hook status
(CF-IF.:F.: I EF::?
and
HOOK?)
are relevant to establishing a communication
session. Reading from and writing to the modem buffers
(t'10D I t·U
and
t'1C!DOUT)
is relevant to VOICE
mode only because
t'10DOUT
sends the commands and
t'1C!D I t·U
reports the responses to the commands.
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