Automatic Dialling

		Automatic Dialling

Automatic dialling is invoked by issuing the D command followed by a valid telephone number. The modem responds to the command by checking that the telephone handset connected to the rear of the modem is not in use and then going off hook (grabbing the telephone line) and dialling the number. If the telephone handset is in use, the modem returns BUSY message. If an answer tone from the remote system is detected, a connection is negotiated and, if successful, established at a level common to both systems. The level of communication, in terms of speed, error correction, result messages etc., is decided by the settings (or limitations) of both systems. When dialling, the modem is said to be in originate mode.

Using D without a dial string causes the modem to check that the telephone handset is actually in use (on a previously established voice call) before entering on-line originate mode, and waiting for an answer tone. If the telephone handset is not in use the modem returns the ERROR message. No dialling takes place because no dial string is attached. This can be useful for switching from an established voice link to a data link. In this situation, the remote system issues the A command to generate an answer tone. The two systems then negotiate a connection.

Dial timing parameters.

Certain functions of the dialling and connection processes can be controlled to a high degree to provide flexibility and compatibility with a wide variety of telephone systems. Several of the S registers are specifically designed to hold timing information and you should be aware of their effect on these processes. Refer to the S registers chapter for more details.

Dial monitoring.

While the modem is attempting to dial, it may encounter a number of tones such as dial tone, busy, engaged etc. These are generated by the exchange equipment. The setting of the X command determines which of these tones the modem can recognise and how it reacts.

The dial process can be monitored when the modem's in-built speaker is enabled. Monitoring is useful for determining the possible failure in establishing a communications link. For instance, if the dial tone is not present when the modem goes off hook it is pointless trying to dial.

Cancelling the dial command.

If you wish to cancel the dial attempt while it is still in progress, press any key at the keyboard. This aborts the attempt and returns the OK message.

Dialling mode.

There are two types of dialling system in use. The original (older) type of dialling is called pulse (or Loop Disconnect, LD) dialling. It can be recognised by the characteristic clicking sound whilst dialling a number. The newer, faster type of dialling is called tone (or Multi Frequency, MF) dialling. This can be recognised by a sequence of musical tones whilst the number is being dialled.

The modem supports both pulse and tone dialling. Tone dialling is the default method of dialling and is compatible with the majority of phone lines.

Command Summary.
D Dial :

The D command without any other parameters instructs the modem to go on-line in originate mode, without dialling a number, and wait for an answer tone. More usually, the D command is used in association with other dialling commands (a dial string) causing further dialling actions to be taken.

These commands have the functions described only when they are used within a dial string. Some of the commands are interpreted differently if they are entered as stand alone commands.

The dialling mode used is controlled by the P (pulse) and T (tone) dial string parameters. If neither of these parameters is present in the dial string then the current dialling mode is used. The current dialling mode is tone by default but is altered whenever the P or T parameters are used.

The valid dial string parameters are described below. Parentheses, hyphens and spaces may be used to aid clarity and are ignored by the dialling process. Throughout this manual spaces have been included in AT commands to improve readability.

0 - 9 :

Dials the digits 0 to 9.

* :

Dials the 'star' digit, tone dialling only.

# :

Dials the 'gate' digit, tone dialling only. Also called "hash" or "square".

@ Wait for silence :

Waits for at least 5 seconds of silence before continuing with the next dial string parameter.

P Pulse Dial :

Selects pulse dialling mode. Only digits 0-9 can be dialled in pulse dial mode. It affects current and all subsequent dialling. It is possible to include both pulse and tone dialling within the same dial string. For example:

ATD T 9 P 01234 567890 <Return>

forces the modem to use tone dialling to dial the 9 then pulse dial the remainder of the number.

R Reverse Dial Mode :

This mode originates the call in answer mode.

T Tone Dial :

Selects tone dialling mode. This is the default dialling mode. The command affects current and all subsequent dialling. Any digit 0-9,*,#,A,B,C,D, may be dialled as tone.

W Wait :

The W command is used to make the modem wait for a secondary dial tone before proceeding to dial the telephone number. This may be required when using an exchange to access an outside line. In such circumstances two dial tones are encountered: the first when the modem goes off hook; the second when an outside line is available. The example below shows the placing of the W in the dial string:

ATD 9 W 01234 567890 <Return>

, Pause :

In certain situations it may be necessary to introduce a pause into the dialling process. An example of this is where the secondary dial tone is not distinguishable and the W command would be inappropriate:

ATD 9 , 01234 567890 <Return>

The default length of the pause for a single comma is 2 seconds.

! Hook flash :

The ! command forces the modem to go on hook for 0.5 seconds, then returns it to off-hook. This can be used as a "hold" facility, useful for transferring calls or accessing outside lines on particular exchanges. For example:

ATD ! 01234 567890 <Return>

The D takes the modem off hook. The exclamation mark then puts it on hook for 0.5 seconds before returning off hook and continuing with the dial string.

; Return to command mode :

The modem can be used to automatically dial voice calls when a telephone is connected to it. Placing a semi-colon at the end of a dial string forces the modem to return to on-line command mode after completing the dial string. The modem does not attempt to establish a data connection. The progress of the call can be monitored via the modem's speaker. If the call is answered, pick up the telephone handset and issue the ATH0 command (or lower DTR) to put the modem on hook. You may now proceed with a normal voice call.