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This chapter describes all of the commands that are available when the modem is in data communications mode. Details of the environments in which they are used is given in previous chapters. See later chapters for descriptions of the applicable commands for fax and voice modes.
Where applicable, the associated options and default for each command is shown. The command line must start with 'AT' and be terminated by <Return> or <Enter>. More than one command may be entered on one line. If commands are entered without a parameter, 0 is assumed, e.g. ATH is equivalent to ATH0. If a parameter outside the permitted range is entered, the ERROR message is returned.
Note: The default settings for your modem may differ from those listed here. See the supplied Installation Guide for details of any differences.
Many AT commands alter the current configuration of the modem. These changes remain in effect until another AT command overrides the settings or the modem is reset.
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A/ Repeat last command :
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This command re-executes the last command string (not to be preceded with AT or followed by pressing the RETURN key).
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A Answer :
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The A command forces the modem to check that the handset is in use or that RING has been detected. It then goes on-line in answer mode, gives out an answer tone and waits for a response. The modem returns the ERROR message if the above condition is not met. Once on-line the modem remains in answer mode for the period specified in register S7 before returning to local mode with the NO CARRIER message, or connecting to an incoming carrier and giving a CONNECT message.
The command is commonly used to switch from a previously established voice link to a data link so that data transfer can take place. The remote modem issues the D command. In this respect, the A and D act as a command pair and are used in conjunction with each other. The A command is not related to the automatic answer facility which allows the modem to respond automatically to incoming calls.
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\A Maximum V.42 block size :
| Options : \A0, \A1, \A2, \A3, \A4
| Default : \A3
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This command sets the maximum block size during connection. This option forces the modem chip set to transmit smaller blocks of data. This command is typically used for poor quality telephone-lines where transmitting smaller blocks of data avoids retransmissions, thus improving throughput.
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| \A0
| selects 64 character maximum block size.
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| \A1
| selects 128 character maximum block size.
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| \A2
| selects 192 character maximum block size.
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| \A3
| selects 256 character maximum block size.
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| \A4
| selects 32 character maximum block size (for ETC).
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%A Set Autoreliable Fallback Character :
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This command allows the user to specify the character that tells the modem to return (or fall back) from auto-reliable (MNP) mode to a normal connection on the answering modem. The default value is 013 |
B Line Modulation Options :
| Options : B0, B1, B2, B3
| Default : B0
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This command is used to configure the modem to the desired communications standard setting at a particular line speed.
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| B0
| selects ITU-T V.22 mode when the modem is at 1200 bits/s, and ITU-T V.21 when the modem is at 300 bits/s.
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| B1
| selects Bell 212A when the modem is at 1200 bits/s, and Bell 103 when the modem is at 300 bits/s.
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| B2
| selects V.23 originate mode receive 1200 bits/s, transmit 75 bits/s; answer mode receive 75 bits/s, transmit 1200 bits/s.
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| B3
| selects V.23 originate mode receive 75 bits/s, transmit 1200 bits/s; answer mode receive 1200 bits/s, transmit 75 bits/s.
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&B Autoretrain :
| Options : &B0, &B1, &B2
| Default : &B1
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This command controls the modem action when it receives a poor signal.
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| &B0
| Hang up on a poor received signal
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| &B1
| Retrain on a poor received signal. Hang up if condition persists longer than the threshold set in register S7.
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| &B2
| Do not hang up; do not retrain (i.e. tolerate any line).
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\B Transmit a break :
| Options : \B1 to \B9
| Default : N/A
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The \B command sends a break to the remote modem. It is equivalent to using the 'BREAK' key on your terminal. This command can be used if your keyboard does not have a 'BREAK' key.
In non-error corrected mode the break length is specified by the number associated with the option. \B1 transmits a break length of 100ms, \B2 200ms and so on.
Under error corrected conditions the break length is not user selectable. If no connection is established the ERROR message is returned.
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C Carrier Control Option (dummy command) :
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This command is used by some modems to control the transmit carrier. This chip set does not support C0 and will respond in error if this command is given. This chip set will accept C1 without error in order to ensure backward compatibility with communications software that issues this command.
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| C0
| Transmit carrier always off (returns ERROR).
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| C1
| Normal transmit carrier switching.
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%C Data Compression :
| Options : %C0, %C1
| Default : %C1
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This command controls the MNP Class 5. The modem can only perform data compression on an error corrected link.
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| %C0
| Disable MNP5 data compression.
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| %C1
| Enable MNP5 data compression.
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&C DCD (Data Carrier Detect) option :
| Options : &C0, &C1
| Default : &C1 |
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This command controls how the state of the DCD or RLSD (receive line signal detect) relates to the carrier from the remote modem. Always use &C1 for Sync mode.
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| &C0
| State of carrier from remote modem is ignored. RLSD circuit is always on.
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| &C1
| State of carrier from remote modem is tracked. RLSD circuit reflects the state of carrier.
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\C Set Autoreliable Buffer :
| Options : \C0, \C1, \C2
| Default : \C0
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This command determines whether the modem chip set buffers the data it receives during link negotiation.
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| \C0
| Does not buffer data.
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| \C1
| Buffers data on answering modem for 4s, until 200 characters have been buffered, and then switches to normal mode and the buffered characters are passed to the serial port. If a SYN character is detected, reliable mode is attempted.
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| \C2
| Does not buffer data on answering modem. Switches to normal mode upon receipt of autoreliable character and passes it to serial port.
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+VCID Caller ID option :
| Options : +VCID=0, +VCID=1, +VCID=2
| Default : +VCID=0 |
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This command controls whether the Caller ID functions of the modem are enabled. When enabled and the modem is connected to a telephone line that supports Caller ID, information relating to the calling number will be output by the modem for each incoming call. Depending on your telephone service provider, not all of the following fields will be available
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| +CID=0 Caller ID functions are disabled.
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+CID=1 Caller ID functions are enabled in formatted form. For example:
RING
DATE/TIME=0303 14:05
NMBR=5045551234
NAME=A N OTHER
RING
RING
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+CID=2 Caller ID functions are enabled in unformatted form. For example:
RING
0412303332323234303539313435353531322333435
RING
RING
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D Dial :
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The D command instructs the modem to go on-line in originate mode. If followed by a telephone number your modem dials the number and waits for an answer tone from the remote modem. Upon receiving an answer tone, connection is attempted. If the connection is established, data transfer can take place.
A number of commands are also valid parameters within the dial string. These are described in detail in the Automatic dialling chapter. The commands are: 0-9 P R T W , (comma) ! @ ; (semi-colon). In addition, the * (star) # (gate) A B C D characters can be used if recognised by your telephone exchange. However, these characters can only be used when tone dialling.
Entering D by itself can be used to switch a previously established voice link to a data link (see the A command). The modem returns the ERROR message if the handset was not previously in use.
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&D DTR (Data Terminal Ready) option :
| Options : &D0, &D1, &D2, &D3
| Default : &D2 |
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This command interprets how the modem will respond to the state of the DTR signal and changes to the DTR signal.
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| &D0
| DTR is ignored.
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| &D1
| Go to command mode on on-to-off DTR transition.
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| &D2
| Hang up and go to command mode on on-to-off DTR transition. Autoanswer is disabled if DTR is low. |
| &D3
| Reset on on-to-off DTR transition. |
E Echo select :
| Options : E0, E1
| Default : E1 |
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The E command is used to enable or disable the echo of AT commands when in local mode. With echo enabled (E1) the modem echoes back commands sent from the DTE. If an echo facility within your DTE software is also enabled you will see two of each character entered as part of the command line. To avoid this either disable local echo within your DTE or issue the E0 (echo off) command to the modem.
With echo off and no DTE local echo the AT command lines that you enter are not visible.
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F On-Line Echo Command (dummy command) :
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This modem will accept F1 without error in order to ensure backward compatibility with communications software that issues this command. Setting F to any other value will cause the modem to respond with an ERROR message.
F0 and values above 1 return ERROR, and F1 disables the ON-line echo. |
&F Restore factory defaults :
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When using the modem you may decide to change some of the S register and AT command settings.
The &F command is used to restore the original factory default settings for AT commands and S registers.
When used on its own the &F command temporarily restores the factory default settings until the modem is next powered up.
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&G Guard Tone Option:
| Options : &G0, &G1, &G2
| Default : &G2 |
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This command determines which guard tone, if any, to transmit while transmitting in the high band (answer mode). This command is only used in V22 and V22bis mode.
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| &G0
| No guard tones.
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| &G1
| 550 Hz guard tone.
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| &G2
| 1800 Hz guard tone. |
\G Set Modem Port Flow Control (only in Normal Mode):
| Options : \G0, \G1
| Default : \G0 |
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This command selects the modem chip set's DCE or port flow control. If the modem chip set is receiving data from the remote modem faster than it can be processed, enabling port flow control allows the modem chip set to handle the data efficiently.
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| \G0
| Disables port flow control.
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| \G1
| Sets port flow control to XON/XOFF flow control.
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%G Auto fallforward/fallback enable :
| Options : %G0, %G1
| Default : %G1 |
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This command selects the auto fallback option. |
| %G0
| Disabled.
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| %G1
| Enabled.
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H On/off hook :
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In the same way that a telephone is said to be on-hook or off-hook, the modem has the same two states. When not in use the modem is on-hook. It automatically goes off-hook when an attempt is made to dial or an incoming call is detected with auto answer enabled.
Issuing the H0 command puts the modem on hook and consequently terminates the call. This can only take place after the escape sequence puts the modem into command mode. H0 also terminates any &T test in progress. Dropping DTR is often a more convenient method of terminating a call.
H1 takes the modem off hook in command mode. The modem returns on-hook after the period specified in register S7.
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"H V.42bis Compression Control :
| Options : "H0, "H1, "H2, "H3
| Default : "H3 |
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This command controls V.42bis data compression over an error correction link.
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| "H0
| Disable V.42bis.
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| "H1
| Enable V.42bis only when transmitting data.
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| "H2
| Enable V.42bis only when receiving data.
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| "H3
| Enable V.42bis for both directions.
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I Identify :
| Options : I0, I1, I2, I3, I4, I5
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The I command is used to request information from the modem as listed below:
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| I0
| Reports the modem's identification followed by its version number.
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| I1
| Reports a 4-digit ROM checksum. Displays the checksum on the DTE.
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| I2
| Performs a ROM check. Calculates and verifies the checksum, " OK" or "ERROR".
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| I3
| Modem Identity string and driver version.
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| I4
| Returns the modem's identification followed by its version number.
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| I5
| Returns the board/firmware ID
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| I9
| Returns the country code in English
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\J DTE auto rate adjustment :
| Options : \J0, \J1
| Default : \J0 |
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The DTE data rate normally remains constant irrespective of the type of connection established. This command lets the modem adjust the DTE speed to match the line speed when the connection is complete.
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| \J0
| disables adjustment of DTE speed to match line speed.
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| \J1
| enables adjustment of DTE speed to match line speed.
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-J Set V.42 Detection Phase :
| Options : -J0, -J1
| Default : -J1 |
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This command determines whether the originating modem sends a sequence to the answering modem to determine the type of error correction it has enabled.
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| -J0
| Disables the V.42 detection phase.
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| -J1
| Enables the V.42 detection phase.
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&K DTE-to-modem flow control :
| Options : &K0, &K3, &K4
| Default : &K3
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Both software (XON/XOFF) and hardware (RTS/CTS) flow control are available when the modem is communicating with the local DTE.
When the modem's internal data buffer is virtually full it issues an XOFF and/or drop CTS instructing the DTE to stop sending data. When the buffer is nearly empty the modem issues an XON and/or raises CTS to resume data flow.
In software flow control (&K4) the XON/XOFF characters are filtered out of the data stream by the modem before being transmitted to the remote modem.
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| &K0
| disables flow control. The XON/XOFF characters are ignored and CTS is held active (HIGH) by the modem.
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| &K3
| enables bi-directional hardware RTS/CTS flow control.
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| &K4
| enables bi-directional software XON/XOFF flow control.
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\K Set Break Control:
| Options : \k0, \k1, \k2, \k3, \k4, \k5
| Default : \k5
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This command selects the way in which the modem handles a break signal. This modem chip set can handle a break signal in one of three ways: expedited destructive, expedited, or queued break.
(In on-line data mode with V.42 protocol or normal mode, then break signal to DTE through RS-232).
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| \k0, \k2, \k4:
| Enter on-line command mode but do not send break to the remote modem.
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| \k1:
| Empty the data buffers and send break to the remote modem immediately.
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| \k3:
| Send break to the remote modem immediately.
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| \k5:
| Send break to the remote modem in sequence with data.
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(In on-line data mode with direct mode, then receive break at serial port).
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| \k0, \k2, \k4:
| Immediately send break and enter on-line command state.
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| \k1, \k3, \k5:
| Immediately send break through.
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(In on-line command mode with V.42 protocol or normal mode, and the DTE issues a transmit break command).
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| \k0, \k1:
| Empty the data buffers and send break to the remote modem.
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| \k2, \k3:
| Immediately send break to the remote modem.
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| \k4, \k5:
| Send break to the remote modem in sequence with data.
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(In on-line data mode with normal mode, receive break signal from remote at modem port, send to serial port).
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| \k0, \k1:
| Empty the data buffers and send break to the local DTE
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| \k2, \k3:
| Immediately send break to local DTE.
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| \k4, \k5:
| Send break to local DTE in sequence with data.
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L Speaker volume :
| Options : L0, L1, L2, L3
| Default : L2
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The L command sets the in-built speaker volume. The M command controls when the speaker is enabled. Take care not to select the HIGH volume setting if using headphones connected to the rear panel socket of a voice modem.
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| L0
| selects LOWEST speaker volume.
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| L1
| selects LOW speaker volume.
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| L2
| selects MEDIUM speaker volume.
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| L3
| selects HIGH speaker volume.
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M Speaker control :
| Options : M0, M1, M2, M3
| Default : M1
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The M command is used to specify when the modem's built-in speaker is active. Speaker volume is determined by the L command.
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| M0
| disables the speaker completely so that no audible call progress is provided.
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| M1
| enables the speaker through dialling. Upon establishing a connection the speaker is disabled.
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| M2
| permanently enables the speaker. Should be used to give an audible indication of the call progress to assist in diagnosing any possible difficulties in establishing a connection.
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| M3
| enables the speaker after dialling. Upon establishing a connection the speaker is disabled.
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&M Select Async Mode :
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This range of modems can only operate in Async Mode, so &M0 is the only setting that can be used. See also &Q
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