Hayes modem AT strings and commands

First developed by Dennis Hayes, the AT commands can specify settings for Hayes compatible legacy modems. If any of these strings happen to cause additional issues with your modem, use the ATZ command to reset the modem back to default factory settings.

Example of generic AT modem strings

AT&F&C1&D2
or
AT&FE1Q0V1X4&C1&D2

General at commands

AT - Gets the modem's attention. This prefix must be included in all commands unless otherwise noted.
+++ - Switches the modem from data to command state. This command is not proceeded by AT.
A - Instructs the modem to attempt to answer a call.
A/ - Re-executes the last modem executed command. This command is not proceeded by AT.
Bn - Sets the operation standard 0=CCITT, 1=Bell.
Cn - Turns the modem transmit carrier signal on and off 0=turn off the modem transmit carrier signal during normal operation. 1=Turn on modem transmit carrier signal during normal operation.
Dn - Dials the number, n. T - Touch-tone dialing, P - Pulse dialing, , - pause 2 seconds, @ - Wait for 5 seconds of silence, R - Dial an originate-only modem, S=n - Dial one of four stored telephone numbers, : - Return to command mode after dialing. ! - Go on hook for 1/2 second.
En - Turns the command echo feature on/off so the commands' either are displayed or are not displayed on the screen 0=off, 1=on.
Hn - Causes the modem to hang up or pick up when in data mode 0=hang up, 1=pick up.
In - Identifies the modem code and the status of the ROM 0=display 3-digit product ID code (preset to 248), 1=display checksum, 2=display checksum and display OK or ERROR.
Ln Sets the internal speaker volume 0,1=lowest, 2=medium, 3=highest.
Mn - Determines when the speaker is on and off. 0=always off, 1=on during call and off when receiving data carrier, 2=always on, 3=off when receiving carrier and during dialing but not when answering.
On - Switches the modem from the command state to the online state. 0=returns the modem to the online state when a connection is still open, 1=returns the modem to the online state and initiates a V.22 bis retain sequence.
Qn - Displays or suppresses (quiets) result codes 0=displays codes (default), 1=suppresses codes
Sr? - Read and display register r
Sr=n - Set register r to value n
Vn - Displays result codes in short form (numbers) or long form (test). 0=Send numeric responses, 1=send word responses.
Xn - Determines result codes that displays the following modem operations. 0=Basic response set with blind dialing, 1=Extended response set with blind dialing, 2=Extended response set with dial tone detection, 3=Extended response set with blind dialing and busy signal detection, 4=Extended response set with dial tone and busy signal detection.
Yn - Enables or disables the long space disconnect 0=disable, 1=enable.
Zn - Resets the modem and recalls a user profile. 0=recall user profile 0, 1=recall user profile.

& commands

Cn - Controls the data carrier detect signal, leaving the DCD interchange circuit on at all times or turning it on only when a data carrier is detected. 0=force DCD signal active, 1=DCD signal indicates true state of remote carrier signal.
&Dn - Controls DTR (data terminal ready) transition. 0=Ignore DTR signal, 1=Return to command mode when after losing DTR, 2=Hang up, turn off auto answer, and return to command mode after losing DTR, 3=Reset after losing DTR.
&Fn - Returns the configuration to the factory settings. 0=Fetch default configuration, 1=Recall factory default configuration, 2=Recall Sierra factory default configuration for auto-reliable MNP, 3=Recall Sierra factory default configuration for auto-reliable V.42bis.
&Gn - Selects the guard tone (not used on calls in the U.S.). 0=Disable guard tone, 1=set guard tone on answering modem to 550 Hz, 2=Set guard tone to 1800 Hz.
&Mn - Sets operation; 0=Sets asynchronous operation.
&Pn - Selects the ratio of the make/break interval that the modem uses for pulse dialing. "make" refers to off-hook, "break" refers to on-hook, 0=39/61 for U.S., 1=33/67 for United Kingdom.
&Sn - DSR override. 0=Force DSR signal active; 1=DSR active according to the CCITT specification.
&Tn - Test and diagnostics. 0=End current test, 1=Start local analog loopback test, 2=Unknown, 3=Start local digital loopback test, 4=Grant remote request for remote digital loopback test, 5=Deny remote request for remote digital loopback test, 6=Start remote digital loopback test, 7=Start remote digital loopback test with self-test, 8=Start local analog loopback test with self-test.
&V - Displays the user profiles and stored numbers.
&Wn - Stores the current configuration as Profile 0 or Profile 1 0=Profile 0, 1=Profile 1.
&Xn - Selects the synchronous clock source. 0=Provide synchronous clock on EIA pin 15.
&Yn - Selects what user profile will be the default at power-on and reset. 0=Profile 0, 1=Profile 1.
&Z=x - Stores telephone numbers x in location n (0-3). x=up to 32 characters. For pulse dialing: 0-9 and dial modifiers for touch dialing, 0-9, dialing modifiers, A-D, #, and *.

MNP commands

\An Selects the maximum MNP block. 0=64-character maximum MNP block. 1=128-character maximum MNP block, 2=192-character maximum MNP block, 3=256-character maximum MNP block.
\Bn - Sends a break command to a remote modem n=0-9 in 100 milliseconds.
\Cn - Specifies whether data is to be buffered or not. 0=Do not buffer data, 1=Buffer data for 3 seconds, until character are received, or until packet is detected.
D/n - Dials one of four stored telephone numbers.
DL - Redials the last telephone number dialed.
\Gn - Disables or enables DCE flow control.
\Jn - Turns off or turns on DTE baud adjustment. 0=Turn off DTE baud rate adjustment.
\K - Sets the break control.
\Nn - Selects connection. 0=Allows standard buffered connection only. 1=Allows direct connection only, 2=Allows reliable MNP connection only, 3=Allows auto-reliable MNP connection.
\P - Stores telephone number.
\Qn - Selects flow control. 0=Turns off DTE flow control, 1=XON/XOFF bi-directional software flow control, 2=CTS signal unidirectional software flow control, 4=XON/XOFF unidirectional flow control (V.42 only)
\S - Displays modem status.
\T - Inactivity timer.
\U - Accepts reliable link from standard link.
\Vn - Selects send or do not send for MNP responses.
\Xn - Sets standard XON/XOFF pass through flow control. This command works only when a standard buffered connection is established and XON/XOFF flow control is being used. 0=Modem acts on XON/XOFF received from the local DTE, but does not pass XON/XOFF to the remote modem. 1=Modem acts on XON/XOFF received from the local DTE and passes XON/XOFF to the remote modem. Using this switch allows the local DTE to XON/XOFF the remote DTE through the established modem link.
\Y - Switches standard link to reliable link.
\Z - Ends the reliable connection and switches to the standard connection.
%A - Sets the auto-reliable fallback character.
%Cn - Disables or enables class 5 MNP operation. 0=Disable class 5 MNP operation, 1=Enable Class 5 MNP operation.
%En - Disables or enables auto retain 0=Disable auto retain, 1=Enable auto retain.