基本ATコマンド

This reference includes all the commands that are found in industry standards as well as commands that appear in the reference modem designs and specifications that are published by the major modem chipset manufacturers or modem firmware designers.

This section describes AT modem commands that do not have a prefix character such as "&". (Commands with prefixes are described in another section.)

After a description of the function of the command or option, the "Standards: section will indicate if the described command is compatible with a standard, such as EIA/TIA-602. The section number of the standard that covers the command is included.

The "Compatibility" entry uses the following symbols to indicate which modems the described command is compatible with:

E
EIA/TIA-602 Standard (Data Mode commands only)
2
Hayes Smartmodem 2400
4
Hayes Ultra 14,400
R
Modems containing Rockwell chipsets or Connexant firmware (Yellow)
U
Modems containing USR/3Com/TI chipsets and firmware (Purple)
S
Modems containing Sierra Semiconductor chipsets and firmware (White) White
L
Modems containing Lucent/AT&T Microelectronics chipsets and firmware (Orange)
T
Modems manufactured by Telebit (Blue)
C
Modems containing Cirrus Logic chipsets and firmware (Red)
The colors are used below when a command or an interpretation of a command is completely unique to the listed vendor. This marking should be treated as a warning that the command is non-portable and should be avoided if possible.

This document cannot include every command available on every modem claiming to be AT Command Set compatible, and the documentation for a specific modem should always be trusted over any other reference when it comes to what commands are available and what actions they perform. For example, a given command may have a completely different function on one brand of modem than on other brands. In cases where conflicts are known, the conflicting commands and their behavior are described below.

Items that are marked in Green are the default numeric setting for that command. Be aware that different vendors may have a different default value for the same command or S-register. When known, this is documented.

AT Command Description
A
(Answer)
Forces the modem into an off-hook state and the modem begins offering answer carriers in the same sequence that the modem would use if it was responding to an incoming call when S1 is non-zero and the number of incoming rings specified in S1 has been reached.
Standards: EIA/TIA-602 6.1.5
Compatibility: 2 4 E R U S L T C
B
(Bell Mode)
Specifies whether the modem should use CCITT/ITU answer tones and modulations, or use modulations compatible with the AT&T Bell 212 transmission system.
Standards: The "B" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
B0
(Default)
The CCITT/ITU V.25 (or later) answer sequence is used. For 1200 baud transmissions, V.22 modulation is used. B0 is the default setting in all modern modems. Some modems select V.21 for 300 baud operation with this command.
Compatibility: - 4 - R U S L T C
Note: Sierra modems that perform V.34 or faster do not support the "B" commands and those that do use "B1" as the default setting.
B1 The Bell 212 answer sequence is used. For 1200 baud transmissions, Bell 212 modulation is used. This mode is for backward compatibility with the proprietary (and very obsolete) Bell 212 transmission system. Some modems select Bell 103J for 300 baud operations with this command.

Setting "B1" when operating at speeds higher than 1200 baud should be ignored by the modem, but some modern modems may malfunction if "B1" is set.


Compatibility: - 4 - R U S L T C
Note: Sierra modems that perform V.34 or faster do not support the "B" commands and those that do use "B1" as the default setting.
B2 The "B2" command dictates that ITU-T V.23 modulations will be used. When the DTE speed is 1200bps and the modem is originating, transmit will be at 75bps and receive at 1200bps. For answer, transmit will be at 1200bps and receive at 75bps.
Compatibility: - 4 - - - S L - -
Note: Sierra modems that perform V.34 or faster do not support the "B" commands and those that do use "B1" as the default setting.
B3 The "B3" command dictates that ITU-T V.23 modulations will be used. When the DTE speed is 1200bps and the modem is originating, transmit will be at 1200bps and receive at 75bps. For answer, transmit will be at 75bps and receive at 1200bps.
Compatibility: - 4 - - - S L - -
Note: Sierra modems that perform V.34 or faster do not support the "B" commands and those that do use "B1" as the default setting.
B15 On Lucent modems, the "B15" command specifies that ITU-T V.21 modulations will be used when the DTE speed is 300bps.
Compatibility: - - - - - - L - -
B16 On Lucent modems, the "B16" command specifies that Bell 103J modulations will be used when the DTE speed is 300bps.
Compatibility: - - - - - - L - -
C
(Carrier
Control)
The "C" command controls how the modem manages carrier. In the original Hayes Smartmodem 1200, this command controlled carrier during Data operations. Years later, some vendors re-used this command to control FAX carrier. On modern modems, the command has no effect on data modem operations.
Standards: The "C" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
C0 The "C0" command forces the modem to not transmit carrier. It can receive data via an incoming carrier. This is meant for 1200bps and slower transmission speeds only and has applications in amateur radio and RTTY operations.
Compatibility: 2 - - - - - - - -
Note: Most modern modems do not accept this command and have no equivalent command available.
C1
(Default)
The "C1" command allows carrier to be transmitted when needed.
Compatibility: 2 - - R - - L - C
Note: Most modern modems accept and silently ignore this command. This behavior is performed entirely to be compatible with an obsolete software packages that issue this command and expect it to be accepted.
D
(Dial)
This command is used to initiate an outgoing modem connection. The modem is placed off-hook if not already in that state. Based on any specified modifiers and previously entered settings, the modem may also listen for dial-tone, then transmit a telephone number to the Public Switched Telephone Network (PSTN). The modem may then listen for and negotiate carrier with a remote modem or simply return to Command Mode.

(For subsequent brevity, the phrase "dials" or "dialing" means to transmit one or more digits or symbols using either pulse or DTMF signaling into the Public Switched Telephone Network or PSTN.)

Any dialing operation (including timed pauses) can be aborted, causing the modem to go on-hook and return to Command Mode, if the entire dialing and carrier negotiation operation exceeds the time limit specified by the S7 register, or the DTE sends a character after the carriage return and before receiving a Result Code from the modem, or by the DTE dropping DTR if &D2 is set.


Standards: EIA/TIA-602 5.4.1, 6.1.1
Compatibility: Not All Dial Modifiers are implemented on all modems or may have different behavior. See individual Dial Modifier descriptions.
Modifiers Description
0 to 9 Dials the specified digit.
Standards: EIA/TIA-602 6.1.1.1
Compatibility: 2 4 E R U S L T C
#, * Dials the # (pound) or * (asterisk) DTMF codes. (DTMF dialing only.)
Standards: EIA/TIA-602 6.1.1.1
Compatibility: 2 4 E R U S L T C
@
(at sign)
Dialing operations pause at this point for silence on the line plus a delay of five seconds before performing the next dial modifier. This is useful for allowing plenty of time for a dial-tone to appear when blind-dialing must be used or when a dial-tone is present that is not reliably recognized by the modem, such as the stuttering dial-tone used in Italy and also used with some "voice-mail" systems.

Depending on the modem, the "X3" or higher Response Code may be required for the modem to listen for silence on the line. In such cases, the only action will be a five-second delay. EIA/TIA-602 allows modem makers to make the "@" modifier behavior to vary based on the current Response Code.


Standards: EIA/TIA-602 6.1.1.7
Compatibility: 2 4 E R U S L T C
,
(comma)
Dialing operations pause at this point for the amount of time specified by the "S8" register, normally two seconds. This is useful for waiting for a dial-tone when blind-dialing must be used or when a dial-tone is present that is not reliably recognized by the modem, such as the stuttering dial-tone used in Italy and also used with some "voice-mail" systems. The "," modifier is similar to the "@" modifier, except that the amount of time the modem pauses is adjustable when using the "," modifier.
Standards: EIA/TIA-602 6.1.1.2
Compatibility: 2 4 E R U S L T C
!
(exclam-
ation
point)
At this point in the dialing operation, the modem goes on-hook briefly before returning to the off-hook state. This action is known as the "Hook Flash" (sometimes called "Flash Hook"). The on-hook time is not adjustable and should be compatible with the PSTN regulations of the country that the modem is certified to be used in.
Standards: EIA/TIA-602 6.1.1.5
Compatibility: 2 4 E R U S L T C
;
(semi-
colon)
Return to Command Mode. Subsequent characters on this command line are treated as Commands rather than Dial Modifiers. If sufficient digits and other parameters preceded the semi-colon and a remote modem answers, the modem will negotiate carrier and other connection parameters despite being in Command Mode, unless a subsequent command places the modem on-hook.
Standards: The semicolon Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: 2 4 - R U S L T C
A, B,
C or D
Dials the specified extra DTMF symbols. (DTMF dialing only.)
Standards: EIA/TIA-602 6.1.1.1
Compatibility: 2 4 E R - S L T C
Note: USR/3Com modems do not implement this part of the EIA/TIA-602 specification.
J On Rockwell modems, the J Dial Modifier causes the modem to perform a MNP 10 link negotiation at 1200bps for this call only.
Standards: The "L" Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: - - - R - - - - -
Note: See the related "*H" command.
K On Rockwell modems, the K Dial Modifier enables power level adjustments during MNP 10 link negotiation for this call only.
Standards: The "K" Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: - - - R - - - - -
Note: See the related ")Mn" command.
L The L Dial Modifier causes the modem to dial the last number dialed. This only works if the last time the modem went off-hook, a number was actually dialed. If the modem goes off-hook, then on-hook, or the modem has been reset, a subsequent L modifier will not dial a number. (The command string "ATDL" provides the same result as the "A/" command.)
Standards: The "L" Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: - 4 - R - S L - -
P Digits dialed after this modifier are to be performed using pulse (also known as rotary) dialing at the standard 10pps pulse rate. This setting remains in effect for subsequent ATD commands and modems with non-volatile memory may save this setting along with other stored parameters.
Standards: EIA/TIA-602 6.1.1.4
Compatibility: 2 4 E R U S L T C
R The R Dial Modifier instructs the modem to use the transmission frequencies normally associated with the answering modem even though this modem is originating the call. The called modem must respond with the originate transmission frequencies.
Standards: The "R" Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: 2 - - - U - L T C
S=n The S Dial Modifier causes the modem to dial a telephone number previously stored in the modem using the "&Z" command. The number of different numbers that can be stored varies by modem model, with the most common number being four, allowing "S=0" through "S=3" to be used in Dial commands.
Standards: The "S" Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: - 4 - R - S L T C
Note: USR/3Com modems use a syntax different than that of other modems. Telebit modems allow the larger range "S=0" through "S=9".
Sn On USR/3Com modems, the S Dial Modifier causes the modem to dial a telephone number previously stored in the modem using the "&Z" command. The number of different numbers that can be stored varies by modem model, with the most common number being four, allowing "S0" through "S3" to be used in Dial commands.
Standards: The S Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Not: All other modems that support stored telephone numbers use the "S=n" command to perform the same operation.
T Digits and Symbols dialed after this modifier are to be performed using DTMF (also known as Touch-Tone) dialing. This setting remains in effect for subsequent "ATD" commands and modems with non-volatile memory may save this setting along with other stored parameters.
Standards: EIA/TIA-602 6.1.1.3
Compatibility: 2 4 E R U S L T C
W Dialing operations pause at this point until dial-tone energy is detected. This can be used to wait for a second dial-tone when dialing through a PBX to an outside line. It is optional for the modem maker to detect a busy-signal while pausing for dial-tone, and this only occurs if busy-detection is allowed by the setting of the "X" command. If the "W" command does not work reliably on a given telephone line, the "@" or "," (comma) Dial Modifiers should be used instead.
Standards: EIA/TIA-602 6.1.1.6
Compatibility: - 4 E R U S L T C
V On Lucent modems, when the "V" modifier is executed, the modem will switch into speakerphone mode and resume processing the dialing string.
Standards: The "V" modifier is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - - L - -
$ On USR/3Com modems, the "$" modifier sends a list of supported Dial Modifiers to the DTE.
Standards: The "$" modifier is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
$ On Telebit modems, the "$" modifier causes the modem to wait for a calling card billing prompt tone. If the tone is not detected in the amount of time specified by the "S7" register, the modem returns a "NO PROMPTTONE" response to the DTE.
Standards: The "$" modifier is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - - - T -
Note: Rockwell modems use the "&" modifier for a similar function.
& On Rockwell modems, the "&" modifier causes the modem to wait for a calling card billing prompt tone. If the tone is not detected in the amount of time specified by the "S7" register on US models or the "S6" register on W-Class models, the modem aborts the call attempts and send an error response to the DTE.
Standards: The "&" modifier is not part of the EIA/TIA-602 standard.
Compatibility: - - - R - - - - -
Note: Telebit modems use the "$" modifier for a similar function.
> On Rockwell modems, the ">" modifier causes certain modems meant to be used in certain countries to generate a grounding pulse on the EARTH relay output. In other situations, using the ">" modifier may be ignored or may return an error.
Standards: The ">" modifier is not part of the EIA/TIA-602 standard.
Compatibility: - - - R - - - - -
^ When dialing in FAX mode, the calling tone will be disabled. During a normal outgoing FAX call, a FAX calling tone is transmitted at regular intervals so that the answering FAX device will know another FAX device is calling. Unlike a data modem, the answering FAX device normally remains silent until it detects the calling tone.
Standards: The "^" modifier is not part of the EIA/TIA-602 standard.
Compatibility: - - - R - - L - -
Note: Rockwell documentation describes this modifier as a toggle. Lucent states only that it disables the calling tone.
\label\ Similar to the "S=n" command, the backslash modifier directs the modem to dial a stored number. When numbers are stored using the ~N3 command, a textual label may be included that can be referenced by the backslash modifier rather than using the stored number directory entry number. For example, if "AT~N2=817-970-5555\IDLINE\" was stored, that stored number could be dialed later by issuing the command "ATDTS=2" or by "ATDT\IDLINE\".
Standards: The "\" modifier is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - - - T -
-
(minus
or dash)
On Hayes Smartmodems, the "-" modifier causes the modem to transmit a 1270Hz tone for 186msec, followed by 62msec of silence. This implements a "DASH" in morse code. The "-" modifier and the other morse code modifiers must appear after a ";" modifier. Before the ";" modifier, the "-" character may appear within a telephone number being dialed and will be ignored.

To demonstrate the morse code capability, the command sequence "ATD;-.-./--.--H0" would take the modem off-hook, transmit the letters "C" and "Q" in morse code, and place the modem on-hook. The resulting code rate would be 20 words per minute and is compliant with FCC regulations in applications such as automated repeater identification.


Standards: The "-" modifier is not part of the EIA/TIA-602 standard.
Compatibility: 2 - - - - - - - -
Note: Almost all modern modems do not implement this capability.
.
(period
or dot)
On Hayes Smartmodems, the "." modifier causes the modem to transmit a 1270Hz tone for 62msec, followed by 62msec of silence. This implements a "DOT" in morse code. The "." modifier and the other morse code modifiers must appear after a ";" modifier. Before the ";" modifier, the "." character may be treated as an invalid character.
Standards: The "-" modifier is not part of the EIA/TIA-602 standard.
Compatibility: 2 - - - - - - - -
Note: See the "-" modifier for an example of the use of the morse code feature. Almost all modern modems do not implement this capability.
/
(slash)
Operations pause at this point for 125 msec. This can be used to implement a pause for character spacing when transmitting in Morse code.
Standards: The slash Dial Modifier is not part of the EIA/TIA-602 specification.
Compatibility: 2 - - - - - L - -
Note: See the "-" modifier for an example of the use of the morse code feature. Almost all modern modems do not implement this capability.
E
(Local
Echo)
When the modem is in Command Mode, this command specifies whether the modem should echo characters received from the DTE back to the DTE.
Standards: EIA/TIA-602 6.2.4
Compatibility: 2 4 E R U S L T C
Modifiers Description
E0 When the modem is in Command Mode, command characters received from the DTE are not echoed back to the DTE.
E1
(Default)
When the Modem is in Command Mode, command characters received from the DTE are echoed back to the DTE. This is the typical default setting.
F
(Online
Echo)
When the modem is in Data Mode (online), this command specifies whether the modem should echo characters received from the DTE back to the DTE. In some documents, this command is also described as placing the DTE interface on the modem into half or full duplex mode.
Standards: The F Command is not part of the EIA/TIA-602 specification.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
F0 When the Modem is in Data Mode, characters received from the DTE are echoed back to the DTE.
Compatibility: 2 - - R U - - - -
Note: Lucent and Cirrus modems do not accept the "F0" command but accept and silently ignore the "F1" command.
F1
(Default)
When the modem is in Data Mode, characters received from the DTE are not echoed back to the DTE. This is the typical default setting.
Compatibility: 2 - - R U - L - C
Note: Lucent and Cirrus modems accept and silently ignore the "F1" command.
H
(Hook
Control)
Forces the modem on or off-hook, regardless of the current activity of the modem.
Standards: EIA/TIA-602 6.1.6 (H0 only)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
H0
(Default)
The modem goes on-hook, terminating any call in progress. If a data connection was in progress, a tear-down of the connection may occur before going on-hook.
Standards: EIA/TIA-602 6.1.6
Compatibility: 2 4 E R U S L T C
H1 The modem goes off-hook. The modem does not attempt to establish a data connection and will not respond to an answer carrier received from a remote modem. If the modem has a speaker which is enabled and the volume is turned-up, the sounds of the telephone line should be heard.
Standards: The "H1" command is not part of EIA/TIA-602.
Compatibility: 2 4 - R U S L T C
H2 In the Hayes Smartmodem, the modem goes off-hook by the modem activating the line relay, but the auxillary relay is not activated. Both relays are activated in the "H1" command. The Smartmodem 1200 documentation suggests ways in which the "H1" and "H2" commands can be used in Amateur Radio and RTTY applications since this modem provides additional commands that allow the DTE to force specific carrier tones to be transmitted.
Standards: The "H2" command is not part of EIA/TIA-602.
Compatibility: 2 - - - - - - - -
Note: See the "S10" register and the "C0" command.
I
(Information)
The "I" commands display information about the modem (DCE) as one or more lines of information text as determined by the manufactuer. The "I" commands are used so inconsistently by the chipset makers and the modem manufacturers that the EIA/TIA-602 explicitly warns against using the returned results from an "I" command to determine the DCE (modem) type or in making any other determination about the DCE capabilities or configuration. Despite this official warning, certain proprietary operating systems routinely try to guess the modem type and its capabilities by using the results of "I" commands.

The most common uses of "I" commands are listed below. Most option values are repeated. This is done when one modem maker uses a particular option number in a way significantly different from the other modem makers shown in this reference.


Standards: The EIA/TIA-602 specification considers the "I" command to be optional and the results unreliable. Be sure to read more about this caution above. (EIA 6.1.0)
Compatibility: Not all options are implemented in all modems and it should be assumed that no two models of modems will perform the same function in response to a given "I" command or return similar results.
Modifiers Description
I0 A numeric value is returned equal to the maximum speed the modem is capable of operating at. Typically, 2400bps modems return "240" (in keeping with all AT-compatible modem registers being 8-bits wide, with a numeric range of 0 to 255). A 1200bps modem should return "120". Later, Hayes and some vendors tried to remain consistent to the three character limit by returning the most-significant digits of the modems maximum speed. That means that the "I0" command returns "144" for 14,400bps modems, "288" for 28,800bps "336" for 33,600bps, and "56" for 56,000bps modems (56 is used because 560 is greater than 255.) However, some modem vendors deviate from this traditional use of the "I0" command, returning a four or five-digit result, such as "33600" for a V.34 modem or "56000" for a V.90 or V.92 modem.

Telebit modems return the model number of the modem rather than a maximum speed indication.


Compatibility: 2 4 - R U S L T C
Note: Being listed for compatibility only indicates that the modem will return a result, but the result will vary by manufacturer or modem model.
I1 A three or four digit checksum of the modem ROM (Read Only Memory) is displayed. In modems with no physical controller (aka "soft" modems), an arbitrary value is usually returned, sometimes a firmware revision code. Some vendors also ensure that their firmware checksum equals "000" or "0000" by padding the executable image. Refer to the manual that came with a modem to determine what value is correct for that modem.


Compatibility: 2 4 - R U S L T -
Note: Being listed for compatibility only indicates that the modem will return a result, but the result will vary by manufacturer or modem model.
I1 Telebit modems return a sum of results of various self-tests in addition to a firmware validity test. Any non-zero value indicates that a fault has been detected.
Compatibility: - - - - - - - T -
I1 Cirrus modems return the firmware revision.
Compatibility: - - - - - - - - C
I2 The modem ROM is checksummed and the result is compared against what the correct checksum should be, a value also stored in the ROM. If the checksum is correct, the modem returns "OK" (or "0"); otherwise, it returns "ERROR" (or "4"). In modems with no physical ROM (aka "soft" modems), the test is not performed and a "OK" (or "0") result is always returned.

On some modems, a second "OK" (or "0") is displayed following the test result.


Compatibility: - 4 - R - S L - C
Note: Being listed for compatibility only indicates that the modem will return a result, but the result will vary by manufacturer or modem model.
I2 On USR/3Com modems, a checksum is performed on the RAM in the modem.
Compatibility: - - - - U - - - -
I2 Telebit modems return the current CONNECT status.
Compatibility: - - - - - - - T -
I3 The modem firmware revision is displayed. On so-called "soft" or "controllerless" modems that have no on-board firmware, the revision of operating system support drivers that emulate the modems controller on the DTE may be displayed and this command may work even if the physical modem is not present. Some versions of "soft" modems display a meaningless string for this command or simply return OK.
Compatibility: - - - R - S L T -
Note: Being listed for compatibility only indicates that the modem will return a result, but the result will vary by manufacturer or modem model.
I3 On Cirrus modems, the device set name is displayed. No other information was provided.
Compatibility: - - - - - - - - C
I4 On Rockwell modems, displays an OEM identifier string.
Compatibility: - - - R - - - -
Note: Being listed for compatibility only indicates that the modem will return a result, but the result will vary by manufacturer or modem model.
I4 Displays the current modem settings.
Compatibility: - - - - U S - -
Note: Being listed for compatibility only indicates that the modem will return a result, but the result will vary by manufacturer or modem model.
I4 Lucent modems return configuration data on the country that the modem is designed for. For example: "North America - M40Y3BSACH03".
Compatibility: - - - - - - L -
I4 Cirrus modems return configuration data on how the modem firmware and hardware are configured, using this table of bit settings:
Bit
Indicates
0
0 == Hayes Escape Sequence with Guard Time
1 == T.I.E.S. Escape Sequence
1
0 == Data/fax/voice without V.42/MNP firmware
1 == Data/fax/voice with V.42/MNP firmware
2
0 == Serial Host interface (probably an external modem)
1 == Parallel Host interface (probably an internal modem)
3
0 == No external Static RAM present
1 == External Static RAM present
4
0 == 8K x 8 Static RAM present (valid only if Bit 3 == 1)
1 == 32K x 8 Static RAM present (valid only if Bit 3 == 1)
5-7
Reserved

Compatibility: - - - - - - - C
I5 On Rockwell modems, displays the country code parameter. No other information was provided.
Compatibility: - - - R - - - -
I5 On USR/3Com modems, "I5" displays the stored modem settings.
Compatibility: - - - - U - - -
I5 Telebit modems return the last number dialed.
Compatibility: - - - - - - - T
I6 Displays the datapump or DSP revision and possibly the datapump capabilities. On most datapumps and DSPs, including essentially all designed prior to 1997, the datapump or DSP usually cannot be reprogrammed or completely changed in the field, so this can be used to determine what types of modulations the physical hardware is capable of and/or what stepping/mask of the silicon is present in this modem.
Compatibility: - 4 - R - - - -
Note: Being listed for compatibility only indicates that the modem will return a result, but the result will vary by manufacturer or modem model.
I6 On USR/3Com modems, this command reports information about V.42/V.42bis negotiation, link diagnostics and other internal information. This command also displays the current (or end-of-connection) transmit and receive speeds.
Compatibility: - - - - U - - -
Note: Not all USR/3Com modems that support this command have it documented.
I6 Sierra modems return information on the DSP and MCP revision.
Compatibility: - - - - - S - -
I7 Rockwell modems display the DAA code. Rockwell documentation states that this value is obtained by interrogating the DAA for auto DAA recognition. A better explanation was not provided. Some of the codes returned are:
000
United States or Canada
016
Japan
033
Belgium
034
Finland
035
France
037
Italy
038
Netherlands
039
Sweden
040
Switzerland
041
United Kingdom

Compatibility: - - - R - - - -
I7 USR/3Com modems display product configuration data.
Compatibility: - - - - U - - -
Note: Not all USR/3Com modems that support this command have it documented.
I9 USR/3Com modems display its Plug and Play status.
Compatibility: - - - - U - - -
Note: Not all USR/3Com modems that support this command have it documented.
I11 On USR/3Com V.34 and faster modems, this command reports information about the loop signal quality. Of particular note, this command displays the current or end-of-connection signal-to-noise ratio (SNR), and the receive signal strength, both key indicators of the basic quality of an established voice path. Also provided is the round-trip-time of the link, which can be used to estimate the physical distance between modems. (See also "I6" and "Y11").
Compatibility: - - - - U - - -
Note: Not all USR/3Com modems that support this command have it documented.
K
(Buffer
Control)
On Cirrus modems, the "K" command selects whether the modem SRAM is used for voice and FAX modes.
Standards: The "K" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - - - - - C
Modifiers Description
K0
(Default)
SRAM buffer disabled. This is the default when no SRAM is present in the modem.
K1
(Default)
SRAM buffer enabled. This is the default when SRAM is present in the modem.
L
(Volume
Control)
If the modem has a speaker or other line monitor, this command adjusts the volume level.
Standards: EIA/TIA-602 6.3.6
Compatibility: - 4 E R U S L T C
Note: Some modems honor this command but also have a manual mechanical volume control that can override or interact with this command.
Modifiers Description
L0 Low volume
L1 Low volume
L2
(Default)
Medium volume.
Note: The EIA/TIA-602 specification does not stipulate what the default value should be, but most manufacturers have used "Medium".)
Note: Lucent modems use "L3" as the default.
L3
(Default)
High volume
Note: Most modems use "L2" as the default, but Lucent uses "L3".
M
(Monitor
Speaker)
If the modem has a speaker or other line monitor, this command adjusts when the speaker is active.
Standards: EIA/TIA-602 6.3.7 (describes only M0, M1, and M2)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Note: Some modems honor this command but also have a manual volume control that can override or interact with this command.
Modifiers Description
M0 The speaker is always off.
Standards: EIA/TIA-602 6.3.7
Compatibility: 2 4 E R U S L T C
M1
(Default)
The speaker is active when the modem is off-hook and carrier is not detected.
Standards: EIA/TIA-602 6.3.7
Compatibility: 2 4 E R U S L T C
M2 The speaker is always active.
Standards: EIA/TIA-602 6.3.7
Compatibility: 2 4 E R U S L T C
M3 The speaker is active after dialing ends and turns off when carrier negotiation is completed.
Standards: The "M3" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U S - T C
N
(Carrier
Negotiation)
The N command directs the modem on how to negotiate carrier speed. Most modern modems utilize a series of S Registers to handle the multitude of combinations possible should it be necessary to override the modems normal negotiation practices.
Standards: The "N" command is not part of the EIA/TIA-602 specification.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
N0 In Originating or Answering a call, the DTE speed is forced to the speed specified in Register "S37".
Compatibility: - - - R - S L - C
Note: Sierra V.32bis and earlier modems support the "N" command.
N1
(Default)
When Originating a call, the DTE speed is auto-bauded starting at the DCE speed specified in Register "S37". When Answering a call, the DTE speed is always auto-bauded.
Compatibility: - - - R - S L - C
Note: Sierra V.32bis and earlier modems support the "N" command.
N2 Same as "N1" with the addition of V.23.
Compatibility: - - - - - S - - -
Note: Sierra V.32bis and earlier modems support the "N" command.
N3 When Originating a call, force DTE speed specified by Register "S37".
Compatibility: - - - - - S - - -
Note: Sierra V.32bis and earlier modems support the "N" command.
N4 Same as N3.
Compatibility: - - - - - S - - -
Note: Sierra V.32bis and earlier modems support the "N" command.
N5 When Originating a call, auto-baud the DTE speed at the DCE speed specified by Register "S37". When Answering, force DTE speed specified by Register "S37".
Compatibility: - - - - - S - - -
Note: Sierra V.32bis and earlier modems support the "N" command.
O
(Online
Data Mode)
The O command directs the modem to switch from Command Mode to Data Mode if a data carrier is currently established. This command is commonly-used following using the Data Mode Escape Sequence.
Standards: EIA/TIA-602 6.1.9 ("O0" only. Non-zero options are explicitly reserved for manufacturer specific operations.)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
O0
(Default)
Switch from Command Mode to Data Mode. Depending on the state of the modem and connection, several Result Codes are possible, including: "CONNECT", "NO CARRIER", "ERROR", and "CONNECT <text>".
Standards: EIA/TIA-602 6.1.9
Compatibility: 2 4 E R U S L T C
O1 Switch from Command Mode to Data Mode. As part of the transition, the modem is forced to retrain and renegotiate the connection. Depending on the state of the modem and connection, several Result Codes are possible, including: "CONNECT", "NO CARRIER", "ERROR", and "CONNECT <text>".
Standards: The "O1" command and other "On" options are allowed by the EIA/TIA-602 specification, but their function is not defined by the standard and are reserved for "manufacturer proprietary use".
Compatibility: - 4 - R U S L T C
O3 On Lucent modems, switch from Command Mode to Data Mode. As part of the transition, the modem is forced to attempt a speed renegotiation before returning online. Depending on the state of the modem and connection, several Result Codes are possible, including: "CONNECT", "NO CARRIER", "ERROR", and "CONNECT <text>".
Standards: The "O1" command and other "On" options are allowed by the EIA/TIA-602 specification, but their function is not defined by the standard and are reserved for "manufacturer proprietary use".
Compatibility: - - - - - - L - -
P
(Pulse
Dialing)
The "P" command causes subsequent Dial commands to dial in Pulse (rotary) mode. The setting remains in effect until a subsequent "T" command or "T" Dial Command modifier is received or the modem is reset. Some modems can also store this parameter in non-volatile memory.

The "P" command behaves the same as the "P" Modifier of the "D" command.


Standards: EIA/TIA-602 6.1.3
Compatibility: 2 4 E R U S L T C
Q
(Result
Code
Supression)
The "Q" command is used to determine if Result Codes to this and subsequent commands or events should be sent to the DTE. When Result Codes are supressed, the modem does not generate any characters in response to the completion of a command or when an event occurs.
Standards: EIA/TIA-602 6.2.5 (Q0 and Q1 only)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
Q0
(Default)
The Modem will send Result Codes to the DTE.
Standards: EIA/TIA-602 6.2.5
Compatibility: 2 4 E R U S L T C
Q1 The Modem will not send Result Codes to the DTE.
Standards: EIA/TIA-602 6.2.5
Compatibility: 2 4 E R U S L T C
Q2 Result Codes are returned when calls are Originated, but not when calls are Answered. This is the preferred setting when modems are used with dial-up systems where the "CONNECT" message from an incoming call might be inadvertantly accepted as a login username or cause other handshake problems.
Standards: The "Q2" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U S L T -
Sr?
(Examine "S"
Register r)
The "Sr?" command displays the numeric content of the specified "S" Register. For example, "S7?" displays the contents of the "S7" register.

If the "S" Register number is not specified, Register 0 is examined.


Standards: EIA/TIA-602 5.2.2.2
Compatibility: The syntax of this command is consistent through all EIA/TIA-602 compliant modems. The available "S" registers and their functions vary considerably. Refer to the description of a given "S" register for its compatibility.
Note: Rockwell modems allow the "S" to be omitted, and recognize number? as an equivalent command. For example, "AT3?" is treated as a command to display the contents of "S" Register "3". Use of this short-cut is not recommended. (RC288ACi/RC288ACL 3.2.1)
Sr=n
(Set "S"
Register
r to n)
The "Sr=n" command sets the content of the specified "S" Register. For example, "S7=60" sets the contents of the "S7" register to "60".

If the "S" Register number is not specified, Register 0 is modified. If the value is not specified, the register is set to zero.


Standards: EIA/TIA-602 5.2.2.2
Compatibility: The syntax of this command is consistent through all EIA/TIA-602 compliant modems. The available "S" registers and their functions vary considerably. Refer to the description of a given "S" register for its compatibility.
Note: Rockwell modems allow the "S" to be omitted, and recognize number=number as an equivalent command. For example, "AT3=7" is treated as a command to load the value "7" into "S" Register "3". Use of this short-cut is not recommended. (RC288ACi/RC288ACL 3.2.1)
Sr.b=n
(Set "S" Bit b
of Register
r to n)
On USR/3Com modems, the "Sr.b=n" command sets the content of the specified bit of the specified "S" Register. For example, "S32.2=1" sets bit 2 of the "S32" register to 1.
Standards: The "Sr.b=n" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Note: The available "S" registers and their functions vary considerably. Refer to the description of a given "S" register for its compatibility.
S$
(List "S" Registers)
On USR/3Com modems, a list of the "S" registers are transmitted to the DTE.
Standards: The "S$" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Note: The available "S" registers and their functions vary considerably. Refer to the description of a given "S" register for its compatibility.
T
(Tone
Dialing)
The "T" command causes subsequent dial commands to dial in Tone (Touch-Tone or DTMF) mode. The setting remains in effect until a subsequent "P" command or "P" Dial Command Modifier is received or the modem is reset. Some modems can also store this parameter in non-volatile memory.

The "T" command behaves the same as the "T" Modifier of the "D" command.


Standards: EIA/TIA-602 6.1.1.3
Compatibility: 2 4 E R U S L T C
V
(Result Code Type)
The "V" command selects whether the modem returns word responses or the numeric responses that some older software that control modems require.
Standards: EIA/TIA-602 6.2.6
Compatibility: 2 4 E R U S L T C
Note: Numeric responses are largely obsolete as the word responses usually return more detailed information on established data connections or error conditions that are encountered.
Modifiers Description
V0 The modem will return numeric responses.
V1
(Default)
The modem will return word responses.
W
(Negotiation
Messages)
The "W" command specifies if the modem should return negotiation progress messages as part of a "D" or "O" Command. It also specifies what transmission speed should be used to send the CONNECT Result Code to the DTE.
Standards: The "W" command is not part of the EIA/TIA-602 specification.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Note: On Lucent modems, the "S95" register interacts with the settings of this command.
Modifiers Description
W0
(Default)
The modem will not send negotiation progress messages. The CONNECT message is sent to the DTE at the DTE speed.
Compatibility: - 4 - R - S L - -
Note: Sierra V.32bis and earlier modems support the "W" command.
W1 The modem will send negotiation progress messages. The CONNECT message is sent to the DTE at the DTE speed.
Compatibility: - 4 - R - S L - -
Note: Sierra V.32bis and earlier modems support the "W" command.
W2 The modem will not send negotiation progress messages. The CONNECT message is sent to the DTE at the negotiated DCE speed.
Compatibility: - 4 - R - S L - -
Note: Sierra V.32bis and earlier modems support the "W" command.
X
(Result Code
Group)
When the modem returns a Result Code, the "X" Command specifies the range of Result Codes that the modem is allowed to return. The "X" command also controls how many tests the modem performs on the line when dialing a telephone number.
Standards: EIA/TIA-602 6.2.7 (specifies X0, X1, X2, X3 and X4 only)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
X0 While dialing, the dial-tone and busy signal detection are disabled. On originated calls, the modem will return CONNECT or NO CARRIER. On answered calls, the modem can return RING, CONNECT and NO CARRIER.
Compatibility: 2 4 E R U S L T C
X1 While dialing, the dial-tone and busy signal detection are disabled. On originated calls, the modem will return CONNECT <text> or NO CARRIER. On answered calls, the modem can return RING, NO CARRIER or CONNECT <text>. The CONNECT message text includes the speed of the connection.
Compatibility: 2 4 E R U S L T C
X2 While dialing, the busy signal detection is disabled and dial-tone detection is enabled. On originated calls, the modem will return NO DIALTONE, CONNECT <text> or NO CARRIER. On answered calls, the modem can return RING, NO CARRIER or CONNECT <text>. The CONNECT message text includes the speed of the connection.
Compatibility: - 4 E R - S L T C
Note: that USR/3Com modems return "NO DIAL    TONE" (with a blank between "DIAL" and "TONE') and as such are not fully EIA/TIA-602 compliant.
X3 While dialing, the busy signal detection is enabled and dial-tone detection is disabled. On originated calls, the modem will return BUSY, CONNECT <text> or NO CARRIER. On answered calls, the modem can return RING, NO CARRIER or CONNECT <text>. The CONNECT text message includes the speed of the connection.
Compatibility: - 4 E R U S L T C
X4
(Default)
While dialing, the busy signal detection is enabled and dial-tone enabled is disabled. On originated calls, the modem will return NO DIALTONE, BUSY, CONNECT or NO CARRIER. On answered calls, the modem can return RING, NO CARRIER or CONNECT. The CONNECT message includes the speed of the connection.
Compatibility: - 4 E R - S L T C
Note: that USR/3Com modems return "NO DIAL    TONE" (with a blank between "DIAL" and "TONE') and as such are not fully EIA/TIA-602 compliant.
X11 On Telebit modems, the "X11" command is similar to "X1", except that the DTE speed is returned in the CONNECT messages instead of the DCE speed.
Compatibility: - - - - - - - T -
X12 On Telebit modems, in addition to the result codes produced by X11, when originating a call the modem can also return NO DIALTONE and BUSY, as well as the progress messages RRING and DIALING. DIALING is returned when the transmission of dialed digits begins. RRING is returned for each ring-back detected.
Compatibility: - - - - - - - T -
Y
(Long Space
Disconnect)
Based on the setting of the "Y" Command, the modem will end a data connection, go on-hook and return to Command mode in response to a long-space disconnect signal (also known as a Modem Break) from the remote modem. The remote modem must send a continuous break signal for 1.6 seconds or longer to be considered a long-space disconnect condition.
Standards: The "Y" command is not part of the EIA/TIA-602 specification.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Note: USR/3Com modems use the "Y" command for unrelated operations.
Modifiers Description
Y0
(Default)
The modem will not respond to a long space disconnect received from the remote modem.
Compatibility: - 4 - R - S L T C
Note: Lucent modems accept and silently ignore this command.
Y1 The modem will respond to a long space disconnect received from the remote modem. If the DTE issues a "H0" command, the local modem will transmit a continuous break signal to the remote modem for four seconds before going on-hook.
Compatibility: - 4 - R - S - T C
Y
(Select power-on
and reset
default con-
figuration)
On USR/3Com modems, the "Y0" through "Y3" commands are used to control which factory-supplied or customer-stored configuration should be loaded by the modem after a reset or power-cycle operation.
Standards: The "Y" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - -
On USR/3Com modems, the "Y" commands are used for two unrelated functions. This description covers "Y0" through "Y3".
Note: All other vendors use the "Y" command for unrelated operations.
Modifiers Description
Y0
(Default)
On reset, the modem loads customer-profile "0" (stored via the &W0 command).
Y1 On reset, the modem loads customer-profile "1" (stored via the &W1 command).
Y2 On reset, the modem loads the factory-provided "generic" profile.
Y3 On reset, the modem loads the factory-provided "hardware flow control" profile. A configuration based on this profile is preferred for PPP connections.
Y4 On reset, the modem loads the factory-provided "software flow control" profile.
Y11
(Display
Spectrum
Analysis
Data)
On USR/3Com modems, the "Y11" command displays a table of received signal levels at regular frequency points across the POTS line spectrum. The results of this command are valid only while a data connection is in progress.
Standards: The "Y" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Note: Most USR/3Com modems that support this command do not have it documented.
All other vendors use the "Y" command for unrelated operations.
Y16
(Display
Spectrum
Analysis
Graph)
On USR/3Com modems, the "Y16" command displays a graph showing received signal levels at regular frequency points across the POTS line spectrum. The graph is created using ASCII printable characters and should be displayed using a fixed-spacing font. The results of this command are valid only while a data connection is in progress.
Standards: The "Y" command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Note: Most USR/3Com modems that support this command do not have it documented.
All other vendors use the "Y" command for unrelated operations.
Z
(Reset)
This command resets the modem and resets all parameters to their power-on factory states. A call that is in progress may be aborted without a complete call tear-down.

On modems with non-volatile RAM, a previously-stored user configuration may also be loaded. Some types of modems with non-volatile RAM only save a subset of all the parameters in non-volatile RAM, so the items that are not saved return to the factory settings when a Z command is issued. Some modems further complicate this by having multiple factory setting "profiles", meaning one or more previously-entered commands may dictate what settings are restored following a modem reset.

The Z commands return an "OK" response code after they have been completed, but EIA/TIA-602 does not specify how long the command may take to execute. Until completely reset, the modem will not respond to new commands, and any unexecuted commands will be discarded when the Z command is executed.


Standards: EIA/TIA-602 6.1.7 (Z0 only)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
Z0
(Default)
After resetting, the modem will load the factory profile, and if there is a customer stored profile #0 (or 'A'), this will be also loaded.
Standards: EIA/TIA-602 6.1.7
Compatibility: 2 4 E R - S L T C
Z0
(Default)
After resetting, the modem will load the customer stored profile previously specified by the "Y" command.
Standards: EIA/TIA-602 6.1.7
Compatibility: - - - - U - - -
Z1 After resetting, the modem will load the factory profile, and if there is a customer stored profile #1 (or 'B'), this will be also loaded.
Standards: The Z1 command is not part of the EIA/TIA-602 specification.
Compatibility: - 4 - R - S - T C
Z1 On USR/3Com modems, after resetting, the modem will load the customer stored profile #0 (or 'A'), this will be also loaded.
Standards: The Z1 command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Z2 On USR/3Com modems, after resetting, the modem will load the customer stored profile #1 (or 'B'), this will be also loaded.
Standards: The Z2 command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Z3 On USR/3Com modems, after resetting, the modem will load factory profile #0 (the same specified by the &F0 command).
Standards: The Z3 command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Z4 On USR/3Com modems, after resetting, the modem will load factory profile #1 (the same specified by the &F1 command).
Standards: The Z4 command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Z5 On USR/3Com modems, after resetting, the modem will load factory profile #2 (the same specified by the &F2 command).
Standards: The Z5 command is not part of the EIA/TIA-602 specification.
Compatibility: - - - - U - - - -
Special Commands Description
A/
(Repeat Command)
This command causes the modem to re-execute the last command performed in Command Mode. Note that this command must not be preceded by the "AT" prefix.

If the last command was a null command (AT[0x0d]), the "A/" command will result in another NULL command being performed. The Escape Sequence is not a command and therefore the "A/" command won't repeat that operation.

Commands that result in the modem resetting ("ATZ", "AT&D3H", etc) will clear the previous command history. Entering "A/" when there is no previous command results in a null command being performed and a successful result code returned.


Standards: EIA/TIA-602 5.2.1.3
Compatibility: 2 4 E R U S L T C
+++
(Original
Hayes
in-band
Escape
Sequence)
When the modem is in Data Mode, the modem can be forced back into Command Mode while maintaining the connection to the remote modem. Hayes modems initially used a sequence of three plus (+) characters sent by the DTE to the modem (in-band signaling) to cause the modem to revert to Command Mode. The actual character used in the escape sequence could be changed by the setting of the "S2" register. This in-band Escape mechanism can be completely disabled on most modems by setting "S2" register to 0, 128 or 255, depending on the modem. Consult the documentation to determine the correct value for the modem in question.

Once in Command Mode, a subset of commands are available. Certain commands (like the "D" command) that don't make sense to use when a data connection is already established return ERROR, or the action won't take effect until the next data connection is initiated.


Standards: The in-band Escape Sequence is not part of EIA/TIA-602.
Compatibility: 2 - - - - - - - -
+++
(Hayes
in-band
Escape
Sequence
with Guard
Time)
When the modem is in Data Mode, the modem can be forced back into Command Mode while maintaining the connection to the remote modem.

To reduce the chance that the modem would falsely switch to Command Mode while transmitting data that happened to include the Escape Sequence characters, this improved version required that the DTE not transmit anything to the modem for a period of time before and after the three escape characters before they would be recognized by the modem as the escape sequence. A subsequent refinement further required that all three escape characters be received within one second of one another.

Hayes obtained a patent on the addition of Guard Time to the escape sequence, and began suing other modem makers who copied this enhancement. Some of these other vendors went back to the original Escape Sequence with no guard time (a system that was not patented), while others devised alternate systems that provided similar protection against switching into Command Mode to avoid infringing the Hayes patent. Rockwell, because they were the primary maker of datapumps for Hayes, obtained free access to the Guard Time patent, and most Rockwell modems still implement this method of Escape Sequence management. USR/3Com paid Hayes for the right to use the Guard Time mechanism in their modems.

Once in Command Mode, a subset of commands are available. Certain commands (like the "D" command) that don't make sense to use when a data connection is already established return ERROR, or the action won't take effect until the next data connection is initiated.

The actual character used in the escape sequence could be changed by the setting of the "S2" register. This in-band Escape mechanism can be completely disabled on most modems by setting "S2" register to 0, 128 or 255, depending on the modem. Consult the documentation to determine the correct value for the modem in question.


Standards: The Escape Sequence is not part of EIA/TIA-602.
Compatibility: - 4 - R U S - T C
Note: By default, Sierra and Cirrus modems come with T.I.E.S. escape sequence code, but if the OEM desires the Guard Time method and is willing to pay for the Hayes patent royalty, the Hayes method is available as an option.
+++
(T.I.E.S.
in-band
Escape
Sequence)
When the modem is in Data Mode, the modem can be forced back into Command Mode using the T.I.E.S Escape Sequence while maintaining the connection to the remote modem.

To reduce the chance that the modem would falsely switch to Command Mode while transmitting data that happened to include the Escape Sequence characters, this implementation requires at least the entire six character sequence be received (+++AT[none or more commands]0x0d) by the modem before the modem considers itself in Command Mode. The modem does respond with a "OK" result code after receiving the three escape characters, and will echo subsequent characters according to the setting of the E command, but unless the confirmation characters of "A", "T", and the ASCII code 0x0d (carriage return) are received, the modem will revert back to Data Mode. An actual command to be performed may also be present following the "AT" and preceding the carriage return character and the Escape Sequence will still be recognized.

The Time Independent Escape Sequence (T.I.E.S.) was developed to make it less likely that the modem would accidentally enter Command Mode in response to the escape characters occurring within data being transmitted. Because of Hayes position to not allow other modem vendors to infringe or license their patent for requiring idle times before and after the escape characters (known as "Guard Time"), Ventel, working with other modem makers, devised the T.I.E.S. system.

There was quite a battle in the early 1990s over this point, with Hayes briefly running the now-infamous "Tick Tick Tick Boom! You're Dead" ads that featured a non-Hayes modem strapped to sticks of dynamite and an alarm clock, and the ad made claims that the T.I.E.S. system was unreliable and bound to destroy customer data, something that would never ever happen if the customer had bought a Hayes modem, etc etc. Hayes got sued instantly. In an attempt to support their claim, Hayes distributed a free test program designed to fail when it encountered a T.I.E.S. modem and only work when it detected a Hayes modem. Then, the terminal software Hayes was distributing with their modems was modified to fail if it was ever used with a T.I.E.S. modem, even though older versions of the same software package worked fine on T.I.E.S. modems.

The entire dispute ended-up in court, with the T.I.E.S. modem makers forcing Mr. Hayes to testify. Mr. Hayes eventually admitted on the stand that T.I.E.S. was not any less reliable than the Guard Time system that Hayes had touted as superior, and evidence submitted in the trial from mathematicians showed that a T.I.E.S. modem had a lower probability of accidentally going into Command Mode, mainly because you had to send a longer specific sequence of characters to cause the Escape to occur.

Of course, all in-band Escape Sequence systems are bound to falsely switch to Command Mode under certain conditions, which is why modems that transmit binary data primarily (like PPP) should disable these in-band escape sequences entirely.


Standards: The Escape Sequence is not part of EIA/TIA-602.
Compatibility: - - - - - S - - C
Note: By default, Sierra and Cirrus modems come with T.I.E.S. escape sequence code, but if the OEM desires the Guard Time method and is willing to pay for the Hayes patent royalty, the Hayes method is available as an option.


Ampersand-Prefixed AT Commands
&$
(Display Ampersand-prefixed commands)
On USR/3Com modems, the "&$" command displays a list of the commands that begin with the ampersand (&) character.
Standards: The "&$" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
&A
(Display ARQ Codes)
On USR/3Com modems, the "&A" command enables or disables AQR Result Codes. These messages display when the modem enters or returns to Data Mode, although other commands can override part or all of this display.
Standards: The "&A" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Note: The "&A" command is used in Sierra modems for an unrelated function.
Modifiers Description
&A0 ARQ Result Codes are disabled.
&A1 ARQ Result Codes are enabled.
&A2 ARQ Result Codes are enabled and the V.32 modulation indicator is also displayed.
&A3
(Default)
ARQ Result Codes are enabled. The V.32 modulation indicatoe and protocol indicators (LAPM/MNP/NONE for Error Control and V.42bis/MNP5 for data compression) are also displayed.
&A
(Automatic Fall Forward and Fall Back)
Sierra V.34 modems use the "&A" command to enable or disable the modems ability to increase or decrease connection speed in response to changing line conditions.
Standards: The "&A" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - S - - -
Note: The "&A" command is not present on V.32bis and earlier Sierra modems. USR/3Com modems use the "&A" command for unrelated operations.
Modifiers Description
&A0 The modem will not perform fall-forward or fall-back operations.
&A1
(Default)
The modem will perform fall-forward or fall-back operations as line conditions change.
&B
(Set DTE Speed)
On USR/3Com modems, the "&B" command specifies how the DTE transmission rate is determined.
Standards: The "&B" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Note: Lucent modems use the "&B" command for unrelated functions.
Modifiers Description
&B0 The DTE speed is variable and adjusts to match the DCE rate when a connection is established.
&B1
(Default)
The DTE rate is fixed to the speed detected on the DTE for the last command.
&B2 The DTE rate is fixed in ARQ Data modes and variable in non-ARQ Data modes.
&B
(V.32 Auto Retrain)
The "&B" command controls whether the modem performs automatic retrains during V.32 connections.
Standards: The "&B" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
&B0 The modem will not perform automatic retrains during V.32 transmissions when they are needed needed.
Compatibility: - - - - - - - - -
Note: Lucent modems accept and ignore the "&B1" command, but do not accept the &B0 command.
&B1
(Default)
The modem will perform automatic retrains during V.32 transmissions when they are needed needed.
Compatibility: - - - - - - L -
Note: Lucent modems accept and ignore the "&B1" command, but do not accept the &B0 command.
&C
(Data Carrier Detect)
The "&C" command instructs the modem as to what (if anything) to report to the DTE via the Carrier Detect signal (DCD or CD). The command takes effect immediately.

In the RS-232 standard using DB25 connectors, the CD signal is on Pin 8.

This electrical signal is also known as the Received Line Signal Detector (RLSD) and Circuit 109 in related standards.


Standards: EIA/TIA-602 6.3.8 (&C0 and &C1 only)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
&C0 The "&C0" command causes the modem to report to the DTE that carrier is always being detected, even if the modem is on-hook or there is no actual carrier signal present.

This mode is useful in certain situations when the DTE operating system or application will not allow commands to be sent to the modem when CD detect signal is false on the RS-232 interface, but there can be no carrier on the phone line until the modem has been instructed to establish carrier.

Most modern operating systems will allow characters to be transmitted regardless of the state of CD, but may require the CTS or DSR RS-232 signals to be true before characters may be sent.


Standards: EIA/TIA-602 6.3.8
Compatibility: 2 4 E R U S L T C
&C1
(Default)
The "&C1" command instructs the modem to have the Carrier Detect signal sent to the DTE by the modem change state generally based on the presence or absence of carrier between this modem and the remote modem.

There is not a precise relationship between the presence of carrier between DCEs. For example, based on a S register setting, the modem may continue to report the presence of carrier to the DTE when carrier on the telephone line is briefly lost. Other protocol layers operating in the modem may also delay reporting a change in carrier status back to the DTE. See the MNP-3, MNP-4, V.42, V.110, V.120 and V.13 protocols for additional related interactions.


Standards: EIA/TIA-602 6.3.8
Compatibility: 2 4 E R U S L T C
&C2 On Sierra V.34 modems, the "&C2" command causes the Carrier Detect signal to indicate the true state of the remote carrier signal. This means that any interruption in the carrier received from the remote modem will cause the DCD line to toggle, even if the modem is able to recover and continue the data connection without data loss.
Standards: The "&C2" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - S - - -
Note: The "&C2" command is not present on V.32bis and earlier Sierra modems.
&C2 On Telebit modems, the "&C2" command forces the Data Carrier Detect signal to a TRUE state, execpt when the actual received data carrier goes from a present to an absent state (disconnect), the Data Carrier Detect signal sent to the DTE toggles briefly by going FALSE, then TRUE again.
Standards: The "&C2" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - - - T -
Note: The duration DCD FALSE condition is dictated by the "S47" register.
&C3 On Telebit modems, the "&C3" command forces the Carrier Detect signal sent to the DTE by the modem to be the inverse of the Clear To Send (CTS) signal.
Standards: The "&C3" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - - - T -
&D
(Data Terminal Ready)
This command instructs the modem on how to respond in response to the DTE changing the state of the Data Terminal Ready (DTR) signal.

In the RS-232 standard with DB-25 connectors, the DTR signal is on Pin 20. This electrical signal is also known as Circuit 108 or 108/2 in related standards.

Note: Per the EIA/TIA-602 specification, any action that the modem takes is to be performed on the TRUE to FALSE transition of DTR, but some non-compliant modems deviate from this, remaining completely unresponsive to subsequent commands from the DTE and unresponsive to incoming calls until DTR becomes TRUE again. Such behavior is incompatible with the EIA/TIA-602 specification.


Standards: EIA/TIA-602 6.3.9 (&D0, &D1, and &D2)
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Modifiers Description
&D0
(Default)
The modem ignores the status of the DTR signal at all times.
Standards: EIA/TIA-602 6.3.9
Compatibility: 2 4 E - U S L T C
&D0
(Default)
On Rockwell modems, the "&D0" command behavior varies based on the settings of other commands. If "&Q0", "&Q5" or "&Q6" is set, DTR is ignored at all times. If "&Q1" or "&Q4" is set and DTR goes FALSE, it will cause the modem to go on-hook and auto-answer is still allowed. If "&Q2" or "&Q3" is set and DTR goes FALSE, it will cause the modem will go on-hook and auto-answer is disabled until DTR goes TRUE. The modem ignores the status of the DTR signal at all times.
Standards: Rockwells implementation of "&D0" is not 100% compatible with EIA/TIA-602 6.3.9.
Compatibility: - - - R - - - - -
&D1 When DTR changes from TRUE to FALSE, the modem will return to Command Mode if it was in Data Mode. Any call in progress is undisturbed, and the behavior is similar to sending the Data Mode Escape character Sequence. Some modems also require the DTR drop for at least the amount of time specified by the "S25" register prior to entering Command Mode.
Standards: EIA/TIA-602 6.3.9 (&D1 is optional)
Compatibility: 2 4 E - - S L T C
Note: USR/3Com modems do not implement this optional command in the EIA/TIA-602 specification.
&D1 On Rockwell modems, the behavior of the "&D1" setting is dependent on other parameters. If "&Q0", "&Q1", "&Q4", "&Q5", or "&Q6" is set and DTR goes FALSE, the modem will switch to Command Mode without going on-hook. If "&Q2" or "&Q3" is set and DTR goes FALSE, the modem goes on-hook, the modem enters Command Mode and auto-answer is disabled.
Standards: Rockwells implementation of "&D1" is not 100% compatible with EIA/TIA-602 6.3.9.
Compatibility: - - - R - - - - -
&D2
(Default USR/3Com Modems)
When DTR changes from TRUE to FALSE, the modem will terminate any call in progress with a complete protocol tear-down, place the modem on-hook and return to Command Mode. While DTR is FALSE, the modem will not go off-hook in response to an incoming call, but will report RINGs received if RING detection is enabled (see the "X" command).

If the "&D3" command is not available on a given modem, this is the second-best preferred setting when modems are used in an auto-answer configuration.

The modem may qualify the DTR signal with a timer, typically controlled by the S25 register. (This varies by manufacturer - see the S registers for details.) If the DTR signal goes from TRUE to FALSE, it must remain in FALSE state for at least the amount of time specified by the timer, or else the transition is ignored. This allows the modem to reject noise and other glitches on the DTR signal that might falsely terminate a call in progress.


Standards: EIA/TIA-602 6.3.9
Compatibility: 2 4 E R U S L T C
Note: "&D2" is the default setting on USR/3Com modems instead of "&D0", the more common default.
&D3 When DTR changes from TRUE to FALSE, the modem will terminate any call in progress with a complete protocol tear-down, place the modem on-hook and reset itself, performing steps similar or identical to those in the Z command. While DTR is FALSE, the modem will not go off-hook in response to an incoming call, but will report RINGs received if RING detection is enabled in the post-reset settings (see the "X" command).

The modem may qualify the DTR signal with a timer typically controlled by the S25 register. (This varies by manufacturer - see the S registers for details.) If the DTR signal goes from TRUE to FALSE, it must remain in FALSE state for at least the amount of time specified by the timer, or else the transition is ignored. This allows the modem to reject noise and other glitches on the DTR line that might falsely reset the modem and incorrectly terminate a call in progress.

Because "&D3" can guarantee that each incoming call gets a freshly-reset modem, this setting is preferred for most unattended auto-answer modem configurations.


Standards: The "&D3" command is not part of the EIA/TIA-602 standard.
Compatibility: - 4 - - - S L T C
Note: A few modems return an unsolicited "OK" Result Code as a consequence of the "&D3" command when a TRUE to FALSE transition of DTR occurs. This is done prior to rebooting the modem.

Also, the typical modem is usually not able to accept subsequent commands for several seconds following a reset. It should be assumed that when &D3 is used, the modem will be briefly unresponsive when DTR transitions from TRUE to FALSE.

&D3 On Rockwell modems, the behavior of the "&D3" command is dependent on other parameters. If "&Q0", "&Q1", "&Q4", "&Q5", or "&Q6" is set and DTR goes FALSE, the modem will go on-hook and perform a soft reset. After the reset is complete, the modem will be in Command Mode. If "&Q2" or "&Q3" is set and DTR goes FALSE, the modem goes on-hook, the modem enters Command Mode and auto-answer is disabled.
Standards: The "&D3" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - R - - - - -
Note: The few modems that do return an unsolicited "OK" Result Code as a consequence of the "&D3" command and a TRUE to FALSE transition of DTR do so prior to rebooting the modem. Also, the modem is typically not actually able to accept subsequent commands for several more seconds. It should be assumed that when &D3 is used, the modem will be briefly unresponsive when DTR transitions from TRUE to FALSE.
&F
(Recall Parameters)
The "&F" command instructs the modem to load a stored configuration into the modems active register area, forcing all parameters to default values. The action may also be impacted by any customer-selected hardware configuration switches or other "manufacturer-defined critera".

So what does all that mean? It means that the "&F" command series may set all or some registers and modem state with pre-defined factory defaults, alterations possibly based on physical jumpers or switches on the modem.

Most modems always restore the stored profile #0 in a reset situation, but a few brands allow the customer to specify that a stored profile other than #0 is to be loaded after a modem reset that was not initiated by an explicit command.

Certain brands of modems offer multiple "&F" commands, with each loading a factory-provided profile that are considered best for certain types of modem use. None of these are part of the EIA/TIA-602 specification, and should be considered unique to a specific brand and model of modem, possibly even unique to certain firmware versions for a given model of modem.


Standards: EIA/TIA-602 6.1.8 (&F0 only - read Note below)
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
Note: The EIA/TIA-602 standard only defined a single command (&F0) which the standard clearly states is to be used to "set parameters to factory defaults". However, many modem makers deviate from this, meaning that a modem reset (either by Z command, a TRUE-to-FALSE transition on DTR when "&D3" is used, or a power reset) followed by a "&F" may be the only way to truly force all modem registers back to their factory settings. This is because some modem makers fail to reset every register and setting in the "&F" command, and many do not store all possible settings in the stored profiles, meaning that they must be re-issued to the modem following a modem reset.

The values that are loaded in response to a given "&F" command vary by manufacturer, and sometimes by individual modem model.

Modifiers Description
&F0
(Default)
On Rockwell/Connexant modems, the "&F0" command sets the following factory defaults: (some commands and registers may not exist on given modem models)
B1, E1, L2, M1, N1, P, Q0, V1, W0, X4, Y0, &C1,
&D0, &G0, &J0, &K3, &L0, &P0, &Q5, &R0, &S0, &T4,
&U0, &X0, &Y0, \C0, \G0, \N6, \Q3, \T000, \V1,
\X0, %A000, %D1, %E1

There are also undocumented registers that are set to factory defaults.
Compatibility: - - - R - - - - -
&F0
(Default)
On USR/3Com modems, the "&F0" command sets the "Generic template" factory defaults. (No other information was provided.)
Compatibility: - - - - U - - - -
&F0
(Default)
On Sierra modems, the "&F0" command sets the following factory defaults: (some commands and registers may not exist on a given modem model)
(V.32bis modems)
B1, E1, L2, M1, N1, P, Q0, V1, W0, X4, Y0, &C1,
&D0, &G0, &J0, &K3, &L0, &P0, &Q5, &R0, &S0, &T4,
&U0, &X0, &Y0, \C0, \G0, \N6, \Q3, \T000, \V1,
\X0, %A000, %D1,

S0=0, S2=43, S3=13, S4=10, S5=8, S6=2, S7=40,
S8=2, S9=6, S10=14, S11=75, S12=40, S14=0xAA,
S16=0x00, S18=0, S21=0x20, S22=0x76, S23=0x07,
S25=5, S26=1, S27=0x49, S36=7, S37=0, S38=20,
S46=138, S48=7, S49=10, S50=200, S63=0, S82=128.

(V.34 modems)
E1, L2, M1, T, Q0, V1, X1, Y1, &C1, &D0, &R1,
&S1, &T0, &U0, &X0, \C2, \F0, \G0, \N3, \Q2,
\T001, \V3, %A3, %D1,

S0=0, S2=0, S3=43, S4=13, S5=10, S6=2, S7=60,
S8=2, S9=6, S10=14, S12=8, S18=75, S25=0, S38=0
There are also undocumented registers that are set to factory defaults.
Compatibility: - - - - - S - - -
&F0
(Default)
On Lucent (formally AT&T Microelectronics) modems, the numeric parameter is ignored, and a factory configuration to perform "V.42bis error correction/data compression" is loaded. No other information was provided.
Compatibility: - - - - - - L - -
&F0
(Default)
On Telebit modems, the configuration is loaded according to the settings of the "S254" register. This setting is used mainly with asynchronous terminals. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q0 V1 X1 Y0
&C0 &D0 &G0 &J0 &L0 &Q0 &R3 &S0 &T4 &X0
S0=0 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=0
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=2 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F0
(Default)
No information was provided on what default settings are loaded.
Compatibility: - - - - - - - - C
&F1 On Rockwell modems, the "&F1" command restores factory configuration #1. No other information was provided.
Compatibility: - - - R - - - - -
&F1 On USR/3Com modems, the "&F1" command sets the "Hardware flow control template" factory defaults. No other information was provided.
Compatibility: - - - - U - - - -
&F1 Sierra V.34 modems list this command only as "Reserved for factory use". The command is not recognized on Sierra V.32bis or earlier modems.
Compatibility: - - - - - S - - -
&F1 On Telebit modems, "&F1" loads the factory defaults considered suitable for asynchronous Front End Processor (FEP host ports, operating at a fixed 9600bps speed. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E0 L2 M1 P Q2 V1 X1 Y0
&C1 &D2 &G0 &J0 &L0 &Q0 &R2 &S4 &T4 &X0
S0=1 S1=0 S2=255 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=2 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F2 On USR/3Com modems, the "&F2" command sets the "Software flow control template" factory defaults. No other information was provided.
Compatibility: - - - - U - - - -
&F2 On Telebit Modems, "&F2" selects "Intelligent Answer Mode". This configuration is geared towards applications that know how to interpret result codes. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q2 V1 X1 Y0
&C1 &D2 &G0 &J0 &L0 &Q0 &R2 &S4 &T4 &X0
S0=1 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=0
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=2 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F3 On Sierra V.32bis modems, the "&F3" command sets the following factory defaults, which are identical to those generated by the &F0 command:
B1, E1, L2, M1, N1, P, Q0, V1, W0, X4, Y0, &C1,
&D0, &G0, &J0, &K3, &L0, &P0, &Q5, &R0, &S0, &T4,
&U0, &X0, &Y0, \C0, \G0, \N6, \Q3, \T000, \V1,
\X0, %A000, %D1,

S0=0, S2=43, S3=13, S4=10, S5=8, S6=2, S7=40,
S8=2, S9=6, S10=14, S11=75, S12=40, S14=0xAA,
S16=0x00, S18=0, S21=0x20, S22=0x76, S23=0x07,
S25=5, S26=1, S27=0x49, S36=7, S37=0, S38=20,
S46=138, S48=7, S49=10, S50=200, S63=0, S82=128.
There are also undocumented registers that are set to factory defaults.
Compatibility: - - - - - S - - -
&F3 On Telebit Modems, "&F3" loads a configuration suited for use with System V HoneyDanBer (HDB) UUCP coounications. For more efficient PEP transmissions, each modem will emulate the handshake the UUCP protocol, with the local modem acknowledging receipt of packets on behalf of the remote system. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q2 V1 X0 Y0
&C1 &D3 &G0 &J0 &L0 &Q0 &R3 &S4 &T4 &X0
S0=1 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=255 S56=17
S57=19 S58=2 S59=0 S60=0 S61=0 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=30 S112=1 S180=3 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F4 On Telebit Modems, "&F4" loads a configuration suited for use with BSD UUCP 4.2 or 4.3, as well as SCO XENIX. This is generally compatible with systems using a Version 2 UUCP protocol. For more efficient PEP transmissions, each modem will emulate the handshake the UUCP protocol, with the local modem acknowledging receipt of packets on behalf of the (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q2 V1 X0 Y0
&C1 &D3 &G0 &J0 &L0 &Q0 &R3 &S1 &T4 &X0
S0=1 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=254 S56=17
S57=19 S58=0 S59=0 S60=0 S61=0 S62=15 S63=0 S64=1
S68=3 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=30 S112=1 S180=3 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F5 On Telebit Modems, "&F5" loads a configuration for transparent synchronous operations and typically used with synchronous terminals or devices. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q1 V1 X0 Y0
&C1 &D2 &G0 &J0 &L0 &Q6 &R3 &S3 &T4 &X0
S0=1 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=7 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=0
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=0 S181=0 S183=25 S190=1
S252=0 S253=0 S254=255 S255=255

Compatibility: - - - - - - - T -
&F8 On Telebit Modems, "&F8" loads a configuration compatible with IBM PC/MAC software flow control (XON/XOFF). (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q1 V1 X1 Y0
&C1 &D2 &G0 &J0 &L0 &Q6 &R3 &S0 &T4 &X0
S0=0 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=0
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=2 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F9 On Telebit Modems, "&F9" loads a configuration compatible with IBM PC/MAC hardware flow control (RTS/CTS). For typical PPP transmissions, this profile is a good starting configuration. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q0 V1 X1 Y0
&C1 &D2 &G0 &J0 &L0 &Q0 &R3 &S0 &T4 &X0
S0=0 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=255 S56=17
S57=19 S58=2 S59=0 S60=0 S61=1 S62=15 S63=0 S64=0
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=2 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F10 On Telebit Modems, "&F10" loads a configuration for leased line operation where this modem will be set to asynchronous originate mode. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E0 L2 M1 P Q2 V1 X1 Y0
&C1 &D0 &G0 &J0 &L1 &Q0 &R3 &S1 &T4 &X0
S0=0 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=255
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=7 S51=5 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=0 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=2 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F11 On Telebit Modems, "&F11" loads a configuration for leased line operation where this modem will be set to asynchronous answer mode. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E0 L2 M1 P Q2 V1 X1 Y0
&C1 &D0 &G0 &J0 &L1 &Q0 &R3 &S1 &T4 &X0
S0=1 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=255
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=7 S51=5 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=0 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=2 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F12 For Telebit Modems, "&F12" loads a configuration for leased line operation where this modem will be set to synchronous originate mode. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E0 L2 M1 P Q2 V1 X1 Y0
&C1 &D0 &G0 &J0 &L1 &Q0 &R3 &S1 &T4 &X0
S0=0 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=255
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=7 S51=5 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=0 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=0 S181=0 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F13 For Telebit Modems, "&F13" loads a configuration for leased line operation where this modem will be set to synchronous answer mode. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q2 V1 X1 Y0
&C1 &D0 &G0 &J0 &L1 &Q6 &R3 &S3 &T4 &X0
S0=1 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=255
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=7 S51=5 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=0 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=0 S181=0 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F32 For Telebit Modems, "&F32" loads a configuration that allows the modem to operate in asynchronous mode when idle, and switches to synchronous mode when a connection to a remote modem is established. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q2 V1 X1 Y0
&C1 &D2 &G0 &J0 &L0 &Q1 &R3 &S3 &T4 &X0
S0=1 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=7 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=255 S112=1 S180=0 S181=0 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F33 For Telebit Modems, "&F33" loads a configuration that is compatible with a HP3000 system that uses ENQ/ACK flow control and this modem is connected to the host. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q0 V1 X1 Y0
&C1 &D2 &G0 &J0 &L0 &Q0 &R2 &S1 &T4 &X0
S0=1 S1=0 S2=255 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=7 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=1
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=40 S112=1 S180=3 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&F34 For Telebit Modems, "&F34" loads a configuration that is compatible with a HP3000 system that uses ENQ/ACK flow control and this modem is connected to a terminal. (Settings shown are from the Telebit 3000/Worldblazer)
B1 E1 L2 M1 P Q0 V1 X1 Y0
&C1 &D0 &G0 &J0 &L0 &Q0 &R3 &S0 &T4 &X0
S0=0 S1=0 S2=43 S3=13 S4=10 S5=8 S6=2 S7=40
S8=2 S09=6 S10=14 S11=70 S12=50 S18=0 S25=5 S26=1
S38=0 S41=0 S45=0 S47=4 S48=0 S50=0 S51=255 S56=17
S57=19 S58=3 S59=0 S60=0 S61=1 S62=15 S63=0 S64=0
S68=255 S69=0 S90=0 S93=8 S94=1 S100=0 S102=0 S104=0
S105=1 S111=41 S112=1 S180=3 S181=1 S183=25 S190=1
S252=0 S253=10 S254=255 S255=255

Compatibility: - - - - - - - T -
&G
(Guard Tones)
The Guard Tone is used with FSK, PSK or DPSK transmission systems. It causes the modem to generate a tone at 550Hz or 1800Hz. It's purpose is to prevent certain telephone switching systems from becoming confused by the modem carrier and prevents calls from being terminated prematurely.

When a guard tone is not used, the "B" command must be set to the CCITT/ITU operation mode ("B0").


Standards: The "&G" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems. See individual option descriptions.
Note: On Sierra modems, the "&G" command only exists on V.32bis models and earlier designs.
Modifiers Description
&G0
(Default)
No Guard tone is generated. This is the correct setting for use in North America (United States, Canada and Mexico).
Compatibility: 2 4 - R U S L T C
Note: On Rockwell "W-Class" modems, "&G2" is the default setting.
&G1 The answering modem will generate a 550Hz guard tone, which is used in some European countries (not the U.K.).
Compatibility: 2 4 - - U S - T C
Note: Although Lucent doesn't document anything other than "&G0", their modems probably really do support guard tones in modem models sold in countries that require guard tone operation.
Note: Some European countries also require that the ITU V.25 mode be selected by issuing "B0" command.
&G1 On Rockwell modems, the "&G1" command disables the guard tone although "&G0" should be used to disable guard tone in a portable fashion.
Compatibility: - - - R - - - - -
Note: Some European countries also require that the ITU V.25 mode be selected by issuing "B0" command.
&G2
(Default)
The answering modem will generate a 1800Hz guard tone, which is used in the United Kingdom.
Compatibility: 2 4 - R U S - T C
Note: Although Lucent doesn't document anything other than "&G0", their modems probably really do support guard tones in modem models sold in countries that require guard tone operation. On Rockwell "W-Class" models, "&G2" is the default.
Some European countries also require that the ITU V.25 mode be selected by issuing "B0" command.
&H
(Set Transmit to DTE flow control)
On USR/3Com modems, this command controls what type of flow control is to be performed between the modem and the DTE.
Standards: The "&H" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Modifiers Description
&H0 The modem sends data to the DTE when it is available. All flow control is disabled.
&H1
(Default)
Hardware Flow Control is to be implemented by monitoring the Clear to Send (CTS) signal from the DTE. When CTS is false, the modem will not transmit data to the DTE.
&H2 Software flow control is used when sending data to the DTE. The modem will transmit data to the DTE until the ASCII character X-OFF is received from the DTE. Transmissions may resume when the an X-ON character is received from the DTE.
&H3 Hardware and Software flow control are used when sending data to the DTE. Refer to the description of "&I1" and "&I2" for how the two types of flow control work. Also see the USR/3Com "&I" and "&R" commands for managing flow control from the DTE to the modem.
&H
(Set Speed Negotiation Mode)
On Sierra V.34 modems, the "&H" command controls the selection of the modulation protocols that may be negotiated with the remote modem. If a specific set of protocols are required and a common protocol cannot be negotiated, the call will fail.

When an external clock is used (see "&X1" command), a fixed modulation protocol should be selected.


Standards: The "&H" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - - S - - -
Note: The &H command does not exist on Sierra V.32bis and earlier modems.
Modifiers Description
&H0
(Default)
Automatic mode. Any speed can be negotiated with the remote modem.
&H1 Negotiate V.34 speeds only (4,800 to 33,600bps).
&H2 Negotiate up to V.32bis speeds (300 to 14,400bps).
&H3 Negotiate V.32bis speeds only (4,800 to 14,400bps).
&H4 Negotiate up to V.32 speeds (300 to 9,600bps).
&H5 Negotiate V.32 speeds only (4,800 to 9,600bps).
&H6 Negotiate V.22bis speeds only (1,200 to 2,400bps).
&H7 Negotiate V.22 speeds only (1,200bps).
&H8 Negotiate Bell 212 only (1,200bps).
&H9 Negotiate Bell 103 only (300bps).
&H10 Negotiate Bell V.21 only (300bps).
&I
(Receive Data Software Flow Control)
This command interacts with the "&R" command on certain modem brands to specify how software flow control is to behave.
Standards: The "&I" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Modifiers Description
&I0
(Default)
Software Flow control is disabled. Any XON/XOFF characters sent by the DTE or received from the remote modem pass through the modem without any action. This setting is required in PPP or SLIP applications. Hardware Flow control should be used instead.
&I1 The local modem responds to XON/XOFF signals received from the DTE, and sends the XON/XOFF to the remote modem.
&I2 The local modem responds to XON/XOFF signals received from the DTE, but does not send the XON/XOFF to the remote modem.
&J
(Jack Selection)
This command controls how the telco leads are utilized by certain specialized modems, mainly those that also support a leased-line configuration.
Standards: The "&J" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or have the same function. See individual option descriptions.
Modifiers Description
&J0
(Default)
The Auxillary CPE telco leads are disabled.
Compatibility: - - - - - - L T C
Note: Rockwell and Lucent modems accept this command but take no action.
&J1 The A/A1 control function is selected.
Compatibility: - - - - - - - T C
Note: Rockwell modems accept this command but take no action.
&J2 The MI/MIC control is selected.
Compatibility: - - - - - - - T -
&K
(DTE Port Flow Control)
The "&K" commands provide a variety of methods to implement flow control between the modem and the DTE. Note that some Modifier values are not used.
Standards: The "&K" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or have the same function. See individual option descriptions.
Note: Most of the modem makers have compatible flow control options, but USR/3Com modems use a completely different set of commands to provide the same functionality.
Modifiers Description
&K0
(Default)
DTE flow control is disabled.
Compatibility: - 4 - R - S L - -
Note: On Rockwell modems, "&K6" is the default setting. For Sierra modems, this command only exists on V.32bis models and earlier designs.
&K3 Bi-directional hardware flow control is used. The modem uses the CTS line to regulate data coming from the DTE. The DTE uses the RTS line to control data coming from the modem. This is the preferred setting for PPP/SLIP connections.
Compatibility: - 4 - R - S L - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&K4 Bi-directional software flow control is used. XOFF characters received from the DTE cause the modem to suspend transmissions to the DTE. An XON resumes transmissions to the DTE. The modem also sends XOFF/XON characters to the DTE to regulate characters being sent to the modem. Software flow control should not be used with PPP/SLIP protocols.
Compatibility: - 4 - R - S L - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&K5 Bi-directional transparent software flow control is used. This mode is used by Hayes Smartcom III communication software that allows software flow control to be used and still allows XON/XOFF characters in binary data to be not be interpreted by the modem.
Compatibility: - 4 - R - S - - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&K6
(Default)
On Rockwell modems, enables both RTS/CTS and XON/XOFF flow control.
Compatibility: - - - R - - - - -
&K
(Enable/Disable Data Compression)
On USR/3Com modems, "&K" is used to specify what types of data compression are to be negotiated with the remote modem.
Standards: The "&K" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Note: This description is valid only for USR/3Com modems. Other brands that have a "&K" command use it for an unrelated function, described above.
Modifiers Description
&K0 Data Compression is disabled.
&K1
(Default)
The modem is allowed to negotiate the best compression method both modems support.
&K2 Data Compression must be used. If compression cannot be negotiated, the call will fail to negotiate.
&K3 The modem will only negotiate LAP-M/V.42bis/V.44 compression. MNP-5 will not be used even if that is the only compression method that the remote modem is willing to negotiate.
&L
(Line Selection)
The "&L" command instructs the modem as to whether a leased line or a PSTN (dial-up) line is to be used.

In leased line/direct connect modes, most modems automatically establish and maintain carrier. If carrier is interrupted, the modem will re-establish carrier without DTE intervention. Some modems support only a limited set of speeds in leased line configurations.


Standards: The "&L" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or have the same function. See individual option descriptions.
Modifiers Description
&L0
(Default)
The modem is connected to the Public Switched Telephone Network (PSTN) via an analog or digital link.
Compatibility: 2 4 - R - S L T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&L1 The modem is connected to a leased line.
Compatibility: 2 4 - R - S L T -
Note: Lucent and Rockwell modems accept this command but treat it the same as the "&L0" command. For Sierra modems, this command only exists on V.32bis models and earlier designs. Some modems only allow specific speeds to be used in leased line mode.
&M
(Asynchronous or Synchronous Mode)
The "&M" commands are used to specify if the modem communicates with the DTE using asynchronous or synchronous communications. Some related operations are controlled by the "&Q" command.
Standards: The "&M" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or have the same function. See individual option descriptions. Internal modems may not support Synchronous modes but the commands may be accepted. The S25 register may also interact with the &M commands.
Modifiers Description
&M0
(Default)
Asynchronous Mode, also known as Direct Mode. All serial transmissions to and from the DTE use Start and Ttop bits. ("&M0" is equivalent to "&Q0".)
Compatibility: - 4 - R - S L T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&M1 Synchronous Mode 1. The modem communicates asynchronously with the DTE in Command mode. After DCD transitions from FALSE to TRUE or the CONNECT message is sent to the DTE, subsequent transmissions to and from the DTE are in synchronous mode. There may be additional delays or criteria that may occur that effect when communication in synchronous mode begins, so you should refer to the documentation for the specific modem. ("&M1" is equivalent to "&Q1".)
Compatibility: - 4 - R - S - T -
Note: For Sierra modems, this description only applies to V.32bis models and earlier designs.
&M1 Sierra V.34 modems document the "&M1" command as being for factory use only and provide no other information.
Compatibility: - - - - S - - - -
&M2 Synchronous Mode 2. The modem communicates synchronously at all times. Modems dial a stored telephone number automatically when DTR goes from FALSE to TRUE. This modem is to be used with synchronous terminals. ("&M2" is equivalent to "&Q2".)
Compatibility: - 4 - R S - - T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&M3 Synchronous Mode 3. The modem communicates synchronously at all times. The modem begins transmissions when DTR goes from FALSE to TRUE. A handset is used to manually originate the synchronous call. ("&M3" is equivalent to "&Q3".)
Compatibility: - 4 - R S - - T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&M
(Select Error Control)
On USR/3Com modems, the "&M" commands specify error control.
Standards: The "&M" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Note: Other manufacturers use the "&M" command for unrelated functions.
Modifiers Description
&M0 Error Control is disabled. This setting is not recommended.
&M1 USR/3Com documents this command as reserved.
&M2 USR/3Com documents this command as reserved.
&M3 USR/3Com documents this command as reserved.
&M4
(Default)
Normal/ARQ Error Control on 1200bps and higher speeds.
&M5 ARQ-only Mode
&N
(Set Special Echo Canceller Training Sequence)
Sierra Semiconductor V.32bis modems use this command to control Echo Cancellation. (No additional description information was available.)
Standards: The "&N" command is not part of the EIA/TIA-602 standard.
Compatibility: - 4 - - - S - - -
Note: USR/3Com modems use the "&N" command for unrelated functions.
Modifiers Description
&N0 Special Echo Canceller Training Sequence is not required.
&N1
(Default)
Special Echo Canceller Training Sequence required.
&N
(Set Connect Speed)
USR/3Com modems use this command to force a specific carrier speed that is to be used during a data connection. If that exact speed cannot be established, the modem will go on-hook. Use of this command is not recommended since it will abort data calls or prevent data calls from being establihed due to changes in line impairments.
Standards: The "&N" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Note: Sierra modems use the "&N" command for unrelated functions.
Modifiers Description
&N0
(Default)
The modem is allowed to select the best carrier rate that the line conditions allow with an acceptable number of bit errors per second. This is the recommended setting.
&N1 The modem is allowed to connect only at 300bps.
&N2 The modem is allowed to connect only at 1200bps.
&N3 The modem is allowed to connect only at 2400bps.
&N4 The modem is allowed to connect only at 4800bps.
&N5 The modem is allowed to connect only at 7200bps.
&N6 The modem is allowed to connect only at 9600bps.
&N7 The modem is allowed to connect only at 12,000bps.
&N8 The modem is allowed to connect only at 14,400bps.
&N9 The modem is allowed to connect only at 16,800bps.
&N10 The modem is allowed to connect only at 19,200bps.
&N11 The modem is allowed to connect only at 21,600bps.
&N12 The modem is allowed to connect only at 24,000bps.
&N13 The modem is allowed to connect only at 26,400bps.
&N14 The modem is allowed to connect only at 28,800bps.
&N15 The modem is allowed to connect only at 31,200bps.
&N16 The modem is allowed to connect only at 33,600bps.
&N17 The modem is allowed to connect at PCM speeds.
&P
(Sets Pulse Dialing Make/Break ratio)
The "&P" command controls the duty cycle of the "make" and "break" pulses used when using pulse dialing (also known as rotary dialing). While most countries use a 39% make 61% break ratios, the United Kingdom and Hong Kong use a 33% make 67% break ratios. Using the wrong make/break ratio will usually cause pulse dialing to fail or dial incorrect numbers.
Standards: The "&P" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
Note: Pulse dialing is typically fixed at 10 digit pulses per second. However, some telephone company switching equipment allows higher rates. Some Rockwell modems allow faster pulse dialing via the &P2 and &P3 commands. This faster dialing is not widely supported by the telephone companies, and this feature should be considered non-portable.
Modifiers Description
&P0
(Default)
The pulse dialing make/break ratio is set to 39%/61%. This is the correct setting for North America.
Compatibility: 2 4 - R U S L - C
Note: Lucent modems vary from others, generating a 36%/64% ratio. For Sierra modems, this command only exists on V.32bis models and earlier designs.
&P1 The pulse dialing make/break ratio is set to 33%/67%. This is the correct setting for the United Kingdom and Hong Kong.
Compatibility: 2 4 - R U S L - C
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&P2 On Rockwell modems, the "&P2" command sets the pulse dialing make/break ratio to 39%/61%, which is the correct ratio for North America. "&P2" also increases the pulse rate to 20 pulses per second, which is not compatible with most telephone switching systems. ("&P0" and "&P1" use 10 pulses per second.)
Compatibility: - - - R - - - - -
&P3 On Rockwell modems, the "&P3" command sets the pulse dialing make/break ratio to 33%/67%, which is the correct ratio for Hong Kong and the United Kingdom. "&P3" also increases the pulse rate to 20 pulses per second, which is not compatible with most telephone switching systems. ("&P0" and "&P1" use 10 pulses per second.)
Compatibility: - - - R - - - - -
&Q
(Asynchronous or Synchronous Operation)
The "&Q" commands are used to specify if the modem communicates with the DTE using asynchronous or synchronous communications. Some related operations are controlled by the "&M" command.
Standards: The "&Q" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or have the same function. See individual option descriptions.
Modifiers Description
&Q0 Asynchronous operation. (This must be set for V.23 connections.)
("&Q0" is equivalent to "&M0".)
Compatibility: - 4 - R - S L T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs. On Lucent modems, this command is equivalent to the "\N1" command.
&Q1 Synchronous Mode 1. The modem communicates asynchronously with the DTE in Command mode. After DCD transitions from FALSE to TRUE or the CONNECT message is sent to the DTE, subsequent transmissions to and from the DTE are in synchronous mode. There may be additional delays or criteria that may occur that effect when communication in synchronous mode begins, so you should refer to the documentation for the specific modem.
("&Q1" is equivalent to "&M1".)
Compatibility: - 4 - R - S - T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&Q2 Synchronous Mode 2. The modem communicates synchronously at all times. Modems dial a stored telephone number automatically when DTR goes from FALSE to TRUE. This modem is to be used with synchronous terminals.
("&Q2" is equivalent to "&M2".)
Compatibility: - 4 - R - S - T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&Q3 Synchronous Mode 3. The modem communicates synchronously at all times. The modem begins transmissions when DTR goes from FALSE to TRUE. A handset is used to manually originate the synchronous call.
("&Q3" is equivalent to "&M3".)
Compatibility: - 4 - R - S - T -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&Q4 The "&Q4" command enabled AutoSync mode. When used in conjunction with the Hayes Synchronous Interface (HSI) capability in the DTE, it provides synchronous communication from an asynchronous terminal. Synchronous Mode 3. This command interacts with the "S19", "S20", and "S25".
Compatibility: - 4 - R - S - - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&Q5
(Default)
The modem should make data connections using V.42 or MNP-2 through MNP-4 error-correction (synchronous line transmission modes).
Compatibility: - 4 - R - S L - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs. On Lucent modems, this command is equivalent to the "\N3" command.
&Q6 The "&Q6" command forces data connections to use standard buffered Data Mode.
Compatibility: - 4 - R - S L - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs. On Lucent modems, this command is equivalent to the "\N0" command.
&Q6 On Telebit modems, the "&Q6" command enables transparent synchronous mode. Register "S253" controls Command Mode operations.
Compatibility: - - - - - - - T -
Note: All other modems with a "&Q6" command perform a different action. See above.
&R
(Clear to Send)
The Clear To Send (CTS) signal is controlled by the "&R" command.
Standards: The "&R" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
Note: USR/3Com Modems use the "&R" command for unrelated operations.
Modifiers Description
&R0
(Default)
The CTS signal generated by the modem follows the RTS signal received from the DTE. The signal is delayed by the time specified by Register "S26".
Compatibility: - 4 - R - S - T -
Note: For Sierra modems, this description applies to V.34 modems only. The Sierra V.32bis command description is below.
&R0 In Sierra modems operating in asynchronous mode, CTS is TRUE when the modem is in Command Mode.
In synchronous mode, when RTS goes TRUE, CTS will go TRUE after a delay.
Compatibility: - - - - - S - - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&R1 When the modem is in Data Mode (when the CONNECT message is sent to the DTE), CTS is TRUE.
Compatibility: - 4 - R - S - T -
Note: For Sierra modems, this description applies to V.34 modems only. The Sierra V.32bis command description is below.
&R1
(Default)
In Sierra modems, "&R1" command selects CTS behavior as follows: In asynchronous Data Mode, CTS is forced off.
In asynchronous Data Mode, CTS follows RTS.
In Command Mode, CTS is forced on.
Compatibility: - - - - - S - - -
Note: For Sierra modems, this command only exists on V.32bis models and earlier designs.
&R2 When in Data Mode, the CTS signal follows the RTS signal. CTS follows RTS by 200msec. When in Command Mode, CTS is FALSE.
Compatibility: - 4 - - - S - T -
Note: On Sierra modems, this command only exists on V.32bis and earlier commands. modems.
&R3 In Telebit modems, "&R3" forces CTS to always be TRUE when hardware flow control is disabled. Otherwise, CTS will control hardware flow control.
Compatibility: - - - - - S - T -
&R3 In Sierra modems, the "&R3" command causes the CTS signal to be forced on.
Compatibility: - - - - - S - - -
&R4 In Telebit modems when "&R4" is used and a data connection is in progress, the CTS signal follows the RTS signal, after the delay specified by the S26 register. When the modem is in Command Mode, CTS follows DTR.
Compatibility: - - - - - - - T -
&R
(Set Receive Data from DTE flow control)
On USR/3Com modems, the Request to Send (RTS) signal is controlled by the "&R" command.
Standards: The "&R" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Note: All other manufacturers use the "&R" command for unrelated functions.
Modifiers Description
&R0 USR/3Com documents this command as reserved.
&R1 On USR/3Com modem, the "&R1" command causes the modem to ignore any transition on the RTS signal from the DTE.
&R2
(Default)
On USR/3Com modems, the "&R2" specifies that data is only transmitted to the DTE when the RTS signal from the DTE is true.
&S
(Data Set Ready)
The DTE DSR signal is controlled by the "&S" Commands.
Standards: The "&S" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
Modifiers Description
&S0
(Default)
The DSR line is TRUE at all times.
Compatibility: - 4 - R U S L T C
&S1 The DSR signal goes TRUE after the modem detects an answer tone and remains on until the call ends.
Compatibility: - 4 - R U S - T C
&S2 DSR signal remains TRUE except when a call disconnects. On disconnect the DSR signal goes FALSE for the amount of time specified by the S47 register, then goes TRUE again.
Compatibility: - - - - - - - T -
&S2 DSR is TRUE when carrier is being detected on the telephone loop.
Compatibility: - - - - - S - - -
&S3 DSR is TRUE if DTR is TRUE or carrier is detected and DTR is true. DSR goes FALSE if DTR goes FALSE or carrier is lost.
Compatibility: - - - - - - - T -
&S3 DSR is TRUE at all times.
Compatibility: - - - - - S - - -
&S4 DSR is TRUE only when carrier is detected from the remote modem.
Compatibility: - - - - - - - T -
&T
(Test Commands)
The "&T" commands are used to perform a variety of diagnostic tests.
Standards: The "&T" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
Note: The commands related to Digital Loopback Tests ("&T3", "&T4", "&T5", "&T6" and "&T7") are not available on all modem models.
Modifiers Description
&T0
(Default)
Any test that is in progress is terminated. When the modem is started or is not performing the a test, this is the default state.
Compatibility: 2 4 - R U S L T C
Note: Both "&T0" and "&T5" are defaults at the same time since they control separate operations. In all other cases, a given "AT" command only has one default setting.
&T1 The modem performs an Analog Loopback test.
Compatibility: 2 4 - R U S L T C
&T3 The modem performs a Digital Loopback test.
Compatibility: 2 4 - R U S - T C
&T4 Instructs the local modem to accept a request from a remote modem for a Digital Loopback Test. Typically, the called modem would execute a "&T4" command, then the calling modem would execute a "&T6" or "&T7" command.
Compatibility: 2 4 - R U S - T C
&T5
(Default)
Instructs the local modem to refuse any request from a remote modem to participate in a Digital Loopback Test. By default, a modem refuses a request for a Digital Loopback Test that comes from a remote modem, and this capability must be enabled
Compatibility: 2 4 - R U S - T C
Note: Both "&T5" and "&T0" are defaults at the same time since they control separate operations. In all other cases, a given "AT" command only has one default setting.
&T6 The local modem performs a Remote Digital Loopback Test, assuming the remote modem accepts the Digital Loopback Test.
Compatibility: 2 4 - R U S L T C
&T7 The local modem performs a Remote Digital Loopback Test, assuming the remote modem accepts the Digital Loopback Test. The local modem also performs a self-test.
Compatibility: 2 4 - R U S - T C
&T8 The modem performs an Analog Loopback test and a self-test.
Compatibility: 2 4 - R U S - T C
&T9 On Sierra V.34 modems, the "&T9" command forces a Self-Test to terminate.
Compatibility: - - - - - S - - -
Note: The "&T9" command is not present on V.32bis and earlier Sierra modems.
&T9 On Telebit modems, the "&T9" command forces a Self-Test to terminate and the internal diagnostics that are performed on a power-up are executed. The results of those tests are available via the "I1" command. This command essentially resets the modem, returning all settings to those stored in non-volatile memory.
Compatibility: - - - - - - - T -
&U
(Trellis Coding)
The "&U" command controls whether Trellis coding is used with V.32 transmissions (9600bps).
Standards: The "&U" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
Note: USR/3Com modems use the "&U" command for unrelated operations.
&U0
(Default)
Trellis Coding will be used for V.32 transmissions.
Compatibility: - 4 - R - S - - C
&U1 Trellis Coding will not be used for V.32 transmissions. QAM will be used.
Compatibility: - 4 - R - S - - C
&U
(Set Speed Floor)
On USR/3Com modems running at 28,800bps or higher, the "&U" command specifies that the carrier speed must be at least the value speed or the call will disconnect.
Standards: The "&U" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
Note: Other modems that have a "&U" command use it for unrelated operations.
&U0
(Default)
No minimum carrier speed is specified.
&U1 The minimum carrier speed is 300bps.
&U2 The minimum carrier speed is 1200bps.
&U3 The minimum carrier speed is 2400bps.
&U4 The minimum carrier speed is 4800bps.
&U5 The minimum carrier speed is 7200bps.
&U6 The minimum carrier speed is 9600bps.
&U7 The minimum carrier speed is 12,000bps.
&U8 The minimum carrier speed is 14,400bps.
&U9 The minimum carrier speed is 16,800bps.
&U10 The minimum carrier speed is 19,200bps.
&U11 The minimum carrier speed is 21,600bps.
&U12 The minimum carrier speed is 24,000bps.
&U13 The minimum carrier speed is 26,400bps.
&U14 The minimum carrier speed is 28,800bps.
&U15 The minimum carrier speed is 31,200bps.
&U16 The minimum carrier speed is 33,600bps.
&V
(View Active Configuration)
The "&V" command displays most or all command and S register settings that are active in the modem, as well as the settings that are stored profiles, including any stored telephone numbers.
Standards: The "&V" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
Note: USR/3Com modems use the I4 command to provide this functionality.
&V0 Displays the contents of the active and stored profile #0 (A).
Compatibility: - 4 - R - S L T C
Note: On Sierra modems, the "V0" command displays all stored profiles and all stored telephone numbers.
&V1 Displays the contents of stored profile #1 (B).
Compatibility: - - - - - - - - C
&W
(Stored Active Configuration)
The "&W" command stores the current or active configuration (also known as the "profile") of the modem into non-volatile memory in the modem. When the modem is reset, the factory configuration is loaded, and then the stored configuration is loaded prior to the modem responding to commands.

Depending on the modem, the "&W" command may store all settings or only a portion of all possible modem parameters. Most consumer-grade modems only store a portion of all paremeters in non-volatile RAM. Parameters not stored will revert to their factory settings after a modem reset occurs.

So-called "Soft" or "Win" modems typically emulate the "&W" command by storing configuration parameters in a file or other data structure managed by the operating system running on the host computer.


Standards: The "&W" command is not part of the EIA/TIA-602 standard.
Compatibility: Not all options are implemented in all modems or behave in the same ways. See individual option descriptions.
&W0 The "&W0" command will save the active configuration in stored profile #0 (or "A").
Compatibility: 2 4 - R U S L T C
Note: On Telebit modems, the configuration is written to "A" or "B" based on the setting of register "S255". If "S255==0", the configuration is stored in "A". If "S255==1", the configuration is stored in "B". If "S255=255", the setting of the front panel A/B switch will determine where the configuration is stored.
&W1 The "&W1" command will save the active configuration in stored profile #1 (or "B").
Compatibility: - 4 - R U S - T C
&X
(Select Clock Source)
When the modem is operating in synchronous mode, the "&X" command selects which clock will be used by the modem transmitter.
Standards: The "&U" command is not part of the EIA/TIA-602 standard.
Compatibility: - 4 - R - S - T -
Note: On Sierra modems, the "&X" command exists on V.32bis models and earlier designs.
&X0
(Default)
The transmit clock signal is generated by the local modem. The clock signal is also transmitted on pin 15 for the RS-232 DB25 connector.
&X1 The DTE device is responsible for providing the clock signal to the modem. The modem receives the DTE clock on pin 24 of the RS-232 DB25 connector.
&X2 The modem accepts clock from the remote modem. The received clock is also sent to the DTE on pin 15 of the RS-232 DB25 connector.
&Y
(Select Stored Profile)
When the modem resets or is power-cycled, the "&Y" command specifies which user-stored profile should be loaded after the factory settings are loaded.
Standards: The "&Y" command is not part of the EIA/TIA-602 standard.
Compatibility: - 4 - R - S - - C
Note: USR/3Com modems use the "&Y" command for unrelated operations.
&Y0 The modem will use stored profile #0 (or "A") on reset.
&Y1 The modem will use stored profile #1 (or "B") on reset.
&Y
(Sets break handling)
On USR/3Com modems, when a break signal (spacing) is received from the DTE, the "&Y" command specifies how the modem will respond.
Standards: The "&Y" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
&Y0 On receipt of a data break from the DTE, all untransmitted characters held by the modem are discarded. A data break is not transmitted to the remote modem.
&Y1
(Default)
On receipt of a data break from the DTE, all untransmitted characters held by the modem are discarded. A data break is immediately transmitted to the remote modem.
&Y2 On receipt of a data break from the DTE, a data break is transmitted to the remote modem ahead of any untransmitted characters held by the modem. After sending the data break, transmission of characters continue.
&Y3 USR/3Com documents this command as reserved.
&Zn=s
(Store telephone number in non-volatile memory)
This command stores string s in one of four areas in non-volatile RAM. N is a value from 0 to 3. The maximum number of digits and modifiers that can be stored varies by modem but is usually at least 30 characters.
Standards: The "&Zn=s" command is not part of the EIA/TIA-602 standard.
Compatibility: - 4 - R U S L - C
&Zn=L
(Stores the last dialed telephone number in non-volatile memory)
This command stores the last dialed telephone number string.
Standards: The "&Zn=L" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -
&Zn?
(Display stored telephone numbers in non-volatile memory)
On USR/3Com modems, the &Zn? command displays the dialing string stored in one of four areas in non-volatile RAM. N is a value from 0 to 3.
Standards: The "&Zn?" command is not part of the EIA/TIA-602 standard.
Compatibility: - - - - U - - - -