The AT Command Set Reference

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.

(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.

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.

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.

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.

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

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

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

000

United States or Canada

016

Japan

033

Belgium

034

Finland

035

France

037

Italy

038

Netherlands

039

Sweden

040

Switzerland

041

United Kingdom

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

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

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

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

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.

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.

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.

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.

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.