THE HAYES MODEM COMMAND SET

The modem initialization string consists of a series of commands. It prepares the modem for communications, setting such features as dialing mode, waits, detection of the busy signal and many other settings. Newer modem communications programs reset the initializations string for you according to which menu options you select, which features you enable, etc.

For many years Hayes modems have been the standard. As the field of modem manufactures has grown, most have adhered at least loosely to the Hayes standard. The following is a partial list of the Hayes command set. (called the AT commands). The Hayes Command Set can be divided into four groups:

Basic Command Set

A capital character followed by a digit. For example, M1.

Extended Command Set

An "&" (ampersand) and a capital character followed by a digit. This is an extension of the basic command set. For example, "&M1". Note that "M1" is different from "&M1"

.

Proprietary Command Set

Usually started by either a backslash ("\"), or a percent sign ("%"), these commands vary widely among modem manufacturers. For that reason, only a few of these commands are listed below.

Register Commands

Sr=n where "r" is the number of the register to be changed and "n" is the new value that is being assigned.

A "register" is computerese for a specific physical location in memory. Modem have small amounts of memory onboard. This fourth set of commands is used to enter values in a particular register (memory location). The register will be storing a particular "variable" (alpha-numeric information) which is utilized by the modem and communication software. For example, S7=60 instructs your computer to "Set register #7 to the value 60."

Note: Although most commands are defined by a letter-number combination (L0, L1, etc.), the use of a zero is optional. In this case, L0 is the same as a plain "L" - keep this in mind when reading the table below!

Here are some of the most important characters that may appear in the modem initialization string. These characters normally should not be changed.

AT/dt>

tells the modem that modem commands follow. This must begin each line of commands.

Z

resets the modem to default state

, (comma)

makes your software pause for a second. You can use more than one , in a row. For example, ",,,," tells the software to pause four seconds. (The duration of the pause is governed by the setting of register S8.

^M

sends the terminating Carriage Return character to the modem. This is a control code that most communication software translate as a "carriage return."

Dial Modifiers

(These are common but most modems will have more.)

P

Pulse dial

T

Touch Tone dial

W

Wait for second dial tone

,

Pause for time specified in register S8 (usually 2 seconds)

;

Remain in command mode after dialing

!

Flash switch-hook (Hang up for a half second as in transferring a call)

L

Dial last number

Modems with volume control knobs will not have these options.

This is usually a good thing to use in your init string, since the &F1-&F3 settings can vary among modems, and they may actually be the cause of connection problems. (Since you never know exactly what Brand X's &F2 really changes.

On the other hand, it pays to try out the other options below; many people's problems can be solved by replacing a complicated init string with a simple &F2 or the like. However, if you're building an init string, it's best to start with a simple &F, and not use the "customized" form of defaults.

Note: A direct connection is a simple straight-though connection without any error correction or data compression. In this case, the computer-to-modem and modem-to-modem speeds must be identical.

A normal connection uses flow control (either software or hardware) to buffer the data being sent or received, so that the modem can transmit data at a different rate than the computer is actually sending or receiving it. For example, a computer may send actual data at 57kbps, but using compression, the modem only actually sends 28.8kbps. This is the mode use by most modems.

A reliable connection is a type of normal connection; if, for some reason, data compression or error correction cannot be established or maintained, the connection will hang up. (In essence, such a modem ensures that all connections are reliable, for it will hang up if the connection isn't.)

Likewise, an auto-reliable connection is virtually the same, except that the modem will try to renegotiate the connection in order to establish a reliable connection. Again, this is the mode that most modems use.

Many modems have dozens, even hundreds, of S registers, but only the first dozen or so are fairly standard. They are changed with a command like ATSn=N, and examined with ATSn? (e.g. "AT S10=70 S1?" would tell the modem not to hang up for seven seconds should it not hear the answering modem, and return the number of times the phone last rang.)