Prior to the introduction of the Bulletin Board System (BBS), modems typically operated on direct-dial telephone lines that always began and ended with a single modem at each end. The user would usually dial the phone manually before connecting, or pick it up if it rang. In a few cases the computers themselves had to call a selection of numbers, and for this task they used a separate peripheral device, a "dialer" plugged into a different input/output port on the computer (typically an RS-232 port).
This method of operation worked satisfactorily in the 1960s and early 1970s, when modems generally connected to only large mainframe computers. However, the microcomputer revolution of the 1970s led to the introduction of low-cost modems and the idea of a semi-dedicated point-to-point link was no longer appropriate. There were potentially thousands of users, who might want to dial any of the other thousands of users and the only solution at the time was to make the user dial manually.
The computer industry needed a way to tell the modem what number to dial through software. The earlier separate dialers had this capability, but only at the cost of a separate port, which a microcomputer might not have available. One solution could have used a separate set of "command pins" dedicated to sending and receiving commands, another could have used a signal pin indicating that the modem should interpret incoming data as a command. But both of these suffered from the similar problem that these pins might not be available, or connected, in many microcomputers.
Hayes Communications introduced a solution in its 1977 product, the Smartmodem, by re-using the existing data pins with no modification. Instead, the modem itself could switch itself between one of two modes:
data mode in which the modem sends the data to the remote modem. (A modem in data mode treats everything it receives from the computer as data and sends it across the phone line).
command mode in which data is interpreted as commands to the local modem (commands that the local modem should execute).
To switch from data mode to command mode sessions sent an escape sequence string of three plus signs ("+++") followed by a pause of about a second (to go to the command mode without losing the connection); and to switch back they sent the online command, O. In actual use many of the commands automatically switched to the online mode after completion, and it is rare for a user to use the online command explicitly.
This use of In-band signaling leads to a potential serious problem: what happens if the data sent in data-mode contains three consecutive plus signs? This could happen randomly quite easily, and it would happen any time someone attempted to send data describing the system – this document for instance. In order to reduce the chance of this problem the pause at the end of the escape sequence was required, if any other data was received within one second of the three plus signs, it was not the escape sequence and would be sent as-is.
The command set includes commands for various phone-line manipulations, dialing and hanging-up for instance. It also includes various controls to set up the modem, including a set of register commands which allowed the user to directly set the various memory locations in the original Hayes modem. The command set was copied largely verbatim, including the meaning of the registers, by almost all early 300 baud modem manufacturers, of which there were quite a few.
The command set itself had no intellectual-property protection, but Hayes Communications patented the concept of a "guard time" after the escape sequence. In the late 1980s Hayes started enforcing the patent, charging $1 per modem that used it. Many later modems thus did not implement the guard time; this eventually (when modems began to be used to dial up to the Internet) led to a nasty denial-of-service attack involving an ICMP echo request ("ping") packet containing three pluses and ATH, the hangup command.The expansion to 1200 and 2400 baud required the addition of a small set of new commands, some of them prefixed with a & to denote those dedicated to new functionality. However Hayes Communications moved only slowly to higher speeds or the use of compression, and three other companies led the way here – Microcom, U.S. Robotics and Telebit. Each of these three used its own additional command-sets instead of waiting for Hayes to lead the way.
Soon a plethora of new models appeared, including new ones from Hayes, following a variety of "standards". Although these shared the same commands for simple instructions such as dialing, the higher-speed options differed dramatically. Nevertheless, all of them referred to themselves as using the "Hayes command set".
Years later, the TIA/EIA introduced a formal standard with the title Data Transmission Systems and Equipment - Serial Asynchronous Automatic Dialing and Control, otherwise known as TIA/EIA-602. TIA/EIA-602 is almost identical to the data-specific commands found in the Smartmodems 1200 and Smartmodem 2400. Of course, by the time the TIA/EIA-602 standard came out, vendors were selling modems with error-correction, compression and far higher speeds. None of these newer capabilities (or the commands needed to control them) are addressed by the TIA/EIA-602 standard, although other standards or drafts of standards exist for commands specific to FAX operations on modems that support FAX transmission or reception, as well as commands specific to voice operations.
Things became simpler again during the widespread introduction of 14.4 and 28.8 kbaud modems in the early 1990s. Slowly a set of commands based heavily on the original Hayes extended set using "&" commands became popular, and then universal. Only one other command set has remained popular, the US Robotics set from their popular line of modems.
Modem initialisation
A string can contain many of Hayes commands placed together, so as to optimally prepare the modem to dial out or answer. This is called the Initialisation String.
Example session
The following represents two computers, computer A and computer B, both with modems attached, and the user controlling the modems with terminal-emulator software. Terminal-emulator software typically allows the user to send Hayes commands directly to the modem, and to see the responses. In this example, the user of computer A makes the modem dial the phone number of modem B at phone number 555-1234. Note that after every command and response, there is a carriage return sent to complete the command.
Description
The following text lists part of the Hayes command set (also called the AT commands: "AT" meaning attention).
The Hayes command set can subdivide 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 extends the basic command set. For example, &M1. Note that M1 is different from &M1.
proprietary command set - Usually starting either with a backslash (“\”) or with a percent sign (“%”); these commands vary widely among modem-manufacturers.
register commands - Sr=n where r is the number of the register to be changed, and n is the new value that is assigned.
A register represents a specific physical location in memory. Modems have small amounts of memory onboard. The fourth set of commands serves for entering values into a particular register (memory location). The register will store a particular variable (alpha-numeric information) which the modem and the communications software can utilize. For example, S7=60 instructs the computer to "Set register #7 to the value 60".
Although the command-set syntax defines most commands by a letter-number combination (L0, L1 etc.), the use of a zero is optional. In this example, "L0" equates to a plain "L". Keep this in mind when reading the table below.
Some of the most important characters that may appear in the modem initialization string follow. Normally one should not change these characters.
AT - "Attention": tells the modem that modem-commands follow. This must begin each line of commands.
Z - Resets the modem to its default state.
, (a comma) - Makes the software pause for a second. More than one comma can appear in succession: for example, ",,,," tells the software to pause four seconds. (The setting of register S8 governs the duration of the pause.)
^M - Sends the terminating Carriage Return character to the modem. This is a control code that most communication software translates as a carriage return. (Note, when this is sent to the modem, it is sent as a single byte, ASCII CR (0x0D), or "Control-M" not the two characters ^ and M.)
; (a semi-colon) - Return to command mode immediately after dialing. This makes it possible, for example, to dial more than 45 digits numbers, or to walk through interactive menus.
W - wait for dialtone. The modem will wait for a dialtone before dialing numbers following the W. For this to work, waiting must not exceed a timeout, generally configured in the S7 register.
! - flash hook. put quickly the modem on/off hook.
When in data-mode an escape sequence can return the modem to command mode. The normal escape sequence is three plus signs ("+++"), and to disambiguate it from possible real data, a guard timer is used: it must be preceded by a pause, not have any pauses between the plus signs, and be followed by a pause; by default a "pause" is one second and "no pause" is anything less.
Syntactical definitions
The following syntactical definitions apply:
<...> Name enclosed in angle brackets is a syntactical element. They do not appear in the command line.
[...] Optional subparameter of a command or an optional part of TA information response is enclosed in square brackets. Brackets themselves do not appear in the command line. When subparameter is not given in AT commands which have a Read command, new value equals to its previous value. In AT commands which do not store the values of any of their subparameters, and so have not a Read command, which are called action type commands, action should be done on the basis of the recommended default setting of the subparameter.
Compatibility
While the original Hayes command set represented a huge leap forward in modem-based communications, with time many problems set in, almost none of them due to Hayes per se:
Due to the lack of a written standard, other modem manufacturers just copied the external visible commands and (roughly) the basic actions. This led to a wide variety of subtle differences in how modems changed from state to state, and how they handled error conditions, hangups, and timeouts.
Each manufacturer tended to add new commands to handle emerging needs, often incompatible with other modems.
For example, setting up hardware or software handshaking often required many different commands for different modems. This undermined the handy universality of the basic "AT" command-set.
Many "Hayes-compatible" modems had serious quirks that made them effectively incompatible. For example, many modems required a pause of several seconds after receiving the "AT Z" reset command. Some modems required spaces between commands, while others did not. Some would unhelpfully change baud-rate of their own "volition", which would leave the computer with no clue how to handle the incoming bits.
As a result of all this, eventually many communiucations programs had to give up any sense of being able to talk to all "Hayes-compatible" modems, and instead the programs had to try to determine the modem type from its responses, or provide the user with some option whereby they could enter whatever special commands it took to coerce their particular modem into acting properly.
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