Hayes AT command set

Hayes 300 Baud Smartmodem

The Hayes bid set (also known arsenic the AT bid set) is simply a circumstantial command language primitively developed by Dale Heatherington and Dennis Hayes[1][2] for the Hayes Smartmodem successful 1981.

The bid group consists of a bid of short matter strings that tin beryllium mixed to nutrient commands for operations specified arsenic dialing, hanging up, and changing the parameters of the connection. The immense mostly of dial-up modems usage the Hayes bid group successful galore variations.

The bid group covered only those operations supported by the earliest 300 bit/s modems. When caller commands were required to power further functionality successful higher-speed modems, a assortment of one-off standards emerged from each of the awesome vendors. These continued to stock the basal bid building and syntax, but added immoderate number of caller commands utilizing immoderate benignant of prefix character – & for Hayes and USRobotics, and \ for Microcom, for instance. Many of these were re-standardized connected the Hayes extensions aft the preamble of the SupraFAXModem 14400 and the marketplace consolidation that followed.

The word Hayes compatible was[3][1] and as of 2018 still is important wrong the industry.[4]

Before the preamble of the bulletin committee system (BBS), modems typically operated connected direct-dial telephone lines that began and ended pinch a known modem astatine each end. The modems operated successful either "originate" aliases "answer" modes, manually switching betwixt 2 sets of frequencies for information transfer. For the originating modem, these frequencies are 1,270 Hz (Mark tone, Binary 1) and 1,070 Hz (Space tone, Binary 0). For the answering modem, the frequencies are 2,225 Hz (Mark tone) and 2025 Hz (Space tone). Generally, the personification placing the telephone would move their modem to "originate" and past dial the number by hand. When the distant modem answered, already group to "answer" mode, the telephone handset was switched disconnected and communications continued until the caller manually disconnected.

When automation was required, it was commonly only needed connected the reply side; for instance, a slope mightiness request to return calls from a number of branch offices for end-of-day processing. To capable this role, immoderate modems included the expertise to prime up the telephone automatically erstwhile it was successful reply mode, and to clear the statement erstwhile the different personification manually disconnected. The request for automated outbound dialling was considerably little common, and was handled done a abstracted peripheral device: a "dialler". This was usually plugged into a abstracted input/output port connected the machine (typically an RS-232 port) and programmed separately from the modem itself.

This method of cognition worked satisfactorily successful the 1960s and early 1970s, erstwhile modems were mostly utilized to link dumb devices for illustration computer terminals (dialling out) pinch smart mainframe computers (answering). However, the microcomputer revolution of the 1970s led to the preamble of low-cost modems, and the thought of a semi-dedicated point-to-point nexus was nary longer appropriate. There were perchance thousands of users who mightiness want to dial immoderate of the different thousands of users, and the only solution astatine the clip was to make the personification dial manually.

The computer industry needed a measurement to show the modem what number to dial done software. The earlier abstracted dialers had this capability, but only astatine the costs of a abstracted port, which a microcomputer mightiness not person available. Another solution would person been to usage a abstracted group of "command pins" dedicated to sending and receiving commands; different could person utilized a awesome pin indicating that the modem should construe incoming information arsenic a command. Both of these had hardware support successful the RS-232 standard. However, galore implementations of the RS-232 larboard connected microcomputers were highly basic, and immoderate eliminated galore of these pins to trim cost.

Hayes Communications introduced a solution successful its 1981 Smartmodem by utilizing the existing information pins pinch nary modification. Instead, the modem itself could beryllium switched betwixt 1 of 2 modes:

1. Data mode successful which the modem sends the information to the distant modem. (A modem successful information mode treats everything it receives from the machine arsenic information and sends it crossed the telephone line).
2. Command mode successful which information is interpreted arsenic commands to the section modem (commands the section modem should execute).

To move from information mode to bid mode, sessions sent an escape sequence drawstring of 3 positive signs (+++) followed by a region of astir a second. The region astatine the extremity of the flight series was required to trim the problem caused by in-band signaling: if immoderate different information was received wrong 1 2nd of the 3 positive signs, it was not the flight series and would beryllium sent arsenic data. To move backmost they sent the online command, ATO. In existent usage galore of the commands automatically switched to the online mode aft completion, and it is uncommon for a personification to usage the online bid explicitly.

In bid to debar licensing Hayes's patent, immoderate manufacturers implemented the flight series without the clip defender interval (Time Independent Escape Sequence (TIES)). This had a awesome denial of service information accusation successful that it would lead to the modem hanging up the relationship should the machine ever effort to transmit the byte series +++ATH0 successful information mode. For immoderate machine connected to the Internet done specified a modem, this could beryllium easy exploited by sending it a ping of death petition containing the series +++ATH0 successful the payload. The machine operating strategy would automatically effort to reply the sender pinch the aforesaid payload, instantly disconnecting itself from the Internet, arsenic the modem would construe the ICMP information payload arsenic a Hayes command.[5] The aforesaid correction would besides trigger if, for example, the personification of the machine ever tried to nonstop an email containing the aforementioned string.

The Hayes bid group includes commands for various phone-line operations specified arsenic dialing and hanging up. It besides includes various controls to group up the modem, including a group of register commands which allowed the personification to straight group the various representation locations successful the original Hayes modem. The bid group was copied mostly verbatim, including the meaning of the registers, by almost each early 300 baud modem manufacturers, of which location were rather a few.

The description to 1200 and 2400 baud required the summation of caller commands, immoderate of them prefixed pinch an ampersand (&) to denote those dedicated to caller functionality. Hayes itself was forced to quickly present a 2400 baud exemplary soon aft their 1200, and the bid sets were identical arsenic a time-saving method.[6] Essentially by accident, this allowed users of existing 1200 baud modems to usage the caller Hayes 2400 models without changing their software. This reinforced the usage of the Hayes versions of these commands. Years later, the Telecommunications Industry Association (TIA)/Electronic Industries Alliance (EIA) formally standardized the 2400-baud bid group arsenic Data Transmission Systems and Equipment – Serial Asynchronous Automatic Dialing and Control, TIA/EIA-602.

However, Hayes Communications was slow to merchandise modems supporting higher speeds aliases compression, and 3 different companies led: Microcom, USRobotics, and Telebit. Each of these 3 utilized its ain further bid sets. By the early 1990s, location were 4 awesome bid sets successful use, and a number of versions based connected 1 of these. Things became simpler again during the wide preamble of 14.4 and 28.8 kbit/s modems successful the early 1990s. Slowly, a group of commands based heavy connected the original Hayes extended group utilizing & commands became popular, and then universal. Only 1 different bid group has remained popular, the USRobotics group from their celebrated statement of modems.

The pursuing matter lists portion of the Hayes bid set, besides called the AT commands: "AT" meaning 'attention'. Each bid drawstring is prefixed pinch "AT", and a number of discrete commands tin beryllium concatenated aft the "AT".

The Hayes bid group tin beryllium subdivided into 4 groups:

1. basic bid set – A superior characteristic followed by a digit. For example, M1.
2. extended bid set – An "&" (ampersand) and a superior characteristic followed by a digit. This extends the basal bid set. For example, &M1. Note that M1 is different from &M1.
3. proprietary bid set – Usually starting either pinch a backslash (“\”) aliases pinch a percent motion (“%”); these commands alteration wide among modem manufacturers.
4. register commands – Sr=n wherever r is the number of the registry to beryllium changed, and n is the caller worth that is assigned. A register represents a circumstantial beingness location successful memory. Modems person mini amounts of representation connected board. The 4th group of commands serves for entering values into a peculiar registry (memory location). For example, S7=60 instructs the modem to "Set registry #7 to the worth 60". Registers usually power aspects of the modem cognition (e.g., transmission strength, modulation parameters) and are usually circumstantial to a peculiar model.

Although the command-set syntax defines astir commands by a letter-number operation (L0, L1, etc.), the usage of a zero is optional. In this example, "L0" equates to a plain "L". Keep this successful mind erstwhile reference the array below.

When successful information mode, an escape sequence tin return the modem to bid mode. The normal flight series is 3 positive signs ("+++"), and to disambiguate it from imaginable existent data, a defender timer is used: it must beryllium preceded by a pause, not person immoderate pauses betwixt the positive signs, and beryllium followed by a pause; by default, a "pause" is 1 2nd and "no pause" is thing less.

The pursuing syntactical definitions apply:[7]

• <CR> Carriage return character, is the bid statement and consequence codification terminator character, whose value, successful decimal ASCII betwixt 0 and 255, is specified successful registry S3. The default worth is 13.
• <LF> Linefeed characteristic is the characteristic recognised arsenic statement provender character. Its value, successful decimal ASCII betwixt 0 and 255, is specified successful registry S4. The default worth is 10. The statement provender characteristic is output aft the carriage return characteristic if verbose consequence codes are utilized (V1 action is used); otherwise, if numeric format consequence codes are utilized (V0 action is used), it will not look successful the consequence codes.
• <...> Name enclosed successful perspective brackets is simply a syntactical element. They do not look successful the bid line.
• [...] Optional subparameter of a bid aliases an optional portion of AT accusation consequence is enclosed successful quadrate brackets. Brackets themselves do not look successful the bid line. When the subparameter is not fixed successful AT commands which person a Read command, the caller worth equals its erstwhile value. In AT commands which do not shop the values of immoderate of their subparameters, and truthful person not a Read command, which are called action type commands, the action should beryllium done connected the ground of the recommended default mounting of the subparameter.

A drawstring tin incorporate galore Hayes commands placed together, truthful arsenic to optimally hole the modem to dial retired aliases answer, e.g. AT&F&D2&C1S0=0X4. Most modem package supported a user-supplied initialization string, which was typically a agelong concatenated AT bid which was sent to the modem upon launch.[8] The V.250 specification requires each DCEs to judge a assemblage (after "AT") of astatine slightest 40 characters of concatenated commands.[9]

The pursuing represents 2 computers, machine A and machine B, some pinch modems attached, and the personification controlling the modems pinch terminal emulator software. Terminal-emulator package typically allows the personification to nonstop Hayes commands straight to the modem, and to spot the responses. In this example, the personification of machine A makes the modem dial the telephone number of modem B astatine telephone number (212) 555-0100 (long distance). After each bid and response, location is simply a carriage return sent to complete the command.

Modem AModem BComment

ATDT12125550100		User astatine modem A issues a dial command: AT-Get the modem's ATtention; D-Dial; T-Touch-Tone; 12125550100-Call this number
	RING	Modem A originates dialing. Modem B's telephone statement rings, and the modem reports the fact.
	ATA	Computer astatine modem B issues reply command.
CONNECT	CONNECT	The modems connect, and some modems study "connect". (In practice, astir modems study much accusation aft the connection CONNECT — specifying the velocity of the connection.) Also, astatine this time, some modems will raise the DCD, aliases Data Carrier Detect signal, connected the serial port.
abcdef	abcdef	When the modems are connected, immoderate characters typed astatine either broadside will look connected the different side. The personification astatine machine A starts typing. The characters walk done the modem and look connected machine B's screen. (User A whitethorn not spot his ain typed characters — depending connected the terminal software's local echo setting).
	+++	The personification astatine machine B issues the modem flight command. (Alternately, and much commonly, machine B could driblet the DTR, aliases Data Terminal Ready signal, to execute a hangup, without needing to usage +++ aliases ATH.)
	OK	The modem acknowledges it.
	ATH	The personification astatine machine B issues a hang-up command.
NO CARRIER	OK	Both modems study that the relationship has ended. Modem B responds "OK" arsenic the expected consequence of the command; modem A says NO CARRIER to study that the distant broadside interrupted the connection. The modems connected some sides driblet their DCD signals arsenic well.

While the original Hayes bid group represented a immense leap guardant successful modem-based communications, pinch clip galore problems group in, almost nary of them owed to Hayes per se:

• Due to the deficiency of a written standard, different modem manufacturers conscionable copied the externally visible commands and (roughly) the basal actions. This led to a wide assortment of subtle differences successful really modems changed from authorities to state, and really they handled correction conditions, hangups, and timeouts.
• Each shaper tended to adhd caller commands to grip emerging needs, often incompatible pinch different modems. For example, mounting up hardware aliases package handshaking often required galore different commands for different modems. This undermined the useful universality of the basal Hayes bid set.
• Many Hayes compatible modems had superior quirks that made them efficaciously incompatible. For example, galore modems required a region of respective seconds aft receiving the "AT Z" reset command. Some modems required spaces betwixt commands, while others did not. Some would unhelpfully alteration the baud complaint of their ain volition, which would time off the machine pinch nary hint really to grip the incoming data.

As a consequence of each this, yet galore communications programs had to springiness up immoderate consciousness of being capable to talk to each "Hayes-compatible" modems, and alternatively the programs had to effort to find the modem type from its responses, aliases supply the personification pinch immoderate action whereby they could participate immoderate typical commands it took to coerce their peculiar modem into acting properly.

The Hayes bid group facilitated automatic baud complaint detection arsenic "A" and "T" hap to person spot patterns that are very regular; "A" is "100 0001" and truthful has a 1 spot astatine the commencement and extremity and "T" is "101 0100" which has a shape pinch (nearly) each different spot set.[10] Since the RS-232 interface transmits least important bit first, the according statement shape pinch 8-N-1 (eight information bits, nary parity bit, 1 extremity bit) is 01000001010001010101 (start and extremity bits italicized) which is utilized arsenic syncword.

The pursuing commands are understood by virtually each modems supporting an AT bid set, whether aged aliases new.

Basic Hayes Command Set Command Description Comments

A0 aliases A	Answer incoming call
A/	Repeat past command	Do not preface pinch AT, do not travel pinch carriage return. Enter usually aborts.
D	Dial	Dial the pursuing number and past handshake P – Pulse Dial T – Touch Tone Dial W – Wait for the 2nd dial tone R – Reverse to reply mode aft dialing @ - Wait for up to 30 seconds for 1 aliases much ringbacks , - Pause for the clip specified successful registry S8 (usually 2 seconds) ; – Remain successful bid mode aft dialing. ! – Flash switch-hook (Hang up for a half second, arsenic successful transferring a call.) L – Dial past number
E0 aliases E	No Echo	Will not echo commands to the computer
E1	Echo	Will echo commands to the machine (so 1 tin spot what 1 types if the machine package does not support echo)
H0 aliases H	Hook Status	On hook. Hangs up the phone, ending immoderate telephone successful progress.
H1	Hook status	Off hook. Picks up the telephone statement (typically you'll perceive a dial tone)
I0 to I9	Inquiry, Information, aliases Interrogation	This bid returns accusation astir the model, specified arsenic its firmware aliases marque name. Each number (0 to 9, and sometimes 10 and above) returns 1 statement of modem-specific information, aliases the connection ERROR if the statement is not defined. Today, Windows uses this for Plug-and-play discovery of circumstantial modem types.
L0 aliases Ln (n=1 to 3)	Speaker Loudness. Supported only by immoderate modems pinch speakers. Modems lacking speakers, aliases pinch beingness measurement controls, aliases ones whose sound output is piped done the sound paper will not support this command.	0 turns disconnected speaker, 1 to 3 are for expanding volumes.
M0 aliases M	Speaker Mute, wholly silent during dialing	M3 is besides common, but different connected galore brands
M1		Speaker connected until distant bearer detected (user will perceive dialing and the modem handshake, but erstwhile a afloat relationship is established the speaker is muted)
M2		Speaker ever connected (data sounds are heard aft CONNECT)
O	Return Online	Returns the modem backmost to the normal connected authorities aft being interrupted by the "+++" flight code.
Q0 aliases Q	Quiet Mode	Off – Displays consequence codes, personification sees bid responses (e.g. OK)
Q1	Quiet Mode	On – Result codes are suppressed, personification does not spot responses.
Sn	Select existent register Note that Sn, ? and =r are really 3 abstracted commands, and tin beryllium fixed successful abstracted AT commands.	Select registry n arsenic the existent register
Sn?		Select registry n arsenic the existent register, and query its value. Using ? connected its ain will query whichever registry was astir precocious selected.
Sn=r		Select registry n arsenic the existent register, and shop r successful it. Using =r connected its ain will shop into whichever registry was astir precocious selected.
V0 aliases V	Verbose	Numeric consequence codes
V1		English consequence codes (e.g. CONNECT, BUSY, NO CARRIER etc.)
X0 aliases X	Smartmodem	Hayes Smartmodem 300 compatible consequence codes
X1		Usually adds relationship velocity to basal consequence codes (e.g. CONNECT 1200)
X2		Usually adds dial reside discovery (preventing unsighted dial, and sometimes preventing ATO)
X3		Usually adds engaged awesome detection.
X4		Usually adds some engaged awesome and dial reside detection
Z0 aliases Z	Reset	Reset modem to stored configuration, and usually besides physically power-cycles the modem (during which it is unresponsive). Z0, Z1 etc. are for aggregate stored profiles. &F is akin successful that it returns to mill default settings connected modems without NVRAM (non-volatile memory), but it does not reset the modem

Note: a bid drawstring is terminated pinch a CR (\r) character

Although not portion of the bid set, a tilde characteristic ~ is commonly utilized successful modem bid sequences. The ~ causes galore applications to region sending the bid watercourse to the instrumentality (usually for half a second), e.g., aft a Reset. The ~ is not sent to the modem.[11]

RegisterDescriptionRangeDefault value

S0	Number of rings earlier Auto-Answer	0–255 (0 = never)	0
S1	Ring Counter	0–255 rings	0
S2	Escape character	0–255, ASCII decimal	43 ("+")
S3	Carriage Return Character	0–127, ASCII decimal	13 (Carriage Return)
S4	Line Feed Character	0–127, ASCII decimal	10 (Line Feed)
S5	Backspace Character	0–32, ASCII decimal	8 (Backspace)
S6	Wait Time earlier Blind Dialing	2–255 seconds	2
S7	Wait for Carrier aft Dial	1–255 seconds	50
S8	Pause Time for Comma (Dial Delay)	0–255 seconds	2
S9	Carrier Detect Response Time	1–255 tenths of a second	6 (0.6 second)
S10	Delay betwixt Loss of Carrier and Hang-Up	1–255 tenths of a second	14 (1.4 seconds)
S11	DTMF Tone Duration	50–255 milliseconds	95 milliseconds
S12	Escape Code Guard Time	0–255 fiftieths of a second	50 (1 second)
S18	Test Timer	0–255 seconds	0 seconds
S25	Delay to DTR	0–255 (seconds if synchronous mode, hundredths of a 2nd successful each different modes)	5
S26	RTS to CTS Delay Interval	0–255 hundredths of a second	1 hundredth of a second
S30	Inactivity Disconnect Timer	0–255 tens of seconds	0 (disable)
S37	Desired Telco Line Speed	0–10 Command options: 0 Attempt car mode connection 1 Attempt to link astatine 300 bit/s 2 Attempt to link astatine 300 bit/s 3 Attempt to link astatine 300 bit/s 5 Attempt to link astatine 1200 bit/s 6 Attempt to link astatine 2400 bit/s 7 Attempt to link successful V.23 75/1200 mode. 8 Attempt to link astatine 9600 bit/s 9 Attempt to link astatine 12000 bit/s 10 Attempt to link astatine 14400 bit/s	0
S38	Delay earlier Force Disconnect	0–255 seconds	20 seconds

The ITU-T established a modular successful its V-Series Recommendations, V.25 ter, successful 1995 successful an effort to found a modular for the bid group again. It was renamed V.250 successful 1998 pinch an annex that was not concerning the Hayes bid group renamed arsenic V.251. A V.250 compliant modem implements the A, D, E, H, I, L, M, N, O, P, Q, T, V, X, Z, &C, &D, and &F commands successful the measurement specified by the standard. It must besides instrumentality S registers and must usage registers S0, S3, S4, S5, S6, S7, S8, and S10 for the purposes fixed successful the standard. It besides must instrumentality immoderate bid opening pinch the positive sign, "+" followed by immoderate missive A to Z, only successful accordance pinch ITU recommendations. Modem manufacturers are free to instrumentality different commands and S-registers arsenic they spot fit, and whitethorn adhd options to modular commands.

V.250 – Defined starring characteristic sequences Leading
characters Includes commands related to

+A	Call power (network addressing) issues, common, PSTN, ISDN, ITU-T Rec. X.25, switched digital
+C	Digital Cellular extensions
+D	Data Compression, ITU-T Rec. V.42 bis
+E	Error Control, ITU-T Rec. V.42
+F	Facsimile, ITU-T Rec. T.30, etc.
+G	Generic issues specified arsenic personality and capabilities
+I	DTE-DCE Interface issues, ITU-T Rec. V.24, etc.
+M	Modulation, ITU-T Rec. V.32 bis, etc.
+P	PCM DCE commands, ITU-T Rec. V.92
+S	Switched aliases Simultaneous Data Types
+T	Test issues
+V	Voice extensions
+W	Wireless extensions

The ETSI GSM 07.07 (3GPP TS 27.007) specifies AT-style commands for controlling a GSM telephone aliases modem. The ETSI GSM 07.05 (3GPP TS 27.005) specifies AT-style commands for managing the Short Message Service (SMS) characteristic of GSM.

Examples of GSM commands:[12][13]

CommandDescription

AT+CPIN=1234	Enter PIN code
AT+CPWD="SC","old","new"	Change PIN codification from 'old' to 'new'
AT+CLCK="SC",0,"1234"	Remove PIN code
AT&V	Status
ATI	Status (Manufacturer, Model, Revision, IMEI, capabilities)
AT+COPS=?	List disposable networks 0-Unknown/2-Current/3-Forbidden, Longname, Shortname, Numerical-ID, "AcT"
AT+CSQ	Get awesome strength. Answer: +CSQ: <rssi (more=better)>, <ber, less=better>
ATD*99#	Dial entree point
AT+CGDCONT=1,"IP","access.point.name"	Defines PDP context[12]

GSM/3G modems typically support the ETSI GSM 07.07/3GPP TS 27.007 AT bid group extensions, though really galore commands are implemented varies.

Most USB modem vendors, specified arsenic Huawei, Sierra Wireless, Option, person besides defined proprietary extensions for power mode action (GSM/3G preference) aliases similar. Some caller precocious velocity modems supply a virtual Ethernet interface alternatively of utilizing a Point-to-Point Protocol (PPP) for the information relationship because of capacity reasons (PPP relationship is only utilized betwixt the machine and the modem, not complete network). The setup requires vendor-specific AT bid extensions. Sometimes the specifications for these extensions are openly available, different times the vendor requires an NDA for entree to these.[14]

Modems pinch sound aliases answering-machine capabilities support a superset of these commands to alteration integer audio playback and recording.

• Access Point Name (APN)
• Command and Data modes (modem)
• ITU-T Recommendations:
  • H.324 (video)
  • T.31 (fax)

• List of AT commands: Basic (Hayes), Extended, Proprietary
• Hayes AT Command Reference Manual
• A database of Hayes AT commands Archived 2008-07-10 astatine the Wayback Machine
• 3gpp.org, 3GPP AT bid group for User Equipment
• Modem initialisation string Archived 2009-03-10 astatine the Wayback Machine
• Extended Hayes AT bid parameters for SMS (dead)
• Determining your Class of Fax / Modem
• Openmoko: AT Commands
• Cell modem commands Archived 2007-10-28 astatine the Wayback Machine
• ITU Standard V.250
• AT Commands Reference Guide from Telit (dead)