Hayes Protocol

Hayes Protocol

An intelligent modem or terminal adaptor can be controlled using special commands sent from the computer. Most devices recognise a standard set of commands known as the Hayes protocol, although this comes in numerous variations.

Most of the time your computer software and modem uses these codes without you knowing about it. However, in a communications application you can send commands to your modem manually. Each command string sent by the computer begins with AT so as to bring the modem to attention.

Dialling a Number

Here’s an example instruction that tells the modem to dial a number:-

ATDT*70,777-2424

These instructions are explained as follows:-

AT	Attention
D	Dial a number…
T	… using tone dialling
*70	Dials *, 7 and 0 to disable call waiting
,	Pauses for two seconds
777-2424	Dials number 777-2424

in which the letter T could be replaced by a P for pulse dialling.

Here’s another example:-

ATDT9,1(323)444-7487

which breaks up into:-

AT	Attention
D	Dial a number…
T	… using tone dialling
9	Dials 9 for outside line from office exchange
,	Pauses for two seconds
1(323)4…	Dials number 13234447487, ignoring other characters

Special Dialling Codes

Some telephone systems employ call waiting, a mechanism that alerts you to another incoming call by generating a bleep in the background. This is a potential nuisance if it happens whilst you’re using a modem. Depending on your country of origin, you can disable call waiting by using one of the following dialling sequences:-

ATDT*70nnnnn

ATDT*70,,nnnnn

ATDP1170nnnnn

ATDP1170,,nnnnn

where nnnnn represents the number you’re dialling. The commas can be added if your phone system needs pauses in the dialling sequence. The letter Wshould be added at the end of a sequence if you need to persuade your modem to wait for dialling tone before actually dialling the number.

The following information applies to the United Kingdom:-

• Dialling ATDT#43#nnnnn, where nnnnn is the phone number, disables call waiting. You can enable it again by dialling *43#. To find out if it’s enabled you can dial *#43#.
• Dialling *40*nnnnn#, where nnnnn is the phone number, makes a call as normal. However, the exchange rings you back afterwards, giving information about the time taken and the cost.
• Dialling 1470,nnnnn#, where nnnnn is the phone number, ensures caller ID information is sent. This is necessary if your number is normally withheld but you use an ISP that provides a free service financed from call charges. Such ISPs usually have a number starting with 0845.
• Dialling 1471 contacts whoever last rang you. Dialling 1571 gives you access to BT Answer, a free ‘answering machine’ service that stores up to ten messages for up to twenty days.

Initialisation and Other Strings

A command string that sets up a modem for particular software is an initialisation string, whilst a string that temporarily modifies the settings for a particular call is a pre-dial initialisation string or pre-dial init. Finally, a deinitialiastion string is used by an application to restore those settings in a modem that existed before it received an initialisation string.

Standard Commands

Every Hayes protocol instruction consists of AT followed by a succession of letters and numbers, all of which should be understood by the modem. For example, sending the following message:

ATA

and then pressing the Return key, instructs the modem to answer an incoming phone call.

There are many different commands, often varying with the type of modem. The following common commands must be preceded by AT and followed by Return to make them work:-

Code	Command
A	Answer incoming call
C0	Disable data compression
C1	Enable data compression (default)
DPnnnnn	Dial number nnnnn using pulse dialling
DTnnnnn	Dial number nnnnn using tone dialling
E0	Echo off (default)
E1	Echo on (for person-to-person link only)
H	Hang up on caller
L0	Modem speaker at lowest volume
L1	Modem speaker at low volume
L2	Modem speaker at medium volume
L3	Modem speaker at high volume
M0	Modem speaker off at all times
M1	Modem speaker on whilst dialling
M2	Modem speaker on at all times
O	Switch from command mode to normal operation
Q0	Enable user's on-screen messages
S0 = n	Answer after n rings (if n = 0 you must use ATA command)
S6 = n	Wait n seconds for dial tone (default = 2)
S7 = n	Wait n seconds for other modem’s carrier tone (default = 30)
S8 = n	Pause n seconds for ‘,’ in dial string (default = 2)
S11 = n	Pause n milliseconds between each touch-tone dialling burst
V1	Verbose messages (textual on-screen messages, not numbers)
W	Wait for dialling tone
X3	Dial without confirming dial tone exists
X4	Dial after confirming dial tone (‘CONNECT’ gives DTE speed)
Z	Reset modem to default settings
&A3	Enable result codes for compression and protocol changes
&B1	Look serial port speed to fixed rate (default)
&C1	Use serial port CTS signal for flow control
&D0	Ignore serial port DTR signal (default)
&D1	Use serial port DTR signal
&D2	Use serial port DTR signal to return to command mode
&D3	Use serial port DTR signal to reset modem
&F	Reset modem to factory settings
&H1	Enable serial port hardware flow control
&R2	Use serial port RTS signal for flow control
&W	Save current settings as default

As you can see, the commands come in two broad groups. Firstly, there are those that contain a letter, such as M0 or H, often followed by a number. Within this group there are special commands that start with S followed by = and another number. Typically, the value of the number n ranges from 0to 27, although higher values are often used. Secondly, there are commands that begin with an &, followed by a letter, again often followed by a number.

A typical initialisation string contains a combination of the above letters and numbers in any order, although some modems may use numbers that aren’t mentioned above. Here’s a typical string:-

ATE0Q0V1X4&C1&D0M0

This makes slightly more sense when divided up as follows:-

AT E0 Q0 V1 X4 &C1 &D0 M0

Here are some other strings for you to consider:-

ATE1V1C0

AT&FE0&B1&H1&R1X4&A3S0=04S7=60&D2

AT&FE0W1Q0V1X4&C1&K3S95=1&D3S7=75S0=0

Your modem should also accept a pre-dial init that temporarily sets it up for a particular service. This kind of string ends with a & and can look something like this:-

AT&F1S0=0E1M1V1X3W2&

In this example the code X3 tells the modem to disregard dialling tone. This is necessary in countries such as Italy where the phone system doesn’t always provide such a tone.

Some modems accept commands for special services, such as this ‘Page me’ init string:-

AT&F0S37=5S4

Finally, if you really want to know the current settings of your modem you can enter:-

AT&V

A really horrible list of information then appears in the window of your communications application.

Special Commands

When working online you can send AT commands to the modem by switching it into command mode. Just type +++, enter your commands and then return to online mode or normal mode by entering ATO. Similarly, if you want to make the modem hang up you can type +++ followed by ATH.

In addition, some types of modem accept really strange commands, such as:-

AT%C0

which turns off modem compression.

Those of a technical disposition may want to send control codes to the modem, as in:-

ATE1V1^M

where ^M represents Control-M, which is actually a standard CR (carriage return). The common codes are as follows:-

Code	Effect
^C	Aborts current operation (Interrupt, also End of Text or ETX)
^H	Deletes last typed character (Backspace or BS)
^I	Completes file and folder names (Tab)
^O	Stops communication (Abort output)
^Q	Continues communication (XON)
^S	Pauses communication (XOFF)
^M	Terminates entry and starts execution (Carriage Return or CR)
^Z	Marks End of file for text file (EOF)

Messages

Most modems can send messages to the user or to the software that’s controlling the modem. In most instances the modem will be set up to createverbose messages that actually mean something in English, including phrases such as RING, NO DIALTONE, NO CARRIER, ERROR, BUSY, NO ANSWER,CONNECT, CARRIER, PROTOCOL: LAP, PROTOCOL: MNP, PROTOCOL: ALT, COMPRESSION: V, COMPRESSION: MNP5, COMPRESSION, DELAYED, REL, ARQ, COMP,V42, 42BIS, V42bis, MNP, MNP5 or any speed rate expressed in bit/s. With most modern software you don’t actually see these messages but they’re acted on by the application that’s in use. In order to force your modem to send verbose messages you should send it a V1 command.

Modem Scripts in the Classic Mac OS

A modem script, also known as Connection Control Language (CCL) script, can be used to automate the setup and control of a particular type of modem or terminal adaptor. In the Classic Mac OS, each script file must go in the Modem Scripts folder, inside the Extensions folder in the System Folder. It then appears in the menus inside the Remote Access or Modem control panels and also in any application that uses Apple’s standardCommunications Toolbox (CTB). Such a file will also appear in the PPP control panel used in older versions of the Classic Mac OS. Note that some older applications may prefer to store these scripts as loose items in the Extensions folder.

In most instances you’ll find that a modem script for your modem has already been provided along with the system. If not, you should be able to get one from the supplier of your modem, assuming they support the Mac OS. Failing this, you may have to create a modem script of your own. To do this you can use Apple’s Modem Script Generator application.

You can also modify a script using a suitable text editor, although to be on the safe side you should work on a copy of the original file. Unfortunately, some editors can’t open modem script files, since they have a type code of mlts. To fix this problem you can use a file utility such as FileTyper to change the code to TEXT. Having done this, you should be able to view the contents in your editor. To modify the modem initialisation string you should proceed as follows:-

1. Locate the line of text that reads @LABEL 3.
2. Find the line beneath the above that begins with AT, ignoring all of the ‘comments’ lines that begin with !. You should see something like this:
     AT&F1&D0&H1&R1&B1S0=0EOV1Q0&A3S7=60L3\13
   Now, if, for example, you want to set your modem’s volume to the lowest setting you should replace the L3 near the end of the string by L0.
3. Having made any necessary changes you can save the file. If you’ve changed the file’s type code you’ll need to use the file utility one more time to change it back again.

Distinctive Ring Service

Some phone companies provide a Distinctive Ring Service (DSR), where different phone numbers operate over a single phone line but with a different ring. Mac OS X 10.3, when used with a compatible modem, can identify such rings and use them, for example, to identify all fax calls. This behaviour is set by the fax configuration script at /usr/bin/fax, which you can edit in the Terminal application by means of pico or a similar text editor. First, enter the following:-

sudo pico /usr/bin/fax

You should then locate the following line (usually line 209):-

INIT="-iZ -i&FE0&D2S7=120 -i&C0"

which contains a normal modem initialisation string. You should change this to

INIT="-iZ -i&FE0&D2S7=120 -i&C0-SDR=n"

where n must be replaced by a number, usually 1 for a normal ring, 2 for the first distinctive ring, and so on.