Before spending a lot of time configuring your modem, you need to make sure it can be found and that AT commands and the like can be sent to it. So I suggest you first give it a very simple configuration using the communication program you will be using on the port and see it it works. If so, then it's been found. If not then see My Modem is Physically There but Can't be Found. A winmodem may be hard to find and will not work under Linux.
While the serial port on which a modem resides requires configuring, so does the modem itself. The modem is configured by sending AT commands (or the like) to it on the same serial line that is used to send data.
Most modems use an AT command set. These are cryptic and short ASCII commands where all command strings are prefaced by the letters AT. For example: ATZ&K3 There are two commands here Z and &K3. Unfortunately there are many different variations of the AT command set so that what works for one modem may or may not work for another modem. Thus there is no guarantee that the AT commands given in this section will work on your modem. Another point is that to get the modem to act on the AT command string, a return character must be sent at the end of the string.
Such command strings are either automatically sent to the modem by communication programs or are sent directly by you. Most communication programs provide a screen where you can type commands directly to your modem. This is good for setting up the modem as you can have it remember how it was set even after its powered off.
If you have a manual for your modem you can likely look up the AT command set in it. Otherwise, you may try to find it on the Internet. One may use a search engine and include some actual commands in the search terms to avoid finding sites that just talk about such commands but fail to list them. You might also try a few of the sites listed in the subsection Web Sites
The examples given in this subsection are from the Hayes AT modem command set. All command strings must be prefaced by the two letters AT (for example: AT&C1&D3 ). When a modem is powered on, it automatically configures itself with one of the configurations it has stored in its non-volatile memory. If this configuration is satisfactory there is nothing further to do.
If it's not satisfactory, then one may either alter the stored configuration or configure the modem each time you use it by sending it a string of commands known as an "init string" (= initialization string). Normally a a communication program does this. What it sends will depend on how you configured the communications program or what script you wrote for it if you use Kermit. You can usually edit the init string your communication program uses and change it to whatever you want. Sometimes the communications program will let you select the model of your modem and then it will use an init string that it thinks is best for that modem.
The configuration of the modem uses when it's first powered on could be expressed by an init string. You might think of this as the default "string" (called a profile). If your communications program sends the modem another string (the init string), then this string will modify the default configuration. For example, if the init string only contains two commands, then only those two items will be changed. However, some commands will recall a stored profile from inside the modem so a single such command in the init string can thereby change everything in the configuration.
Modern modems have a few different stored profiles to choose from that are stored in the modem's non-volatile memory (it's still there when you turn it off). In my modem there are two factory profiles (0 and 1, neither of which you can change) and two user defined profiles (0 and 1) that the user may set and store. Your modem may have more. Which one of these user-defined profiles is used at power-up depends on another item stored in the profile. If the command &Y0 is given then in the future profile 0 will be used at power-on. If it's a 1 instead of a 0 then profile 1 will be used at power-on.
There are also commands to recall (use it now) any of the 4 stored profiles. One may put such a command in an init string. Of course if it recalls the same profile as was automatically loaded at power-up, nothing is changed unless the active profile has been modified since power-up. Since it could have been modified It's a good idea to use some kind of an init string even if it does nothing more than recalling a stored profile.
Recalling a saved profile (use 1 instead of 0 for profile 1):
Z0 recalls user-defined profile 0 and resets (hangs up, etc.)
&F0 recalls factory profile 0
Once you have sent commands to the modem to configure it the way you
want (including recalling a factory profile and modifying it a little)
you may save this as a user-defined profile:
&W0 saves the current configuration to user-profile 0
Many people don't bother saving a good configuration in their modem, but instead, send the modem a longer init string each time the modem is used. Another method is to restore the factory default at the start of the init string and then modify it a little by adding a few other commands to the end of the init string. By doing it this way no one can cause problems by modifying the user-defined profile which is loaded at power-on.
You may pick an init string supplied by someone else that they think is right for your modem. Some communication programs have a library of init strings to select from. The most difficult method (and one which will teach you the most about modems) is to study the modem manual and write one yourself. You could save this configuration inside the modem so that you don't need an init string. A third alternative is to start with an init string someone else wrote, but modify it to suit your purposes.
Now if you look at init strings used by communication programs you may see symbols which are not valid modem commands. These symbols are commands to the communication program itself (such as meaning to pause briefly) and will not be sent to the modem.
Future editions of Modem-HOWTO may contain more AT commands but the rest of this section is mostly what was in the old Serial-HOWTO. All strings must start with AT. Here's a few Hayes AT codes that should be in the string (if they are not set by using a factory default or by a saved configuration).
E1 command echo ON
Q0 result codes are reported
V1 verbose ON
S0=0 never answer (uugetty does this with the WAITFOR option)
Here's some more codes concerning modem control lines DCD and DSR:
&C1 DCD is on after connect only
&S0 DSR is always on
If your modem does not support a stored profile, you can set these
through the INIT string in a config file (or the like). Some
older modems come with DIP switches that affect register settings. Be
sure these are set correctly, too.
Greg Hankins has a collection of modem setups for different types
of modems. If you would like to send him your working configuration,
please do so:
mailto:gregh@cc.gatech.edu You can
get these setups at
ftp://ftp.cc.gatech.edu/pub/people/gregh/modem-configs.
Note: to get his USR Courier V.34 modem to reset correctly when
DTR drops, Greg Hankins had to set &D2 and S13=1 (this
sets bit 0 of register S13). This has been confirmed to work on USR
Sportster V.34 modems as well.
Note: some Supra modems treat DCD differently than other modems.
If you are using a Supra, try setting &C0 and not
&C1. You must also set &D2 to handle DTR correctly.