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1. Introduction

1.1 DSL, Cable, and ISDN Modems in other HOWTOs

This HOWTO covers conventional analog modems for PCs on the PCI, USB, LPC, and ISA buses. USB and ISDN coverage is weak. For other types of modems see:

1.2 Also not well covered: PCMCIA Modems, PPP

For modems on the PCMCIA bus see the PCMCIA-HOWTO: PCMCIA serial and modem devices. This HOWTO also doesn't cover the details of PPP (used to connect to the Internet via a modem) or communication programs. If you want to use a modem to connect to the Internet then you need to use a program that will automatically set up PPP for you (such as wvdial). More documentation on ppp should be found in /usr/doc/ppp, /usr/share/doc/ppp or the like.

1.3 Copyright, Disclaimer, Trademarks, & Credits


Copyright (c) 1998-2005 by David S. Lawyer

Please freely copy and distribute (sell or give away) this document in any format. Send any corrections and comments to the document maintainer. You may create a derivative work and distribute it provided that you:

  1. If it's not a translation: Email a copy of your derivative work (in a format LDP accepts) to the author(s) and maintainer (could be the same person). If you don't get a response then email the LDP (Linux Documentation Project):
  2. License the derivative work in the spirit of this license or use GPL. Include a copyright notice and at least a pointer to the license used.
  3. Give due credit to previous authors and major contributors.

If you're considering making a derived work other than a translation, it's requested that you discuss your plans with the current maintainer.


While I haven't intentionally tried to mislead you, there are likely a number of errors in this document. Please let me know about them. Since this is free documentation, it should be obvious that I cannot be held legally responsible for any errors.


Any brand names (starts with a capital letter such as MS Windows) should be assumed to be a trademark). Such trademarks belong to their respective owners.

"Hayes" is a trademark of Microcomputer Products Inc. I use "winmodem" to mean any modem which originally required MS-Windows and not in the trademark sense. All other trademarks belong to their respective owners.


The following is only a rough approximation of how this this document was created in the year 2000: About 1/4 of the material here was lifted directly from Serial-HOWTO v. 1.11 (1997) by Greg Hankins. (with his permission). About another 1/4 was taken from that Serial-HOWTO and revised. The remaining 1/2 is newly created by the new author: David S. Lawyer Since 2000 much more has been added by the current author so that little remains of the modem coverage in the old Serial-HOWTO.

1.4 Contacting the Author

Since I don't follow the many different brands/models of modems please don't email me with questions about them (or suggestions of which one to buy). If you are interested in a certain model (to find out if it works under Linux, etc.) see the huge list at Web Sites. Also, please don't ask me how to configure a modem unless you've looked over this HOWTO and still can't do it. I've no personal experience with software-based modems.

Please let me know of any errors in facts, opinions, logic, spelling, grammar, clarity, links, etc. But first, if the date is over a month or two old, check to see that you have the latest version. Please send me any other info that you think belongs in this document.

1.5 New Versions of this HOWTO

New versions of this Modem-HOWTO should come out every few months. Your problem might be solved in the latest version. It will be available to browse and/or download at LDP mirror sites. For a list of such sites see: If you only want to quickly compare the date of this the version v0.38, June 2005 with the date of the latest version go to:

1.6 New in Recent Versions

For a full revision history going back to the first version see the source file (in linuxdoc format) at

1.7 What is a Modem ?

A modem (or analog modem) is a device that lets one send digital signals over an ordinary telephone line not designed for digital signals. If telephone lines were all digital then you wouldn't need a modem. But sometimes, a substitute for an analog modem, connected to a digital phone line, is imprecisely called a "digital modem". A modem permits your computer to connect to and communicate with the rest of the world. When you use a modem, you normally use a communication program or web browser to utilize the modem and dial-out on a telephone line. Advanced modem users can set things up so that others may phone in to them and use the computer remotely. This is called "dial-in".

Oversimplified, there are four basic types of analog modems for a PC: external serial (RS-232), USB (= external USB), internal, and built-in. The external serial and USB set on your desk outside the PC while the other two types are not visible since they're inside the PC. The external serial modem plugs into a connector on the back of the PC known as a "serial port". The USB modem plugs into a USB cable. See USB Modems. The internal modem is a card that is inserted inside the computer. The built-in modem is a chip on the motherboard used primarily in laptops. What is said in this HOWTO regarding internal modems will generally apply also to built-in modems. Internal modems are further subdivided into PCI, ISA, and AMR, depending on whether they are designed for the PCI or ISA bus, or for an AMR slot.

For an external vs internal comparison see External vs. Internal. When you get an internal or built-in modem, you also get a dedicated serial port (which can only be used with the modem and not with anything else such as an external modem or console terminal). In Linux, the common serial ports are named ttyS0, ttyS1, etc. (or tts/0, tts/1 for the device file system (devfs). These ports usually corresponding respectively to COM1, COM2, etc. in Dos/Windows). But in special cases, the names are longer such as: ttySHCF0 is the 0th serial port for a type of winmodem (HCF = Host Controlled Family). New types of serial ports just add some more letter to ttyS.

See Modem & Serial Port Basics for more details on how modems and serial ports work. With a USB modem, the driver simulates a serial port at for example /dev/ttySHCFUSB or /dev/usb/asm/0 (for devfs).

Modems usually include the ability to send Faxes (Fax Modems). See Fax for a list of fax software. "Voice" modems can work like an automatic answering machine and handle voicemail. See Voicemail Software.

The v.92 protocol can put the modem "on hold" when someone makes an ordinary voice call to your telephone, provided that you have "call waiting" from your telephone company. Thus you can get a phone call while online. As of Jan. 2003 Linux doesn't seem to support it. If this is the latest version of this HOWTO, let me know about any Linux support for it. Some linmodem drivers may support it (but what if you have a hardware modem that doesn't use any linmodem driver?).

1.8 Does My Computer Contain an Internal Modem ?

Internal modems usually have a pair of modular telephone jacks on the back of the computer. They should be right next to each other and each one looks like a jack on the interior wall of a building where a telephone plugs in. One of the pair should be labeled "line" (or the like) which is where you plug in the telephone line.

Network cards also have modular jacks, but they are seldom in pairs and are slightly wider since they normally have 8 pins. Internal DSL "modems" exist and also have modular telephone jacks, but I think they are not very common (most DSL modems are external) as of 2002.

1.9 Quick Install

Very Quick Install

If you think your modem will work under Linux and needs no special driver, then just physically install/connect it. Start you computer, watch the boot-time messages for Linux to find the modem. Note it's the serial port number such as ttyS2 (/dev/ttyS2). Connect a phone line to it and dial out with say wvdial (after configuring wvdial). If the above doesn't work, read on.

Will my modem work under Linux?

So called "winmodems" will work under Linux only if a driver for it exists and gets installed. In this case it's called a "linmodem" since it can be made to work under Linux. See modem list and Software-based Modems (winmodems). There's no point of installing a modem that will not work with Linux.

External Serial Modem Install

At one time (2002 ?) no external serial modem was a winmodem but that's no longer the case. With a straight-thru or modem cable, connect the modem to an unused serial port on the PC. Make sure you know the name of the serial port: in most cases COM1 is ttyS0, COM2 is ttyS1, etc. You may need to check the BIOS setup menu to determine this. Plug in the power cord to provide power to the modem. See All Modems for further instructions.

Internal Modems (ISA, PCI and AMR)

If the modem is both PnP and directly supported by the serial driver (kernel 2.4 +) or by a winmodem driver that you've installed, then there is no configuring for you to do since the driver should configure it.

To physically install a modem card, remove the cover of the PC by /removing some screws. Find a matching vacant slot for the card next to the other adapter cards. Before inserting the card in the slot, remove a small cover plate on the back of the PC so that the telephone jacks on the card will be accessible from the rear of the PC. Then carefully align the card with the slot and push the card all the way down into the slot. Attach the card with a mounting screw (usually 3mm, .5mm pitch --don't use the wrong size).

You may watch the boot-time messages to see if your modem is detected. Use "dmesg" to see them or shift-page-up to scroll the screen back after they have flashed by.

Internal Modems: Manual configuration

Normally, you don't need to do this manual configuration since the modem's serial port may be detected and assigned a port at boot-time. For example: ttyS14 at I/O 0x6450 (IRQ = 10). Otherwise (or if there is some special reason to change the configuration) then you need to configure it yourself (or perhaps update your kernel to increases the likelihood that the modem gets detected). If your modem has no ttyS number assigned to it, it can't be used until it gets a ttyS number (like ttyS10). It thus can't be detected by application programs such as dialers or minicom. But it might be found by using say "lspci -v" if it's on the PCI bus.

Finding a lost modem may not be easy and you may need to read a lot more of this HOWTO. Once found, you need to use the "setserial" program to manually assign it to an available ttyS? port of your choice . For this you need to know both it's IO address (such as 0x6450) and its IRQ (such as 10). In the worst case, the modem has been disabled by a failing to be detected and enabled by the BIOS (or Linux) and doesn't have any IO address nor IRQ number. But you may still be able to find it. Older modems could be disabled by a jumper on the card or in rare cases by MS software.

You may have some choice of IRQs and IO addresses (including the case where you are able to change what the BIOS has set). See Choosing Serial IRQs and Choosing Addresses.

Old ISA Modems

ISA modems normally use ttyS0 - ttyS3. For old modems with jumpers look at the modem manual or look for printing on the modem card that tells you what the jumpers do. They have standard IO addresses corresponding to the ttySx. For example you may find it feasible to use /dev/ttyS2 at IO address 0x3e8 and IRQ 11.

If it has no jumpers then it's likely a Plug-and-Play modem which the BIOS may configure when you power one your PC. Typing "pnpdump --dumpregs" should find it. If you need to set or change them use "isapnp". Use the "pnpdump" program to see what changes are possible.

Both PCI and ISA: Use setserial to tell the serial driver

You must find the file where "setserial" is run at boot-time and add a line something like: "setserial /dev/ttyS2 irq 5 port 0x0b8". For setserial v2.15 and later the results of running "setserial" on the command line may (or may not) be saved to file named serial.conf or autoserial.conf. It might be in say the /etc directory or in the /var/lib/setserial directory (use "locate to find it). it runs each time you boot. See What is Setserial for more info. See the next subsection All Modems for further instructions on quick installation.

Use MS Windows to set the BIOS (A last resort method)

If you are using the BIOS to configure you may attempt to use MS Windows9x to "force" the BIOS to set a certain IRQ and/or IO. It can set them into the PnP BIOS's flash memory where they will be used to configure for Linux as well as Windows. See "Plug-and-Play-HOWTO and search for "forced" (occurs in several places). For Windows3.x you can do the same thing using the ICU under Windows 3.x. A few modems have a way to disable PnP in the modem hardware using software (under Windows) that came with the modem.

All Modems

Plug the modem into a telephone line. Then configure a dialing program. If you have an Internet Service Provider (ISP) you might configure one of these : wvdial, pppconfig, gnome-ppp, modem lights (Gnome) or kppp. They not only dial out but start PPP, which you need to connect to the Internet. Otherwise, you might try configuring the minicom dialer which isn't designed for connecting to the Internet, but is good for testing. Whether it's minicom or a dialer that starts PPP, set the serial port speed to a baud rate a few times higher than the bit rate of your modem. See Speed Table for more details on the "best" speeds to use. Tell it the full name of your serial port such as /dev/ttyS1 (or /dev/ttys/1).

Minicom is one way to set up and test your modem. Set hardware flow control (RTS/CTS). With minicom you may check to see if your modem is there (and ready to dial). Once you've set up minicom, type the command: AT, hit enter and you should see an "OK" response which comes directly from the modem. See Dialing Out with Minicom.

1.10 dev/modem

If your modem is on say /dev/ttyS2, you may want to link that to /dev/modem. It's not really necessary to do this since you can write down (or remember) say ttyS2 and tell it to programs that use the modem. It may be simpler to just link it. To link it, type say ln -s /dev/ttyS2 /dev/modem . Note "ttyS2" is just for example. It might actually be ttyS14, etc. Or use Red Hat's modemtool (or the like) to link it. But once you link it, be sure that all programs that use the modem use /dev/modem and not /dev/ttyS2, otherwise two programs may try to use the modem at the same time without knowing they are doing this. System software was written around 2000 to fix this problem but it may not be in recent kernels (like 2.6).

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