2.4External Modems

Do they all work under Linux?

At one time (2002 ?) all external modems would work under Linux. But then came the controllerless external modem which wouldn't. If the box says it requires Windows with no mention of Linux it could mean just that. Could it be that Windows software is provided for "modem on hold" and for use as an answering machine, etc., but that otherwise it will work under Linux?

Linux may not support these features very well if at all. If this is a recent version of Modem-HOWTO, let me know of your experience with this.

PnP External Modems

Many external modems are labeled "Plug and Play" (PnP). If they are hardware modems, they should all work as non-PnP modems. While the serial port itself may need to be configured (IRQ number and IO address) unless the default configuration is OK, an external modem uses no such IRQ/IO configuration. You just plug the modem into the serial port.

The PnP modem has a special PnP identification built into it that can be read (thru the serial port) by a PnP operating system. Such an operating system would then know that you have a modem on a certain port and would also know the id number. If it's a controllerless modem, it could try to locate a driver for it. It could also tell application programs what port your modem is on (such as /dev/ttyS2 or COM3). But Linux may not be able to do this. Thus you may need to configure your application program manually by giving it the ttyS number (such as /dev/ttyS2).

Some programs like wvdial can probe for a modem on various ports.

Cabling & Installation

Connecting an external modem is simple compared to connecting most other devices to a serial port that require various types of "null modem" cables (which will not work for modems). Modems use a straight through cable, with no pins crossed over. Most computer stores should have one. Make sure you get the correct gender and number of pins. Hook up your modem to one of your serial ports. If you are willing to accept the default IRQ and IO address of the port you connect it to, then you are ready to start your communication program and configure the modem itself.

What the Lights (LED's) Mean (for some external modems)

• TM Test Modem
• AA Auto Answer (If on, your modem will answer an incoming call)
• RD Receive Data line = RxD
• SD Send Data line = TxD
• TR data Terminal Ready = DTR (set by your PC)
• RI Ring Indicator (If on, someone is "ringing" your modem)
• OH Off Hook (If off, your modem has hung up the phone line)
• MR Modem Ready = DSR ??
• EC Error Correction
• DC Data Compression
• HS High Speed (for this modem)

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