i have FC 5 installed on my P-4 and now i have purchased a US-Robotics external modem. how do i configure it on my system

What do you want to do with your modem? The range of possibilities is quite large. On the simple end, you can use a communications program such as minicom or C-Kermit (my preference) to dial out to another host, and establish a login, probably with a text-based interface. I recommend this approach as a first step anyway, just to make sure your cabling and serial port device identities are established. You probably want to use /dev/ttyS0 as your serial port.
The next step might be to set up the modem as a PPP client interface. This would allow you to auto-dial a modem-connected ISP on demand. In FC5, use system-config-network to do this.
You can also configure the modem to auto-answer and support logins from remote hosts. I think system-config-network can set this up as well.
If you want to use any built-in fax capabilities, perhaps someone else can offer some guidance.
--- rod.

is there any other option for serial ports then /dev/ttyS0? what if i have a USB modem.
can u plz send me some tutorial on the whole process. i will be greatly obliged.

Sorry, never dealt with USB modems. Did your modem come with a device driver for linux? If not, I'm not sure you will be able to use it at all. If it did, then it should provide some device file '/dev/????' that you can use just like a regular ttySx device.
--- rod.

Try Googling for scanModem. Download and follow instructions for installation. It will identify the modem and tell you what drivers you need. If your modem does not work after the drivers are installed, it will give you hints as to what steps you need to take to get it running.

thanx, both of u fellows.theNbomr,GoinEasy9

Hi again fellows,
this time a installed FC 5 from scratch, with my USR 5631 plug and play serial port modem, but its not detected yet. do i have to use scanModem?

I don't really know what scanModem is/does, but you should be able to use pretty much any RS-232 modem without it. Try running minicom, which comes with FC5. See if the LEDs blink when you connect to the modem port and press keys. Try using a simple dial command:
Things to check if it isn't working: cable, is it connected and is it the right type of cable. For serial modems a straight cable with all DTE (computer) signals directly connected to the corresponding DCE (modem) signals. If both ports use 25-pin connectors, this will simply be a 1<->1, 2<->2...25<->25 cable. If one is a 9-pin, then most conductors connect to different pins at each end. Also, make sure that the cable connects the modem to the same serial port that the software is trying to use. For most people these days, that will be /dev/ttyS0. Serial mouse users will have to figure out what serial port is free. Use a low bitrate, 9600 or less, if you can't see the LEDs blinking. High bitrates might make the blinks too short to see.

--- rod.