Linux and the 3Com/USR 5687-03 Modem

September 1, 1999
Tux the Penguin
So you have purchased or are thinking of purchasing the 3Com/U.S. Robotics 5687-03. Here are some things you would probably like to know:

Q1: Does the 5687-03 modem work with Linux?

Yes. The 5687-03 modem works perfectly with Linux. But, you do have to expend a little energy to configure it (but don't worry, it's easy).
Q2: Why did you write this document?
I was the first person to get the 5687-03 modem to work with Linux and report it to the Linux/Modem Compatibility Knowledge Base. As a result, I get lots of email about this modem, since it appears to be a rather popular model right now, particularly at CompUSA. I figured it would be a good idea to write this document to cover the main points of installation.
Q3: How do I install the 3Com/USR 5687-03 modem?
When you take the 5687-03 modem out of the box, it is in Plug-n-Play mode. Linux doesn't work well with Plug-n-Play mode. You need to configure the modem to not be in Plug-n-Play mode. The modem must be configured to use a certain COM port and IRQ. Put very simply, Linux talks to the modem through the COM port and the modem talks to Linux through the IRQ.

Down at the bottom of the modem, near the connectors, there is a bank of jumpers. When you take the modem out of the box, they look like this:

                ||| .|...
                ... .....

This is Plug-n-Play mode. There is a booklet that comes with your modem that illustrates all of the valid settings for these jumpers. Try setting the modem to COM1 (and the proper corresponding IRQ) by configuring the jumpers like this:

                ||| ::|::

Then, install the modem in the motherboard, turn on the computer and boot up Linux. Most Linux kernels already have support for serial devices (a modem is a serial device) compiled in, so you don't have to worry about setting up the kernel.

After you boot up the kernel, test out the modem. There are a number of ways to do this, and my favorite is the program minicom. Start up minicom, make sure that it's set to the proper COM port, and type atz and enter. 'atz' is the standard modem reset command. The modem should respond with 'OK'. If it does, then you're all set! Congratulations!

Q4: The modem doesn't appear to work; what now?
Don't despair; I've seen people come across several problems while configuring this modem, but they've all eventually succeeded in making the modem work.

Try a different COM port: Sometimes, another serial device (like a mouse) is already using COM1. Shut down the computer, adjust the jumpers to COM2 and test it again.

Make sure the motherboard isn't hogging the COM ports: I'll never forget the day my family got a brand new 486SX/33MHz PC and I tried transplanting my brand new 14,400bps modem from the old 286. It didn't work. Eventually, I figured out that the motherboard was hogging both of the COM ports and I had to set a certain jumper to make it give up one of the COM ports. At least one person I've helped to set up the 5687-03 had the same problem with their motherboard. So read the motherboard documentation to make sure you don't have a similar problem.

Q5: The modem works, so now what? How do I get on the internet?
This really isn't my department, but I can point you in the right direction. To use your PC and modem to get on the internet, you need to set up point-to-point protocol (PPP) networking. Here is the PPP-HOWTO.
Q6: Would you recommend this modem?
I'm no expert on modems, but I also haven't had any trouble whatsoever with this modem. I took it out of the box, read the accompanying booklet, configured the jumpers, installed it, and turned on the computer. Since I was upgrading from another modem I gave away, Linux was already configured to use PPP so I just had to run my dialup script. In summary, I think this is a really good modem.

In case you want the specs, the modem is a 56K, V.90, 16-bit ISA modem. At the time of this writing (mid-1999), it's a great modem but I suspect the demand for the information in this document will die down in a few months as the next big bandwidth thing comes along. In the meantime, I hope this document answers some questions.

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