Cable modems and IRQs

Cable modems and IRQs

By Richard Heller

Cable modems and IRQs

A client who has had a cable modem installed recently contacted me. Instead of the (up to) 100 times faster speed of a regular dial-up Internet connection, the computer and the cable modem were locking up.

The client made the comment that he had experienced a similar problem when he was an AOL customer; the computer would lock up when he was online, and that is why he had switched over to cable. One of the things I discovered on his system was that the modem was not using its own interrupt, that it was sharing one with the universal serial device (USB) controller in the computer.

The computer has 16 of these interrupts or IRQs that enable such things as your floppy drive, hard drive and video to work together. A problem can develop when you install modems and sound cards in your computer: You run out of these IRQs. Windows is able to get around this problem by allowing some devices to share an IRQ. Your sound card and printer can usually get along with this arrangement your modem and mouse can’t.

If the modem tries to share an IRQ with the mouse, the modem will not work every time you move the mouse, a signal is sent through the computer which confuses the modem. The USB controller was causing the same problem with the modem in this computer. The solution was simple; just move the modem to another slot in the computer that let it use a different IRQ, and the problem disappeared.

The problem with the cable modem was a little more involved. The computer would freeze after being on the Internet after a short period of time. Trying to download anything but small files caused the freeze, as did going from web site to web site. We contacted @home, and they downloaded updated drivers to the cable modem. After the update was done, we again went online; the computer was still acting the same way.

The cable had been connected to the computer using a USB Ethernet network adapter. Remember, the modem and the USB controller were using the same IRQ until the modem was moved to a different slot. When you are using a cable modem, you are actually on a computer network, and your cable modem connects to your computer through a network card or adapter. The problem appeared to be that the adapter being used was defective; it would somehow lose connection with the modem.

Trying to find updated drivers for the adapter yielded no results. Reading the instruction manual for the modem didn’t offer any clues. The @ home tech did not know what to tell us. Finally, I disconnected the adapter from the computer and removed the driver from the control panel. I then reconnected the adapter and reinstalled the driver.

After this was done, we went back online. We were downloading files, going from web site to web site for over an hour without lock-ups. When the driver was re-installed, it determined the new settings and corrected the problem. All of this because of a modem installed incorrectly when the computer was made.

Richard Heller is an independent computer specialist who specializes in repairs, installation, upgrades, technical support, Internet sharing, data recovery and diagnostics. If you have any computer or service-related questions, please send them to The Rock River Times or e-mail

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