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Home Troubleshooting PCI Modem Installation & Troubles

PCI Modem Installation & Troubles (Updated 26-May-02)

Warning! Save time and aggravation: PCI Modems must be 'enumerated' and driver correctly installed to define the COM port for the modem. With Windows, this is supposed to be automatic, plug and play - but it doesn't always work that way. IF Windows thinks a driver is correctly installed, even if it is not, it may become almost impossible to load a good modem driver (Win 98,Me,2K and XP). See this page for some real-world PCI hassles involving Conexant Soft56/HSF modem. Many PCI modems can not be successfully installed using Windows Add New Hardware or Add Modem in Control Panel.

The majority of internal modems sold today are PCI cards as opposed to the older ISA (industry standard architecture). In fact, many new PCs sold today come only with PCI expansion slots - there are no ISA slots at all.

In addition to providing a more efficient interface, the PCI bus overcomes one of the old DOS/original Intel x86 architecture: the limited number of IRQs (interrupts). The PCI bus allows IRQ sharing, solving many conflict problems that would arise with ISA add-ons.

In the original PC design, 2 COM ports were defined by an I/O address and interrupt. (I/O address is not significant here as there are plenty of I/O addresses available.) COM1 used IRQ4 and COM2 used IRQ3. COM3 and COM4 were added - but in standard configuration, COM3 used IRQ4 and COM4 used IRQ3 - the same IRQs as COM1 and COM2. This means that in the standard ISA architecture, a conflict would arise if COM1 and COM3 or COM2 and COM4 were in use at the same time.

PCI modems are provided with a driver or 'enumerator' that provides the bridge between the newer PCI bus and the traditional COM port assignments: the driver allows the system to recognize the modem as a traditional COM port with non-standard IRQ assignment.

Most PCI modems are software-based modems requiring an Operating System specific driver! There are a few - notably some USR models - hardware-controller based PCI modems - these will typically cost at least twice as much as software-based or 'Winmodem' models.

Some PCI modem installation problems may be solved by putting the modem in a different PCI slot. Depending upon your system design and BIOS, a conflict may prevent proper modem operation in some, but not all, of your PCI expansion slots. In addition, some vendors have motherboard BIOS updates that address PCI issues - check for updated BIOS for your system.

Other PCI devices may produce conflicts - including some that result in intermittent problems including disconnections. The [PCI] video controller and driver might be involved:

Some graphics drivers may continuously write data to the PCI based video graphics controller even if the controller is not ready. This may cause clicks and pops in audio (sound-card conflict), display anomalies (unintended artifacts on your screen), and possibly modem disconnections or connectivity problems. This problem was identified by Intel in 1998, with a recommendation that graphics board vendors address it with a driver modification. Check for video adapter upgrades for your system if you are experiencing these problems.

You can get more information about your PCI modem card (as well as all other PCI devices in your system) with a utility like PCI Tree.

OTHER CONFLICTS - USB Optical mouse causes modem problem: see this Forum56 Post.

IF Windows shows your modem installed, but you cannot access or use it: An incorrect or "bad" driver package may have been used by Windows. Often, this cannot easily be corrected! Modems - such as Conexant-based HCF/HSF - once detected and "installed" by Windows may not accept an "upgrade" to a working driver successfully. The problem: the only way to re-install (any) drivers is to remove the modem in ControlPanel's Device Manager; you then have to re-boot, and on the re-boot, Windows detects and automatically re-installs the "bad" driver package without giving you a chance to tell it to use "good" drivers you've downloaded or obtained. This situation is discussed further with specific example involving Conexant HSF.

ALSO SEE: Microsoft KB Q182604 - Description of PCI Bus IQR Steering - applies to Windows 95, 98 and Me.

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