Bus Contention and PCI, EISA, and ISA

• Microsoft Windows NT Advanced Server versions 3.5 and 3.51
• Microsoft Windows NT Workstation versions 3.5 and 3.51

SYMPTOMS

If you run a nightly backup of your system and during that period of time you get network disconnect errors in the event log, it is very likely that you are encountering bus contention between your network card and the SCSI controller that may be hosting your tape backup unit.

Because SCSI controllers are most often involved in this type of issue, this article focuses on SCSI, but bus contention can potentially affect the function of any card or built-in accessory port in the system including mice, video, keyboard, and any other I/O system.

MORE INFORMATION

There are hardware issues that occur rarely and are on the surface indistinguishable from software-based problems. These issues most often occur when there are a number of same function cards in a given computer. For example a given hardware platform has several different SCSI cards in the system, one or more in the slots and perhaps an onboard (built-in) SCSI controller in some of the more modern boards. This can occur with any group of cards in a multi-interface system board (any combination of PCI, EISA and ISA).

WORKAROUND

The best test (and potential workaround) for this problem is to choose an ISA board to replace your PCI (or EISA) board's function (for example, replace a PCI network card with an ISA network card) and see whether the problem ceases. If substituting the ISA board eliminates the problem, you may want to contact your various hardware vendors to investigate why and where this conflict is occurring if you want to continue to use the card in question.

An alternative is to simplify your setup. For example, you could place your tape unit on the onboard SCSI chain and eliminate one of the extra SCSI controllers. This issue should not occur in an all-ISA or all-EISA configuration. Although with the ISA configuration you will have the familiar problem of I/O address as IRQ conflict resolution, most of these conflicts are self-evident.