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ExpressBox 3200 | 8 Appendix A Multiple Expansion System Configurations 75
8 Appendix A Multiple Expansion System Configurations
The PCI/PCIe Local Bus Specification defines the PCI/PCIe bus as a
hierarchical bus , where PCI to PCI Bridges (PPBs) may be used to add "levels"
to the PCI bus hierarchy. Hierarchies are organized systems arranged into different levels , you can take advantage of this automatic organizing
and layering to expand the number of available PCIe slots beyond the number available in your computer’s motherboard.
You can easily add two or more OSS expansion systems to your current system in either a "fan-out " or "daisy-chain " configuration. Each of
these configurations has advantages and uses. To determine which type of configuration you wish to use, you should first understand a few
basic facts:
In a desktop computer, the bios enumerates automatically behind PCI/PCIe bridges. In the OSS configuration, the bios finds the OSS host card in
a PCIe slot. The bios configures it, and then looks beyond it to find any PCIe cards installed in the expansion chassis. The bios then configures
the cards and allocates resources. Empty PCIe slots are ignored during configuration.
In theory, your computer “should” be able to travel across up to 255 PCI buses to identify and configure all installed PCIe
cards. Unfortunately, not all computers are created equal. The 255 PCI bus limit is actually a theoretical maximum. The
practical limit is somewhat lower, but should still be a fairly large number.
In a desktop computer the bus numbers tend to be lower (0, 1, 2, etc.). Once the bios is finished configuring everything and
assigning resources, the Operating System starts loading and activates the PCIe cards found.
When adding more Expansion Systems to an existing system, be sure to connect and test them “one-at-a-time.” This will
allow you to quickly resolve any connectivity, or other, issues right at the start.
DO NOT INSTALL any 3
rd
Party PCI cards into
any system until ALL attached systems are working correctly.
For easier understanding, the PCI/PCIe Local Bus should be viewed as a “Resource Toll road” that runs from the computer CPU (resource home),
through the “Toll Booths” (the PCI bridges), to the “Resource Users” (PCI Cards). As in the case of a real Toll Road, the more Toll Booths you
have to slow down a “nanosecond” for, the longer it will take you (your data) to get from the PCIe Card to the CPU and back again. Under most
circumstances the effect cannot even be detected.