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PE3RR PCI Expansion| 69
11 Appendix B Need More Slots?
11.1 Multiple PCI Expansion System Configurations
The PCI Local Bus Specification defines the PCI bus as a hierarchical bus , where PCI to PCI Bridges (PPBs) may be used to add "levels" to the PCI
bus hierarchy. Because hierarchies are organized systems arranged into different levels , you can take advantage of this automatic organizing
and layering to expand the number of available PCI 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:
1. In a desktop computer, the bios enumerates automatically behind PCI bridges. In the OSS configuration, the bios finds the OSS host
card in a PCI slot. The bios configures it, and then looks beyond it to find any PCI cards installed in the expansion chassis. The bios
then configures the cards and allocates resources. Empty PCI 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 PCI
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 PCI 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.
2. The 4 and 7 Slot OSS Expansion Systems each add two (2) PCI to PCI Bridges to the PCI Local Bus. The 13 Slot adds three (3) PCI to PCI
Bridges to the PCI Local Bus. The 6 Slot adds one (1) bridge. These bridges are the communication paths between the PCI cards and
the computer resources. Most computers allow multiple bridges (and thus PCI buses) to be correctly configured. Some computers,
unfortunately, place an arbitrary limit on the number of bridge levels that can be traveled during power-on configuration. Your
computer “power-on-software” should be PCI Specification 2.1 compliant in order to effectively use a OSS PCI Expansion System.
For "fan-out" and "daisy chained" system configurations, where more than one PCI expansion system is being used, you should
count all of the bridge levels to your most deeply nested PCI bus to determine the maximum number of bridge levels that must be
traveled.
3. For easier understanding, the PCI 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 PCI Card to the CPU
and back again. Under most circumstances, the effect cannot even be detected. However, under other conditions, it can be
important to know about.