M A G M A
Fan-Out
For reasons described in bullet #3 above, we recommend using a fan- out configuration when you need to add successive PCI expansion systems to the host computer. This addition to the PCI bus hierarchy is at the "peer" level of other existing expansion buses, thus adding "breadth" to the PCI bus hierarchy, since these buses will be at the same hierarchical level within the system configuration.
You may use either PCIe or PCI host cards in your host computer, depending on available slots and your performance requirements. You must ensure that you have the corresponding PCIe to PCI Expansion System, or PCI Expansion System, chassis to connect with your host card and support your desired configuration.
In this configuration, you have fewer “Toll Booths” (PCI Bridges) to pass through; therefore there are fewer opportunities for concern.
For clarification, it is necessary to understand that the amount of latency caused by any PCI Bridge (even those built into a 3rd Party PCI card or the computer motherboard itself) is negligible (“nanoseconds”) and will not produce any visible effect on most systems. It is only mentioned here because on a few rare configurations which use some unique PCI cards with special software drivers, the total number of PCI buses that the data must pass through COULD make a difference. See Chapter 6 Troubleshooting for more information if you suspect this could be an issue with one of your specialty PCI cards.
38Appendix B Need More Slots?