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38 Appendix B Need More Slots?
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 3
rd
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.