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57
Fan-Out
For reasons described in bullet #3 above, we recommend using a fan-
out configuration when you need to add successive PCIe 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 must ensure that you have the corresponding PCIe to PCIe
Expansion System, or chassis to connect with your host card to
support your desired configuration.
In this configuration, you have fewer “Toll Booths” to pass through;
therefore the latency is shorter, however you’ve “sacrificed” another
PCIe slot from the host computer, which may not always be an option.
For clarification, it is necessary to understand that the amount
of latency caused by a PCIe switch (or a PCIe-to-PCI bridge
built into some 3
rd
Party PCIe cards) is negligible
(“nanoseconds”) and will not produce any visible effect on most
systems. It is only mentioned here because of certain
specialized configurations involving unique PCIe cards with
special software drivers. The total number of logical 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 PCIe cards.