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ExpressBox 3200 | 9 Appendix B PCI Express General Information 80
9.5 Form Factor
There are several different physical connections, each of which can function electrically as a slot with a lower number of lanes and can
accommodate a physically smaller cards. A physical PCIe x16 slot can accommodate a x1, x2, x4, x8 or x16 card, and can run a x16 card @ x16,
x8, x4 or x1. Some slots are open-ended sockets to permit physically longer cards and negotiate the best available electrical and logical
connections.
9.5.1 PCI Express Sample Connectors
The number of lanes actually connected to a to a slot may also be less than the number supported by the physical slot size. An example is a x16
lot that runs at x4, which will accept any x1, x2, x4, x8 or x16 card, but only provides four lanes. Its specification may read as “x16 (x4 mode)”
while “xsize” @ xspeed” notation (“x16 @ x4”) is also common. The advantage is that such slots can accommodate a larger range of PCI
Express cards without requiring motherboard hardware to support the full transfer rate.