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PCI-Express not backward compatible with PCI and PCI-X
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In comparison to PCI and PCI-X, PCI-Express is a newer standard with a different form factor. PCI-Express adapters are not backward compatible to PCI or PCI-X.  Not only are the data rates on PCI-Express different from PCI/PCI-X, but PCI-Express uses serial data transfers versus PCI/PCI-X transferring its data in a parallel fashion. For these reasons, a PCI-Express type motherboard connector (or "slot") is physically different from a PCI or PCI-X type connector. Therefore a PCI-Express adapter will not physically fit in a PCI or PCI-X slot, and vice-versa. This is true regardless of the adapter card's manufacturer.