One goal of the PCI Express add-in card electromechanical spec was to encourage migration from the PCI architecture found in many desktop and mobile devices today by making the migration path straightforward and minimizing the required hardware changes. Towards this end, PCI Express add-in cards are defined to be very similar to the current PCI add-in card form factor, allowing them to readily coexist with PCI slots in system boards designed to the ATX or micro-ATX standard. PCI Express features like automatic polarity inversion and lane reversal also help reduce layout issues on system boards, so they can still be designed using the four-layer FR4 board construction commonly used today. As a result, much of an existing system board ...

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