Indeed, among the 18 pins in a PCI-E x1 slot, only four wires forms the actual data link, two for RX and another two for TX [0]. In terms of space, A PCI-E conn

Indeed, among the 18 pins in a PCI-E x1 slot, only four wires forms the actual data link, two for RX and another two for TX [0]. In terms of space, A PCI-E connection is really a piece of cake. And interestingly, to make a designer's life easier, PCI-E does have a reference clock signal, but it doesn't directly control the data transfer, it's only here for establishing a stable frequency reference, so a card doesn't have to include its own clock (which is a possible option). Since PCI-E Gen 2, its use is entirely optional because data recovery was possible, see [1]. [0] https://en.wikipedia.org/wiki/PCI_Express#Pinout https://en.wikipedia.org/wiki/PCI_Express#Pinout [1] https://www.silabs.com/documents/public/white-papers/PCIe-Clock-Source-Selection.pdf https://www.silabs.com/documents/public/white-papers/PCIe-Cl...