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The M.2 form factor uses keying (M, B, etc.) to prevent incorrect insertion. Revision 5.0 retains the physical "M key" for PCIe x4 devices but updates the . Specifically, the spec redefines the high-speed transmission lines (PETp/n and PERp/n) to support the higher Nyquist frequency of 16 GHz (for 32 GT/s in PCIe 5.0). The PDF includes new layout guidelines to minimize crosstalk between adjacent pins. pci express m.2 specification revision 5.0 version 1.0 pdf