PCIe 3.0 relied on a signaling method called , which encodes one bit per clock cycle. Moving to 16 GT/s using NRZ pushed the frequency of the signal to the physical limits of standard FR-4 printed circuit board (PCB) material.
The PCI Express 4.0 specification defines a 16 GT/s bit rate, doubling the speed of PCIe 3.0 to allow for up to 32 GB/s per direction on a x16 slot. It outlines detailed requirements for the physical, data link, and transaction layers to ensure high-speed data integrity while maintaining full backward compatibility.
One of the most significant additions to the ecosystem (formalized alongside PCIe 4.0 compliance) is the widespread adoption of .
The specification does not mandate expensive new PCB materials for motherboards. Instead, the specification engineers had to optimize the receiver and transmitter logic. PCIe 4.0 introduces aggressive requirements.