Guide to PCIe - Peripheral Component Interconnect Express

Peripheral Component Interconnect Express, commonly known as PCI Express and abbreviated as “PCIe”, is a high speed computer bus architecture which has become the industry standard for nearly all internal computer component connections.

PCIe provides the connections from a computer’s processor(s) and memory to other components and peripherals. This interface is a highspeed serial transport designed to replace older parallel bus architectures including PCI and ISA (Industry Standard Architecture). PCIe uses a point-to-point access bus that provides outstanding data transfer speed improvement over its predecessors. 

What Does PCIe Mean?

By the early 2000’s, demands on computing bandwidth by internal hardware and peripherals had led to the next generation of PCI, known as PCI Express, or PCIe. The biggest advantage of PCIe over conventional PCI is that it utilizes a serial interface, rather than the parallel interface used in PCI architecture. Another advantage is the use of individual, rather than shared busses, for each connected device.

PCIe Architecture

The breakthroughs included in PCI Express architecture have enabled substantially faster transfer rates, as compared to PCI, and each successive version of PCIe has continued to build upon these improvements. Whereas conventional PCI utilized common address, data and control lines, the point to point topology of PCIe provides separate links to each connected device, freeing each from the potential constraints of the others.

PCIe communication utilizes a mode of data transfer known as “packets”. The transaction layer of the PCIe port performs the task of packetizing and de-packetizing the data. This difference in electrical signaling inherent to PCIe vs. conventional PCI required distinct electronic and connector architecture to be developed, making PCIe slots incompatible with PCI cards.

PCIe Formats

A variety of formats have been developed for PCI Express, each with slightly different nomenclature. Understanding what these PCIe formats indicate in terms of size and version is essential information for understanding what expansion cards will (or will not) work with your system.

PCIe Versions

Since the introduction of PCIe 1.0 in 2002, successive versions have been introduced to meet the growing demand for bandwidth and speed. Version 1.0 had an available bandwidth of just under 8GB/sec for a combined 16 lanes of traffic while the transfer rate for PCIe 1.0 was 2.5 gigatransfers per second (GT/s). With each new release, the total bandwidth has doubled, eventually leading to 128GB/sec of bandwidth for PCIe 5.0, released in 2019. At the same time, the transfer rate has incrementally risen from 2.5 to 32.0 GT/s. Along with performance improvements, each version has introduced new features and increased energy efficiency.

With a lofty standard of doubling bandwidth every few years to avoid becoming the bottleneck as processor speed and memory continues to increase rapidly, each revision presents new challenges for designers. Potential for crosstalk and electrical discontinuity increases along with bandwidth, so new materials and design innovations continue to push the envelope.

The Future of PCIe

The immediate future of PCI Express has been shaped by the release of PCIe 5.0 in 2019. This revision, like its predecessors, doubles the available bandwidth of version 4.0. In addition to the performance upgrade, physical differences inherent to PCIe 5.0 include shorter keys and integration of improved materials on the printed circuit board. PCIe 5.0 remains backwards compatible with PCIe 3.0 and 4.0. For this reason, motherboards and adapter cards do not require immediate upgrades. As computer hardware designers and manufacturers continue to integrate PCIe into their latest offerings, the entrenched position of a de facto standard becomes more secure.

The demands on computing power and memory continue to evolve at breakneck speed. So far, the PCI Express bus has managed to keep pace, but the gap is narrowing. PCIe 6.0, expected to be released by 2021, will provide an incredible 256 GB/sec of bidirectional bandwidth and data rates of 64 GT/s to meet the demands of machine learning, artificial intelligence (AI) and other emerging, cutting-edge applications. Data transfer performance in the not-so-distant future may seem like a Hyperloop compared to our familiar and reliable bus, and necessarily so. Perhaps no computer technology plays as important a role in meeting the demand for speed and bandwidth, so the impetus on designers for ongoing breakthroughs in PCIe performance will continue into the foreseeable future.