PCI Express, a guide to understanding the bus and its variants

PCI Express is a computer communication standard designed as a local Input/Output bus. You will see it abbreviated as “PCI-E” or “PCIe” and it is used both for internal connection on the integrated circuits of the motherboards and for connecting external cards that are punched into the corresponding slots.

The PCI Express standard is the responsibility of the “PCI Special Interest Group” (PCI-SIG) and was developed to completely replace older buses such as ISA, AGP or the same PCI as it is based on as a single standard. PCIe offers a fundamental advantage over PCI as it is structured as point-to-point, full-duplex rails working in series.

Basically, each individual PCIe port and its installed cards can obtain the maximum performance of the bus, as opposed to the slower and more saturated PCI when the equipment mounts multiple connectors.

Obviously this is a fundamental standard in today’s computer structure, because it is used for graphics card communication and can be used for other types of expansion cards, network, sound, video editing and in recent times to connect the fastest SSDs on the market. It is still early because the market is not mature and the “memory war” has cut the price of solid state solutions, but it is likely that PCI Express will also end up in the future with the SATA ports we use for hard drives or SATA, imposing itself as a single interface.

PCI Express versions

The most widely used version is currently v3.0. An upgrade from the original PCI 1.0, it quadruples its transfer rate to 8 GT/s; its total bandwidth to 126 Gbit/s (15.8 GB/s) and its bandwidth per lane to 15.8 Gbit/s (1969.2 MB/s).

Recently, the PCI-SIG consortium announced the final specifications of the next generation PCI Express Gen 4.0. A version that will offer higher performance than the current PCIe 3.0, increasing the number of channels through which the signal passes to double the bandwidth to 16 Gigatransfers per second.

It will also offer lower latency, superior RAS capabilities, improved I/O virtualization, to address the increasing needs of the industry especially in the graphics market with truly photorealistic games, but also in professional applications with high workloads and bandwidth as technologies related to artificial intelligence. Another of the improvements will come from the smaller physical size of the bus, which will allow smaller cards and not in the monstrous sizes that we can find -for example- in today’s high-end dedicated graphics.

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The first products under PCI-Express Gen 4.0 will be available in 2019. The consortium responsible is also working on the next version of the standard, PCI Express Gen 5.0, another major advance over previous versions of the standard with a substantial performance increase, using a frequency of 32 GHz to reach a bandwidth of 128 GB/s in full duplex, doubling that of PCI Express 4.0 and in turn quadrupling that of PCI Express 3.0.

PCIe Gen 5.0 will arrive somewhat later than expected. The organization attributes this to “the stagnation of the PC market”. Let us bear in mind that the current PCI Express 3.0 has been available since 2010 and since then the needs of the industry have increased, especially in the graphics market, but also in other applications with high workloads and bandwidth, such as technologies related to artificial intelligence.

PCI Express Types

PCIe has gone through several revisions as we have seen, but they all have a common denominator, they use the same physical connections that you will see in four primary sizes: x1, x4, x8 and x16. There are also x32 ports, but these are extremely rare and usually not seen on consumer hardware.

The different physical sizes allow different amounts of connections and data to be moved simultaneously to the motherboard.

The larger the port, the greater its maximum capacity. These connections are colloquially known as “lines” or “rails”, where each PCI-E rail consists of two signal pairs, one for sending data and the other for receiving. In practice, the greater the number of lanes, the greater the performance and capacity and the faster the data can flow between the peripheral and the rest of the computer system.

In version 3.0 of PCI Express (the most widely used at present), the theoretical maximum performance per line is 8 GT/s which in practice translates into just under 1 Gbyte per second per lane. Not all devices need the same capacity and although there are no established guidelines on what type of slot to use, we can point out some practical examples of its use.

For a common sound card or Wi-Fi, a PCI-E x1 is sufficient, while a high-end network card, RAID controllers or USB 3 expanders use the x4 or x8. Graphics cards typically use x16 for maximum transfer capacity. SSDs in M.2 format for PCIe are usually connected to x4 ports, but all indications are that they will outgrow them in future generations.

PCI-E type and rail considerations

One of the parts of the PCI-E configuration that can confuse a consumer is that a x16 size port may not offer the maximum number of lanes allowed by the standard. The explanation is that while PCI-E can accommodate individual connections in unlimited quantities, there is a practical limit to the performance of the board’s chipset.

This leads us to a conclusion I’m sure you’re aware of: not all motherboards are the same. Economy boards may have x16 slots but their performance equals x8 for example. High-end boards for PCs dedicated to games or professional workstations usually have several x16 slots that in addition to size take full advantage of the performance and bandwidth allowed by the standard.

If you put a high-end graphic (especially in SLI or Crossfire multigraphic systems) in a slot that -even if it is x16 size- does not offer the maximum number of lines, you can have a bottleneck and not get the maximum performance. Another aspect to consider is that many boards with 2 x16 slots only offer the maximum of lines if you use one of them, going down to x8 if you use both together.

Note that smaller x1 and x4 boards can be installed on the x8 and x16 (obviously not the other way around). Also, some x8s have a different set of pins and cannot be installed in x16 slots.

In short, details to be taken into account when buying your motherboard, a fundamental component to pay attention to beyond knowing the socket and chipset, as we have seen in this approach to what has become your most important local I/O bus: PCI Express.

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