PCI Express is an In/Out bus that is an essential part of every PC’s current architecture. It is more commonly used for GPUs and can be used for putting network and sound components, and storage units like the fastest SSDs on the market to replace hard drives and SATA SSDs.
PCI Express (you will see it as “PCI-E” or “PCIe”) is used both for the internal connection of chipsets on the motherboard and the connection of external GPUs on the respective slots. The most widely used version is the 3.0, although the 4.0 and 5.0 versions (which are delayed) will improve performance and turn the standard into a single bus that will be the future of the PC. We will go over the features of the interface, the existing types and its future.
What is the origin of PCI Express?
The PCI Express standard was developed by PCI-SIG (PCI Special Interest Group), an organization formed by 750 members representing every major tech company on the industry. The organization’s objective was to develop a single standard to replace previous buses like ISA, AGP and PCI (on which PCIe is based).
PCIe provides an essential advantage over PCI because it supports point-to-point topology, full-duplex and serial links. Basically, each single PCIe port and its installed cards can get the best performance out of the bus over the slower and saturated PCI in the case of multiple masters.
PCIe 3.0 is the latest update on the market and the one you can currently find when you buy a motherboard. This is a major improvement over PCI 1.0 because it quadruplicates the data transfer up to 8 GT/s; its total bandwidth up to 126 Gb/s (15.8 GB/s); and its bandwidth per lane up to 15.8 Gb/s (1969.2 MB/s).
Types of PCI Express
PCIe has gone through several revisions, and all of them have one thing in common. They use the same physical connections and have five main widths: ×1, ×4, ×8, ×12 and ×16. There are also x32 ports, but they are extremely rare and are usually not seen on consumer-grade hardware.
The different physical widths allow you to transfer different amounts of connections and data to the motherboard simultaneously. The wider the port, the higher its maximum capacity. These connections are typically known as “lanes” or “slots.” Each PCIe slot has two directions: one for outgoing data and one for incoming data. In practical terms, a higher number of slots provides better performance and capacity, and data can travel smoothly from a peripheral to the rest of the system.
In PCI Express 3.0, the maximum theoretical performance per lane is 8 GT/s, but it actually is less than 1 GB/s per slot. Not every device needs the same capacity. Although there are no pre-established norms about the type of slot to use, we can give you some practical examples of usage.
A typical PCIe x1 slot is enough for a typical sound card or a Wi-Fi card, whereas high-end network cards, RAID controllers and USB extenders use x4 or x8 slots. GPUs usually use x16 slots for a maximum transfer rate. PCIe M.2 SSDs are typically connected to x4 ports, but it seems it will not be enough for future generations.
Elements to take into account in terms of types of PCIe slots
One of the things that can confuse consumers when setting up PCIe is that an x16 port might not provide the maximum amount of slots allowed by the standard. While PCIe supports unlimited individual connections, there is a practical limit in terms of the performance of the motherboard’s chipset.
This leads to a conclusion you certainly ignore: not every motherboard is the same. Cheap motherboards may have x16 slots but with the performance of x8 slots, for example. High-end motherboards for gaming PCs or professional workstations usually have several x16 slots. Besides their width, these slots make the most out of the performance and bandwidth provided by the standard.
If we put a high-end GPU (especially on multi-GPU setups like SLI and Crossfire) on a slot that fails to offer the maximum number of lanes despite having an x16 width, you may experience bottlenecking and might not get the GPU’s maximum performance. Another aspect to take into account is that many motherboards with two x16 slots only provide the maximum amount of lanes if you use just one slot. If you use both lanes together, they will be similar to x8 slots.
Smaller x1 and x4 cards can be installed on x8 and x16 slots (but obviously not the other way around). Besides, some x8 cards have a set of different pins and cannot be put on x16 ports.
The future of PCI Express
PCI Express 3.0 is available since 2010. Since then, the industry’s needs have expanded especially in terms of graphics and apps with heavy workloads and bandwidth like the ones related to AI.
This is why the PCI-SIG unveiled the final specs of the next PCI Express 4.0 last year. This version will provide a better performance than the current PCIe 3.0, increasing the number of channels for the signal in order to double the bandwidth up to 16 GT/s.
It will also have a lower latency, better RAS capabilities and better I/O virtualization capabilities to handle the rising needs of the industry in terms of graphics, as there are really photorealistic videogames and professional apps. Another expected improvement is the reduction of the size of the bus to fit smaller GPUs and avoid the enormous size of current dedicated high-end GPUs, for example.
The initial requirements for its adoption have not been met, so it will arrive later than expected. The first PCI Express 4.0 products will be available in 2019. We will have to wait and see. Given the situation, PCIe 5.0 has also been delayed. This version would also provide a substantial increase in performance at 32 GHz for a bandwidth of 128 GB/s at full-duplex, doubling the rates of PCIe 4.0 and quadruplicating those of PCIe 3.0.
Such a big delay, and there is no official launch date yet. The organization says this is due to “the stagnation in the PC market.” However, the new standard would make manufacturing motherboards easier by using a single bus for everything. This would mean replacing legacy interfaces that include SATA-based storage units in terms of motherboard slots and formats like M.2 that take advantage of the great speed provided by PCIe.