What are Peripheral Component Interconnect Express (PCIe) explained

What is PCI?

As the name suggests, PCI are for the peripheral components. It is an expansion slot on the motherboard. It features very high speed data transfers between the peripherals and the motherboard. These components may be a Graphics Card, NVMe SSD, SATA, Network, USB, WIFI, etc.

PCI Express
PCI Express

PCI express have been around since 2003. Latest PCIe 6.0 (2019) can offer speed up to 126 GB/s. Most popular PCIe 3.0 (2010) offers speed up to 16 GB/s. Let us know more about PCI Express (PCIe).

History of PCI

Ever since personal computers developed in 1981, computers needed expansion slots to extend motherboard capabilities.

First such was ISA (Industry Standard Architecture) on 8 bit version. It was made for and by IBM PC. Later, IBM XT PC was released in 1984 with ISA 16 bit.

In 1987, IBM PS/2 machine with MCA (Micro Channel Architecture) was launched with 10MB/s. But not much was being achieved.

IBM collaborated with other companies for EISA (Extended Industry Standard Architecture). EISA gave throughput of 33 MB/s. Still, there was a hitch in speed.

Then in 1992, Intel spear headed and developed VLB (VESA Local Bus). It jumped on the speed to 132 MB/s. But it was made for then best processor Intel 80486 Chip. Same year, it re-launched it as PCI. Now, it was platform independent.

Intel then collaborated with affiliates for further development. This group is known as PCI-Special Interest Group (PCI-SIG). They handle all PCI related released and forums.

With rapid improvement in Graphics Card, Intel redesigned them as AGP (Accelerated Graphics Port). It was exclusive for Graphics Card.

Finally, PCI-SIG developed PCIe in 2003 and brought fundamental change in bus connection. They moved from parallel to serial connection. This brought massive throughput. Since then, PCIe has improved over its previous versions.

What is the speed of PCIe and other Expansion slots?

Throughput speed = Clock Cycles (per second) * Data Rate (per clock cycle) * No. of Bits (per Data Rate) * Encoding

Below table shows the working speed of all Expansion Slots :-

x bits – the transfer speed per Data Rate. Hence, x8 bits means 8 bits or 1 MB is being transferred per Data Rate. Also, note 1 MB = 8 bits.

For Example, PCIe 3.0 x16 has Real World Throughput = 8 GHz Clock Cycles (per second) * 1 Data Rate (per clock cycle) * 16/8 (convert to MB) (per Data Rate) * 128/130 Encoding = 15.8 GB/s.

What is the difference between PCI and PCIe?

PCI Express (PCIe) has moved from parallel to serial communication. That means it has a point to point dedicated link between the expansion slot and chipset. While, PCI uses a parallel shared bus that can carry more bits but it is way slower.

Each PCI Express has lanes for each connection. For Example, a x16 lane means 1 connection with 16 lanes for 16 bit transfers each clock cycle. Each PCIe lane is a full duplex. That means, each lane has a pair to send and receive bits also. Whereas, PCI is half duplex, both send and receive happens over the same lane.

pci express

Thus, it also illustrates how PCIe works.

Is there any compatibility issue with PCI Express?

All PCI Express are backward compatible same as for USB. Even the parallel connection can work with serial. So, PCIe slots can work in conjunction with PCI. This is because every slot has dedicated lines. All new motherboard will work for older PCI on any card.

Even slots can work with one another. Only difference if for example, x16 card is put on x8 slot then speed will be as per x8.

All expansion slots look same size, how to differentiate?

Kindly look into the contact metal pins inside the slots. If there are half the contact pins as in x16 slot then it is x8, if quarter then x4.

What are the applications of PCI Express?

PCIe 3.0 (2010) version is the most widely used PCIe in 2020. Listed below are some popular applications on PCIe 3.0. Some motherboards like X570 chipsets have started rolling PCIe 4.0. We will soon see more adaption.

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