PCI Express: PCIe x1, x4, and x16 Slots

Types of PCI Express: Data Speeds to 16 G/s -- 32 GB/S Coming Soon!

The latest bus for expansion cards is PCI Express (PCIe), with both v1.1 and v2.0 hardware now available. Proponents hope it will replace PCI, PCI-X, and AGP. PCIe uses a serial architecture, but it retains software compatibility with PCI, which means that the same programs and operating systems you used with PCI are still usable with PCIe boards, motherboards, and backplanes.

A single PCIe serial link is made up of two pairs of wire, one to transmit and one to receive. Together, these two pairs are called a lane. Theoretically, each pair in a PCIe v1.1 lane can transmit up to 2.5 GB/s, so together they transmit 5 Gb/s or 635 MB/s. However, there is overhead in the system to track clock speed, transmit error codes, and provide DC balancing so that the wires don't acquire electrical charge. Bottom line: each lane has a net transmission speed of 500 MB/s (250 MB/s simultaneously in each direction).

In the PCIe concept, actual transmission speed depends on the number of lanes. A v1.1 PCIe x1 (pronounced "by one," and meaning one lane) slot has a net speed of 500 MB/s, so a x2 slot is 1 GB/s, a x4 is 2 GB/s, a x16 is 16 GB/s, and so forth. Slots up to x32 are defined in the standard, but at this writing we have seen hardware only up to x16. PCIe v2.0 doubles that speed to 32 GB/s, but note that some motherboards provide a mix of v1.1 and v2.0 slots. For example, our new MXEA motherboard has one v1.1 x1 slot, one v1.1 x16 slot, and two v2.0 x16 slots.

A useful feature of the PCIe standard is that you can put low-speed boards in high-speed slots. For example, the three PCIe slots on the PBER 06R backplane (shown above) could be filled with three x1 boards. Similarly, a v1.1 x1 board works perfectly well in a v2.0 slot; however, you only get v1.1 x1 performance.