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PCI Express 2.0 Vs PCI Express 3.0

The success of the PCI Express as a worthy successor to the PCI-X bus has been phenomenal. The PCI Express simplifies electrical hardware design by requiring lower number of pins, increases speed by using differential signaling and is scalable by providing varying number of lanes. It is able to do so, even while maintaining the compatibility on the protocol level with the earlier PCI-X Bus.

The earliest PCI Express 1.0 has a raw data rate of 2.5 GT/seconds. The PCI Express 2.0 doubles it to 5 GT/seconds. The PCI Express 3.0 further increase it to 8 GT/seconds. Interestingly, the PCI Express 3.0 doubles the through put even when it increases the raw data rate by only by 60%. To understand the reason how it is able to do so, you should know that PCI Express 2.0 uses a 8b /10b coding scheme. Which means the for every 8 coded bits there are 2 additional bits. PCI Express does not use the 8b/10b coding and is therefore able to increase the throughput rate.

The PCI Express 3.0 at 8 GT/seconds poses some Signal Integrity Challenges, especially in lossy lines. The eye diagrams start closing down as the signal moves down a long lossy FR4 line. The system designers have to take special care when designing PCI Express 3.0 systems.

The release of the PCIe 3.0 specification on November 17, 2010 leads to a new era of high speed PC components interconnect. While it will take some time for the component vendors to implement the PCIe 3.0, it definitely looks promising.

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