Nutshell Comparison of PCI, PCI-X and PCI Express.

by Aleksey Tsalolikhin, 22 May 2006

Purpose: I looked at the different I/O bus standards to improve I/O performance of one of my systems. I wanted to figure out what's the next step up from PCI. Here is a summary of the standards.

Peripheral Component Interconnect, a bus for connecting peripheral devices (IC's or add-on cards) to motherboard.
The original standard. 33 MHz 32 bit bus, peak transfer rate 133 MB/s.
Variation on the original: the bus faster and wider and slightly different protocol.
PCI Express
Next generation architecture. Point to point topology, rather than a shared bus. Each point-to-point connection is called a "link", and a link is composed of "lanes". Each lane is composed of two lines, one upstream, one downstream. A single lane can transmit 2.5 Gbps in each direction; with a sustained transfer rate of 250 MB/s. You can have multiple lanes in a link. The multiples are 1x, 2x, 4x, 8x, and 16x. The more lanes you have, the longer the PCI Express slot. Lanes are fixed in the motherboard, you can't add or remove lanes.
  Signalling Bus Width Peak Transfer Rate (bytes)
PCI (original) 33 MHz 32 133 MB/s
PCI 33 MHz 64 266 MB/s
PCI 66 MHz 64 533 MB/s
PCI-X 133 MHz 64 1066 MB/s
PCI-X 2.0 266 MHz 64 2.133 GB/s
PCI-X 2.0 533 MHz 64 4.266 GB/s
PCI Express 1x n/a n/a 250 MB/s
PCI Express 4x n/a n/a 1 GB/s
PCI Express 8x n/a n/a 2 GB/s
PCI Express 16x n/a n/a 4 GB/s

Up to: Aleksey's tech notes