What is a PCI Bus and How Does PXI Handle Bus Extension?

Updated Nov 13, 2018

Reported In


  • PXI Controller
  • PXI Chassis
  • VXI-MXI-Express

Issue Details

How are PXI chassis and other devices recognized on my computer? What is a PCI bus and how will it impact my PXI system?


To maintain signal integrity, a PCI bus signal can only propagate around 8 PXI slots for 33MHz signaling or 5 slots for Compact PCI 66 MHz signaling. A computer has similar limitations and once this limit is met, a PCI-PCI bridge is used to (in simplest terms) rebuffer the transmission. The addition of a PCI-PCI bridge changes the PCI bus number (incrementing your total number of buses). National Instrument's larger PXI chassis contain two PCI-PCI bridges to maintain signal integrity through the longer backplane.

Note: The PCI specification governed by the PCI-SIG allows for up to 256 PCI buses in a single host.

If your computer has a limit on the number of PCI-PCI bridges, you can estimate the number of bridges based on your hardware. There are usually four sources of PCI to PCI bridges in your PXI system: internal to the PC or laptop, MXI PXI Extension, PXI backplane, and PXI modules. A few examples are:

MXI-Express kits utilize the MXI links between the contolling PCI slot and PCI bus links on the PXI(e) chassis. Most PXI chassis will have 1 or 2 links that will introduce 1, 2, or 3 PCI-PCI bridges. More recent PXIe chassis will have up to 4 links that will introduce 7 or 8 PCI-PCI bridges.

Note: Complete MXI-Express kits consist of a host board (PCIe or ExpressCard) and a PXI system controller board (PXI-8360). Each individual component is listed below.

PXI Chassis:
PXI chassis can be constructed using 1 PCI-PCI bridge or be split into multiple PCI bus segments that require more PCI-PCI bridges. The number of PCI bus segments for each chassis is included in the specifications and user's manual for that chassis. For example, the PXI-1044 chassis has 3 PCI bus segments which would utilize 3 PCI-PCI bridges.

PXI Modules: 
Some PXI peripheral cards will introduce additional PCI-PCI bridges. For example, GPIB-ENET cards introduce 1 PCI-PCI bridge.

Additionally, you can determine the number of buses available on your PC using the MXIe Bus Detect Utility, linked below.


Not Helpful