PXI - PCI Extensions for Instrumentation

PCI Extensions for Instrumentation – PXI for short – is an updated version of the CompactPCI (cPCI) standard with extensions for measurement technologies. The standard was originally defined by National Instruments; meanwhile, it is updated as an open industry standard by the PXI System Alliance, or PXISA for short, with has more than 70 member companies. According to the technological progress in the area of PC backplanes, there is still a version available which is based on PCI Express (PCIe) under the name of PXIe.

A PXI system consists of a mainframe with integrated power supply and slots for a defined number of PXI modules. Each mainframe needs a module which will function as a so-called controller. This may be an embedded computer, but could also be a PCI or PCIe bridge or a similar module. The controller always has to be plugged into a defined slot of the chassis.

For a typical PXI system with a PXI bridge between the external PC and the PXI mainframe, the data transfer rates that can be achieved between the PXI modules and the PC are similar to those achieved with PC plugin cards. It is also possible to operate cPCI modules in a PXI mainframe; these can achieve bandwidths of up to 132MB/s.

For a PXIe system, a point-to-point bus topology with a shared PCIe switch replaces the shared bus of the PCI standard. This way, each module has direct access to the bus and thereby also to a dedicated data line. Several PCIe lines may be combined in order to achieve high bandwidths of up to 24GB/s.

In addition to dedicated PXI or PXIe mainframes, hybrid mainframes are also available, permitting a combined system design based on PXI as well as PXIe cards.

With PXI and PXIe, the PC hardware and the mainframe are separate, each with its own power supply and cooling fans; this has many advantages regarding EMC behavior, supply voltage quality, cooling and mechanical properties. If the PC is replaced due to technical progress or maintenance, it is not necessary to remove and re-install all plugin cards.

The PXI standard defines integrated timing and synchronization functions for the internal routing of synchronization clocks and triggers. A PXI chassis contains a dedicated 10MHz system reference clock, a PXI trigger bus, a star trigger bus and a local bus between the separate slots, whereas a PXI express chassis has an additional 100MHz system clock, differential signal transmission and differential star trigger, which permits the utilization of advanced timing and synchronization functions.