PCI eXtensions for Instrumentation
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PCI eXtensions for Instrumentation (PXI) is one of several modular electronic instrumentation platforms in current use. These platforms are used as a basis for building electronic test equipment, automation systems, modular laboratory instruments in science, and the like. PXI is based on industry-standard computer buses and permits flexibility in building equipment. Often modules are fitted with custom software to manage the system.
PXI Systems Alliance
PCI eXtensions for Instrumentation (PXI) is a modular instrumentation platform originally introduced in 1997 by National Instruments. PXI is promoted by the 54-member PXI Systems Alliance (PXISA), whose sponsor members are (in alphabetical order) ADLINK, Aeroflex, Agilent Technologies, Geotest, Gigatronics, National Instruments, Pickering Interfaces and Teradyne. (Sep 2010) 
Executive Members of the alliance include Alfautomazione, BAE Systems, CHROMA ATE Inc, GOEPEL electronic, LeCroy, MAC Panel, Virginia Panel Corp, and ZTEC Instruments. Another 40 associate member organizations that do not have voting rights are supporting PXI and use the PXI logo on their products and marketing material. (Sep 2010) 
Two major players in 2006
- Acqiris; was acquired by Agilent Nov 2006.
- PXIT; an early PXI entrant (acquired by Agilent Nov 2006).
- Keithley Instruments; launched a range of 35 Data Acquisition and Instrumentation PXI cards in Nov 2006.
The PXI worldwide market size, estimated in 2007, was 284 million USD and was expected to grow to over 520 million USD by 2010. The PXI market is also larger than the VME eXtensions for Instrumentation (VXI) market at an estimated size in 2007 of 233.2 million USD. The LAN eXtensions for Instrumentation (LXI) standard has grown to 200 million USD within 2 years.[dubious ].
PXI is designed for measurement and automation applications that require high-performance and a rugged industrial form-factor. With PXI, one can select the modules from a number of vendors and integrate them into a single PXI system, over 1150 module types available in 2006. A typical 3U PXI module measures approximately 100 x 160 mm (4x6") in size, and a typical 8-slot PXI chassis is 4U high and half rack width, full width chassis contain up to 18 PXI slots.
PXI uses PCI-based technology and an industry standard governed by the PXI Systems Alliance (PXISA) to ensure standards compliance and system interoperability. There are PXI modules available for almost every conceivable test, measurement, and automation application, from the ubiquitous switching modules and DMMs, to high-performance microwave vector signal generation and analysis. There are also companies specializing in writing software for PXI modules, as well as companies providing PXI hardware-software integration services.
PXI is based on CompactPCI, and it offers all of the benefits of the PCI architecture including performance, industry adoption, COTS technology. PXI adds a rugged CompactPCI mechanical form-factor, an industry consortium that defines hardware, electrical, software, power and cooling requirements, leaving nothing to chance. Then PXI adds integrated timing and synchronization that is used to route synchronization clocks, and triggers internally. PXI is a future-proof technology, and is designed to be simply and quickly reprogrammed as test, measurement, and automation requirements change.
Most PXI instrument modules are register-based products, which use software drivers hosted on a PC to configure them as useful instruments, taking advantage of the increasing power of PCs to improve hardware access and simplify embedded software in the modules. The open architecture allows hardware to be reconfigured to provide new facilities and features that are difficult to emulate in comparable bench instruments. PXI system data bandwidth performance easily exceeds the performance of the older VXI test standard. There is debate within the technical community as to whether newer standards such as LXI will surpass PXI in both performance and overall cost of ownership.
PXI modules providing the instrument functions are plugged into a PXI chassis which may include its own controller running an industry standard operating system such as Windows XP, Windows 2000, or Linux (which is not yet PXI System Alliance approved), or a PCI-to-PXI bridge that provides a high-speed link to a desktop PC controller. Likewise, multiple PXI racks can be linked together with PCI bridge cards, to build very large systems such as multiple source microwave signal generator test stands for complex ATE applications.
CompactPCI and PXI products are interchangeable, i.e. they can be used in either CompactPCI or PXI chassis, but installation in the alternate chassis type may eliminate certain clocking and triggering features. So for example you could mount a CompactPCI Network interface controller in a PXI rack to provide additional network interface functions to a test stand. Conversely, a PXI module installed in a CompactPCI chassis would not utilize the additional clocking and triggering features of the PXI module.
The PXI Systems Alliance has passed specification PXI-5, which specifies how to integrate PCI Express performance into both CompactPCI and PXI while preserving backward compatibility. Because PXI is based on CompactPCI, work began first on CompactPCI Express in early 2004. The CompactPCI Express specification defines the fundamental mechanical and electrical features of CompactPCI Express systems and, therefore, PXI Express systems. This includes the selection of connectors to support PCI Express, definitions of slots and board mechanicals, definitions of slot/board electrical signals, and compliance-testing requirements. The CompactPCI Express specification was released on June 27, 2005 and the PXI Express specification was completed in August 2005. PXI Express incorporates CompactPCI Express technology with enhancements for PXI compatibility, timing and synchronization, and system software frameworks.
The attached figure shows the basic layout of a CompactPCI/PXI Express backplane and illustrates how PCI Express is integrated into backplanes while preserving compatibility with current PXI modules. PCI Express electrical lanes run from the system slot (either an embedded controller or MXI link) to the hybrid slots, providing a high-bandwidth path from the controller to the backplane slots. In addition, the installation of an inexpensive PCI Express-to-PCI bridge on backplanes provides PCI signaling to all PXI and PXI Express hybrid slots. This ensures compatibility with PXI modules on the backplane. The system controller slot is capable of supporting up to a x16 PCI Express link, plus a x8 link, providing a total of 5.5 GByte/s transfer capacity to the PXI backplane. This is a more than 45 times improvement in PXI backplane throughput.
A key new feature is the PXI Express hybrid slot, which delivers support for both PCI and PCI Express signaling by taking advantage of available pins on the high-density backplanes. This creates a level of backward compatibility not available in even desktop PC card-edge connectors. In this hybrid peripheral slot, you can install PXI modules that use PCI signaling, as well as future high-performance PXI Express modules, in the same slot. However PXI modules that have implemented the J2 backplane connector require minor factory modification by the instrument vendor to allow operation in the PXI Express hybrid slot (the majority of PXI cards implement J2).
PCI Express uses the same driver and OS model as PCI, which ensures complete software compatibility among PCI-based systems (such as PXI) and PCI Express-based systems (for example, PXI Express). Neither vendors nor customers need to revise driver software or application software for PCI Express-based systems. Therefore, in addition to providing hardware compatibility through the hybrid slot, PXI Express systems also help engineers preserve existing software investments.
A MXI link consist of a PXI card and a PCI/CompactPCI card that provides a PC with direct control over the PXI backplane and connected cards. It's transparent to software. Throughout at maximum 208 MByte/s using fiber-optic or copper cabling. With fiber-optics the maximum length is 200 m.
PXI Express patent
Further standards based on PXI
- In September 2009 the PXImc standard was launched  Which enables multiple processors.
- pxisa.org - Overview of PXI
- pxionline.com - PXI Test and Technology magazine
- pickeringtest.com - PXImate Book provides overview of the PXI Standard, module types and cabling
- PXI / PXI Express / PXImc tutorial