THE PXI industry standard has quickly gained adoption and grown in prevalence in automated test systems since its release in 1998. One of the key elements driving the rapid adoption of PXI is its use of PCI in the communication backplane.
Now, as the commercial PC industry drastically improves the available bus bandwidth by evolving PCI to PCI Express, PXI has the ability to meet even more application needs by integrating PCI Express into the PXI standard. To help ensure the successful integration of PCI Express technology into the PXI and CompactPCI backplanes, engineers within the PCI Industrial Manufacturers Group (PICMG), which governs CompactPCI, and the PXI Systems Alliance (PXISA), which governs PXI, have worked to ensure that the PCI Express technology can be integrated into the backplane while still preserving backwards compatibility with the large installed base of existing systems.
Enabling new applications
BY taking advantage of PCI Express technology in the backplane, PXI Express increases the available PXI bandwidth from 132 MB/s to 6GB/s for a more than 45X improvement in bandwidth while still maintaining software and hardware compatibility with PXI modules.
With this enhanced performance, PXI can reach into many new application areas, many of which were previously only served by expensive and proprietary hardware. For example, with PCI Express, a digitizer achieves a direct path to the CPU module, through either an embedded controller or a MXI controller to a PC, with a bandwidth of 1 GB/s.
This is approximately an 8X improvement over the throughput offered by the 32-bit, 33 MHZ PCI. Thus, with PCI Express technology, a high-resolution 16-bit IF digitizer or generator can potentially stream continuously to the CPU at bandwidths up to 500 MHz without bus limits or sharing bandwidth with adjacent modules.
Specifically, in automated test for the military and aerospace industry, the higher bandwidth available in PXI Express provides new solutions for many applications:
• High bandwidth IF instruments for communications systems test.
• Interfaces to high-speed digital protocols including LVDS-based proprietary protocols, FireWire, fibrechannel, and others.
• Large channel count data acquisition systems for structural and acoustic test.
• High-speed image acquisition.
Bringing PCI Express technology to CompactPCI and PXI
IN order to successfully integrate PCI Express into CompactPCI and PXI and still maintain backwards compatibility, the PICMG (www.picmg.org) and the PXISA (www.pxisa.org) are carrying out coordinated plans to ensure a smooth transition. Since PXI is based on the CompactPCI specification, these efforts to integrate PCI Express first began with CompactPCI Express in early 2004.
Defining the fundamental mechanical and electrical features of CompactPCI Express systems, the CompactPCI Express specification in turn defines the mechanical and electrical features of PXI Express systems.
Released June 27, 2005, the CompactPCI Express specification 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. In May 2005, work on the PXI Express specification began with a targeted passage in the fourth quarter of 2005. The PXI Express specification takes the CompactPCI Express technology and adds specifications for PXI compatibility, timing and synchronisation, and system software frameworks.
Because the CompactPCI/PXI Express backplane integrates PCI Express while still preserving compatibility with current PXI modules, users benefit from increasing bandwidth with backwards compatibility with existing systems. PXI Express specifies PXI Express hybrid slots to deliver signals for both PCI and PCI Express. With PCI Express electrical lines connecting the system slot controller, either an embedded or MXI controller, to the hybrid slots of the backplane, PXI Express provides a high bandwidth path from the controller to backplane slots.
Using an inexpensive PCI Express-to-PCI bridge, PXI Express provides PCI signalling to all PXI and PXI Express slots to ensure compatibility with PXI modules on the backplane. With the ability to support up to a x16 PCI Express link, plus a x8 link, the system controller slot provides a total of 6 GB/s bandwidth to the PXI backplane representing a more than 45X improvement in PXI backplane throughput.
By taking advantage of the available pins on the high-density PXI backplane, the PXI Express hybrid slots are capable of delivering signals for both PCI and PCI Express. In doing so, these PXI Express hybrid slots provide backwards compatibility that is not available with desktop PC card-edge connectors, where a single slot cannot support both PCI and PCI Express signalling. Thus, the hybrid slot allows you to install a PXI module that uses PCI signalling or a future high-performance PXI Express module that uses PCI Express signalling.
Maintaining software compatibility
IN addition to providing hardware compatibility through hybrid slots, PXI Express systems also provide software compatibility so that engineers can preserve their investment in existing software. Because PCI Express uses the same driver and OS model as PCI, the specification guarantees that engineers have complete software compatibility among PCI-based systems, for example PXI, and PCI Express-based systems, such as PXI Express. As a result, both vendors and customers do not need to change driver or application software for PCI Express-based systems.
By maintaining software compatibility between PCI and PCI Express technology, the specification drastically reduces cost for vendors and integrators to insert new PCI Express technology into existing test systems. With hardware compatibility provided by the hybrid slot and software compatibility, the cost of adding PXI Express technology is minimal.
Providing additional timing and synchronisation features
PXI Express not only retains the timing and synchronisation features of PXI, but it also adds several new synchronisation features by taking advantage of the existing differential connectors required in PXI and technological advances that provide higher performance, low-cost differential signalling.
Building on these existing capabilities in PXI, PXI Express provides the additional timing and synchronization features of a differential system clock, differential signalling, and differential star triggers. By using differential clocking and synchronisation, PXI Express systems benefit from increased noise immunity for instrumentation clocks and the ability to transmit at higher frequency clocks. In addition to allowing engineers to improve the performance of the system, high-frequency clocks also match well with modern processes and allow lower cost products to remove clock multiplication circuits.
The Future of PXI
WHILE the integration of PCI Express technology into PXI will allow PXI Express to reach new applications, many existing PXI applications will not benefit from the enhanced performance of PXI Express.
For example, hardware such as digital multimeters (DMMs), switches, industrial I/O, bus interfaces, and many mainstream generators and analysers will not benefit from the additional backplane bandwidth. Thus, one of the most valuable aspects of the PXI Express specification is its ability to route both PCI and PCI Express signalling to new slots. As result, engineers should not expect instrument manufacturers to redesign all current boards for PXI Express; rather, many instrument manufacturers will continue to base PXI products on PCI signalling since the current PCI architecture serves the need and PCI signalling is provided to all slots.
Expected timeline for PXI Express
THE PXISA anticipates passage of the PXI Express specification in the fourth quarter of 2005.
As a result, you can expect to see the first PXI Express chassis, controllers, and modules featuring hybrid peripheral slots to enter the market in 2006. With these new high performance PXI Express products, engineers will benefit from solutions for many new applications. At the same time, the compatibility of PXI Express also guarantees the growth of the current PXI architecture with new product releases.
 CompactPCI Express PICMG EXP.0 R1.0 Specification, PCI Industrial Manufacturers Group, July 27 2005.
 PXI-5 PXI Express Hardware Specification Revision 0.5, PXI Systems Alliance, July 15 2005.
 G. Caesar, PXI Embraces New Commercial Standards, Instrumentation Newsletter, Q2 2005. http://ni.com/news/inst_news_q2_05.htm.
 J Titus, PXI gets the Express Treatment, ECN, July 1 2005. www.ecnmag.com/article/CA623714.html.
 L Gutterman, PXI Express?, PXI Technology Review, Spring 2005. www.pxionline.com/columns/PXISA/.
This whitepaper was released as part of NIWeek 2005, held in Austin, Texas, USA from 15-18 August by National Instruments .