PXI Express Increases Bandwidth 45X, Preserves Compatibility

PXI has rapidly grown to be one of the most successful industry-standard platforms for measurement and automation. As the commercial PC industry evolves PCI into PCI Express to drastically increase the bandwidth of the bus, PXI has the opportunity to serve even more applications by adopting this new technology. For several years, engineers within the PCI Industrial Computer Manufacturers Group (PICMG) governing CompactPCI, as well as the PXI Systems Alliance (PXISA) governing PXI, have been working to ensure that PXI and CompactPCI backplanes can integrate PCI Express technology, while preserving backward compatibility with the huge installed base of existing systems.

Integration of PCI Express into CompactPCI and PXI
The PICMG (picmg.org) andPXISA (pxisa.org) are executing coordinated plans 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.

PXI Express specification work began in May 2005, with targeted passage in the fourth quarter of 2005. PXI Express incorporates CompactPCI Express technology with enhancements for PXI compatibility, timing and synchronization, and system software frameworks.



Figure 1 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 6 GB/s bandwidth 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.




As shown in Figure 2, the compatibility of the hybrid slot is made possible by retaining the PCI signaling and PXI timing and synchronization signals of PXI today in the P1 and XP4 connectors (respectively). In addition, the new XP3 connector provides connectivity for x8 PCI Express, as well as pins for additional timing and synchronization.

Software Compatibility
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, PCI 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.





New Applications
Three main market segments have broadly adopted PXI systems -- test and measurement, real-time control and simulation, and industrial data acquisition. In all areas, PXI has seen tremendous industry expansion, with vendors investing heavily in new PXI product introductions. For example, since 2003, PXI vendors have released, on average, more than one new PXI product per week. Each new product expands PXI platform capability, bringing PXI into new areas of performance and lowering the PXI system cost to drive broader industry adoption.

The increased PXI backplane bandwidth -- from 132 MB/s to 6 GB/s -- fueled by PXI Express represents a more than 45X improvement in bandwidth while maintaining software and hardware compatibility with PXI modules. This new performance is expected to extend PXI into new application areas, many of which are currently served by only expensive and proprietary hardware. For example, a digitizer based on PCI Express will have a 1 GB/s direct path to the CPU module (either embedded or MXI to a PC), which is approximately 8X the throughput of 32-bit, 33 MHz PCI today. To translate this into real measurement terms, a high-resolution 16-bit IF digitizer or generator could potentially stream up to 500 MHz bandwidth continuously to the CPU without bus limits or sharing bandwidth with adjacent modules.

PXI Roadmap
While PXI Express expands PXI to serve new applications, many existing PXI applications do not benefit from new PXI Express performance. Specifically, hardware such as digital multimeters (DMMs), switches, industrial I/O, bus interfaces, and many mainstream generators and analyzers do not require additional backplane bandwidth. For this reason, the most powerful aspects of the PXI Express specification are the compatibility and increased bandwidth. Because of the compatibility feature, do not expect instrument manufacturers to redesign current boards for PXI Express. Instrument manufacturers will continue to base many existing and future PXI products on PCI signaling because the current PCI architecture completely serves the need and all slots have PCI signaling.

PXI Express Timeline
PXISA has announced a fourth quarter 2005 target for PXI Express specification passage.
In 2006, expect to see the first PXI Express chassis, controllers, and modules enter the market, featuring the new hybrid peripheral slots. These will provide a solution for new applications and new high-performance PXI Express modules not possible today. However, the strong compatibility of PXI Express ensures that the current PXI architecture will continue to grow with new product releases and applications.

Greg Caesar
PXI Product Manager
greg.caesar@ni.com


View a complete technical tutorial on the CompactPCI Express and PXI Express specifications.

This article first ran in the Q4 2005 issue of Instrumentation Newsletter.

 

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