The Peripheral Component Interconnect (PCI) bus has emerged as the universal I/O bus standard for desktop computers. In addition to the leap in performance PCI has delivered, the leverage this unifying standard has given to the computer industry is significant. The benefits of mainstream PCI technology now extend into form factors targeted at industrial and instrumentation applications.
CompactPCI, for example, with its rugged industrial form factor, is a fast-growing standard for industrial computers. Now, a formal specification for a new instrumentation architecture called PXI (PCI eXtensions for Instrumentation) extends CompactPCI with instrument-specific features to create a platform for modular instrumentation at PC prices.
The motivation for the PXI specification is quite simple. Today’s test challenges demand instrumentation at PC prices. From the test engineer’s standpoint, test complexity is increasing, customers are demanding higher quality, profit margins are decreasing, and time to market is more important than ever before.
In short, engineers today must test more for less money and in less time. The PXI specification leverages the mainstream technologies of the multi-billion dollar PC industry to increase performance and flexibility, decrease costs, and improve ease of use for instrumentation applications.
What is PXI?
The PXI specification defines a modular instrumentation platform that also is a mainstream PC. The only difference between PXI and mainstream desktop PCs is that the computer and plug-in instrument modules are repackaged in an industrial form factor with numerous expansion slots (Figure 1). Unlike desktop PCs, the PXI specification designates system-level guidelines for clean, reliable power; forced air cooling; electromagnetic and environmental compatibility, and multi-module timing and synchronization.
PXI System Components
In terms of hardware, the PXI specification defines a standard chassis (Figure 1). The chassis contains the power supplies, cooling, and slots for installing modules. The PC-compatible computer is a PXI module installed in the left-most slot of the chassis. The PXI instrument modules are installed in the other seven slots to the right. The PXI backplane inside the chassis provides a standard PCI bus interconnection between the computer and instrument modules.
Once the computer is installed, the PCI bus is connected via the PXI backplane to the other modules. In other words, installing a PXI instrument in the chassis is exactly the same, from a software standpoint, as installing a plug-in card into a desktop PC. PXI devices are recognized automatically and configured as standard plug-and-play PCI devices by the standard Windows device manager and run the same device drivers and software as their desktop PCI counterparts.
PXI Modules
The PXI specification defines two module sizes: 3U and 6U (Figure 2). These are the same size as A- and B-size VXI, respectively.
Interoperability among PXI-compatible products and standard CompactPCI products is an important feature. For example, you may want to use a standard CompactPCI network interface card in a PXI chassis or a PXI-compatible plug-in card in a standard CompactPCI chassis.
The PXI Backplane
Because the PXI specification calls out the industry-standard PCI bus, PXI offers the same performance features defined by the desktop PCI specification. This includes 32- and 64-bit data transfers with 132 Mbytes/s (32-bit) and 264 Mbytes/s (64-bit) peak data rates, transparent system expansion via PCI-PCI bridges, and plug-and-play operation.
In addition to the standard PCI bus, the PXI specification describes instrumentation-specific signals on the PXI backplane. Like VXI, these signals include a 10-MHz common reference clock, eight bused trigger lines, and a daisy-chained local bus.
The PXI specification defines a star trigger bus, taken directly from D-size VXI, which offers high-performance synchronization features. If you need advanced timing features, an optional star-trigger controller module can be installed in the system.
The PXI specification leverages mainstream, off-the-shelf software, unlike both GPIB and VXI which waited many years after their introduction to address software issues. Vendors who already offer desktop PCI products only have to port their hardware to the PXI form factor. The software is already done.
A PXI controller must be 100% PC compatible and fully support Windows 95 or Windows NT and all off-the-shelf PC software, including Microsoft and Borland C++, Visual Basic, and National Instruments’ LabVIEW and LabWindows/CVI. PXI requires that all peripheral modules ship with plug-and-play Windows device drivers or VXIplug&play-style instrument driver software.
PXI systems that control external GPIB, VXI, and serial instruments should use the VISA software standard. The VISA standard facilitates using PXI, GPIB, and VXI in hybrid systems and paves the way for GPIB and VXI vendors to offer PXI versions of their products with minimal investment in software.
Where Does PXI Fit?
PXI fills the void that exists between low-cost desktop PCs and relatively more expensive GPIB and VXI-based test systems. For example, a state-of-the-art Pentium multimedia extension (MMX) desktop PC with three PCI expansion slots costs between $2,000 and $3,000 today. The entry-level cost for VXI with a comparable embedded VXI controller and 13-slot chassis is over $10,000.
The entry cost for PXI with an eight-slot PXI chassis and a fully loaded embedded Pentium MMX computer running Windows 95/NT fits in the middle at $5,000. This cost also compares favorably to existing industrial computers on the market today.
Making PCs Better
PCs already are popular in test systems. They are the most common platform for controlling industry-standard GPIB and VXI instruments. Another aspect of the PC, however, is fundamental in the creation of the PXI specification. The PC with plug-in instrument boards is a popular, low-cost alternative to traditional GPIB and VXI instruments and test systems.
In just the past year or so, the mainstream PC crossed an important threshold with respect to instrumentation. Virtually every aspect of today’s PCs has surpassed the capabilities of most dedicated instruments. This includes the data transfer rates of PCI, the processing power of the latest Pentiums, the speed and resolution of graphics displays, and the mainstream familiarity and ease of use of Windows software. In other words, instruments built today with a PC may actually surpass dedicated traditional instruments in terms of performance, flexibility, ease-of-use, and low cost.
If you want to take advantage of PC-based instrumentation but also want your investment protected by an industry-standard specification, PXI can help. The PXI specification is an open industry standard that spells out how any vendor or user can build modular, interoperable instruments with PC technology.
The specification also includes strict environmental requirements to ensure that PXI instruments and systems can withstand harsh environments common in many test areas. With more expansion slots than desktop PCs, PXI systems can address larger and more complex applications. And since PXI combines the mainstream PCI bus with instrumentation features borrowed from VXI, PXI instruments can range from simple to sophisticated.
Today, with the widespread adoption of PCI and Microsoft’s leadership, the computer industry has truly delivered on plug-and-play interoperability and ease of use. For instrumentation software, the most popular test programs run on Windows platforms. PXI follows this trend because it is a PC and runs the same software and drivers as desktop PCs.
Where Does PXI Fit Into Test?
PXI and Mainstream PCs
While PXI supports mainstream PC technology, mainstream PCs will remain a viable choice for many test applications. PXI offers benefits over desktop and laptop PCs, but these come at a slightly higher price. For example, while some PC users may not need PXI’s benefits, it is an attractive alternative if you already are using industrial or rack-mount computers. If you are a PC user, the move to PXI is a hardware-only investment because existing software automatically runs as is on PXI.
PXI and GPIB
Today, GPIB is the most popular instrument option. GPIB offers a huge variety of instruments, familiar software and development tools, and a large installed base of successful applications and users. Since a PXI system is merely a ruggedized PC with many expansion slots, you can use PXI with a GPIB interface board as the GPIB system controller and existing test software and development tools.
PXI and VXI
VXI has generated a lot of excitement over the past decade. When introduced, many predicted it ultimately would surpass GPIB as the mainstream platform of choice.
While VXI grows, GPIB continues to outsell VXI by a wide margin, partially because many mainstream users perceive VXI to be relatively expensive. In recent years, the cost of VXI has lowered, and more is sure to come.
But the reality is that VXI’s strength is in high-end test applications. If you need VXI’s capabilities, there is no better platform, and it delivers the value.
Numerous VXI vendors recently announced products that use proprietary submodule approaches to package multiple functions inside a single VXI module. This indicates that many of today’s most common instrument functions can fit in the smaller PXI format. Some high-end instruments, however, will be available only in VXI because vendors always will be able to package more functionality into the larger VXI form factor.
While PXI can accommodate many of today’s most common test applications, most of the highest-end applications will continue to be best served by VXI. As with GPIB, mixed systems of PXI and VXI can be viable test solutions. In addition, because both platforms use the same backplane timing protocols, a carrier module can allow multiple PXI submodules to be installed directly into a VXI system.
Finally, the low prices of the PXI should result in more people embracing the modular instrument approach. Many of these users, once comfortable with the concept, can apply their experience with PXI into the VXI platform for their higher-end test needs.
Conclusion
Just over 20 years ago, GPIB came into existence as a multivendor instrumentation standard. Ten years ago, VXI emerged as a multivendor standard for high-end modular instrumentation.
Over the past decade, the mainstream PC has played an increasingly important role in instrumentation. Today, the direct application of the PC as the core technology for instrument and test applications has been formalized by the PXI specification. This new PC-based instrumentation platform can create opportunities for both vendors and users to take advantage of modular instrumentation at PC prices.
About the Author
Ron Wolfe, the market strategist at National Instruments, has been with the company since 1982. He also represents National Instruments on the VXIbus Consortium Board of Directors and in the VXIplug&play Systems Alliance. Mr. Wolfe, the author of numerous articles on instrumentation topics and papers presented at conferences worldwide, received a B.S. degree in electrical engineering from the University of Texas. National Instruments, 6504 Bridge Point Parkway, Austin, TX 78730-5039, (512) 794-5466.
Copyright 1997 Nelson Publishing Inc.
October 1997