Facing legacy issues in PXI systems
By Richard A. Quinnell, Contributing Editor -- Test & Measurement World, 3/1/2009 2:00:00 AM
In the 10 years since its introduction, PXI has seen its share of technical changes, including rising performance specifications for modules and the evolution of the PC-oriented hardware and software at PXI’s core. Applications have also changed, with many PXI systems becoming embedded parts of larger test and measurement systems. The resulting combination of long-term fixed installations and changing technology has raised a specter of legacy issues in PXI designs that test engineers will need to exorcise.
It’s not a particularly frightening specter. Maintaining compatibility has been a key goal of the PXISA (PXI Systems Alliance) as it has handled technical evolution throughout its history. Even major changes such as a move from the parallel bus of PXI to the serial bus of PXIe (PXI Express) have provided for the reuse of existing system components.
 Fig. 1 The PXI standard has maintained a high level of compatibility as it evolves, such as the definition of a hybrid card slot to accept both the new PXI Express as well as legacy CompactPCI and PXI modules. |
For the move to PXIe, the PXISA defined a hybrid system slot that would accept cards of either bus type (Figure 1). This hybrid slot allows developers to populate a system with older cards and then replace them with PXIe upgrades as needed without altering system configuration. Even software could remain unaffected; with PXIe, the bus change is transparent to the OS (operating system) and applications layers of software.
Despite these efforts, legacy issues are beginning to arise in PXI systems. In part, these issues stem from an increasing use of PXI as an embedded component of test and measurement systems. The intergeneration compatibility built into PXI allows virtually unlimited reuse of boards and applications software and simplifies the introduction of new capabilities when an engineer is reconfiguring a PXI chassis for various bench uses. When the system elements and configuration need to remain stable for many years as an embedded instrument, however, individual changes that have little impact on bench reconfigurations can accumulate to create significant problems.
Legacy concerns vary
The types of legacy concerns that arise depend on what the test engineer is trying to accomplish. Simply changing a connector to make a PXI card compatible with a hybrid slot has little or no effect on the card’s functionality (Figure 2). But when the goal is to maintain an older existing system essentially unchanged during a long installed life, engineers may find it difficult to obtain the technical support or replacement cards they need. The continual evolution and obsolescence of PC processors and OSs—particularly Windows—affect PXI-based systems, especially those that include a PCI or CompactPCI card.
 Fig. 2 Modifying an older PXI module (left) to make it compatible with a hybrid slot can be as simple as swapping a connector (upper right); most vendors will perform the swap for a modest charge. |
Parts obsolescence becomes a concern when identical replacements rather than functional equivalents are needed to avoid long and costly recertification testing, as in military systems. “The biggest issue we have is with embedded control processors on modules,” said Mike Dewey, senior product marketing manager at PXI equipment maker Geotest. “We are at the mercy of the Intel roadmap and can expect only a three- or four-year run life on a specific component.”
Board vendors can generally handle the situation by maintaining functional equivalence through processor generations. When exact component replacement is a requirement, however, or changes in clock speed and other processor timing create problems elsewhere in the system, test engineers may have a problem despite equivalent functionality. “Sometimes it is better to stick with what you have and go to the secondary market that is reselling older products,” said Dewey.
Although software doesn’t “wear out,” the obsolescence of an OS can create system-maintenance challenges. “Many [PXI] vendors support a couple of OS generations back,” said Matt Friedman, PXI platform manager at National Instruments, “but it is a moving target, and older versions lose support over time.” Geotest’s Dewey also pointed out that third-party applications software providers may also gradually drop support for older versions and OSs.
The support issue is also a concern for developers seeking to upgrade existing systems with minimal impact. Replacing a module in order to get higher performance or gain additional features may not be possible if the legacy system’s OS is unsupported.
“New module introduction becomes problematic under an older OS,” said Tim Carey, PXI product manager at instrument maker Aeroflex. “Our new products aren’t even engineered for Windows 95 or NT anymore, so we don’t know what their behavior would be in a legacy OS system.” Even if a new product supports a legacy OS, some new card features may not be accessible under that older version.
Upgrades create challenges
Developers contemplating upgrades to system hardware also face additional issues. An older card cage, for instance, may not offer the power mix that newer boards require. “Logic interfaces have moved from 5 V to 3.3 V,” noted Geotest’s Dewey. If an engineer moves a system to a new chassis to gain PXIe support, older cards that use the local bus will lose that backplane connection in a hybrid slot. Even if not using the local bus, older cards that include the J2 connector will not fit into a hybrid slot, although that can be readily corrected. “Most vendors will take their card back and swap out connectors to preserve their customer’s investment, even if it is several years old,” said Dewey.
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If it is the software that needs to be upgraded—for instance, to gain access to new board features or because of a new release for third-party application software—then older boards generate a different concern. “When updating the OS, developers need to make sure their vendor provides drivers for their legacy boards that support the latest and greatest,” noted NI’s Friedman. Also, developers will need to make sure the processors in older cards can handle the demands of the upgraded OS.
This interaction of hardware and software can quickly snowball, with one change triggering a need for additional changes elsewhere in the system. Engineers facing such an extended system upgrade may want to look beyond their immediate need to avoid repeating the experience.
“You need to take stock of the whole system status such as where in their life cycle all the components are and how many years the system will need to run, then ask suppliers their support policy and how long things have been in production,” said Dewey. “If it’s more than five years, they [the components] may not be available much longer. The last thing you want to do is make a whole system upgrade, then find out later that a few more parts became obsolete.”
Future-proofing design
As PXI increasingly becomes an embedded test element, such legacy issues will continue to grow in importance. Engineers creating new systems may want to take steps to mitigate these legacy concerns and future-proof their designs.
Aeroflex’s Carey pointed out, for instance, that the system controller is the technology element changing at the fastest pace, making embedded Windows-compatible processors riskier. Added Dewey, “If someone really wanted to get longevity out of their system design, they might want to use an external controller based on an industrial PC.”
Another factor to consider is the potential of PXIe. Not every PXI module will migrate to the standard, but many that need high performance eventually will. Developers should consider including hybrid slots in their test system to allow for such migration and should also make sure that the modules they choose are hybrid compatible.
In addition, developers may want to partition their system functionality among modules to simplify future upgrades. Digital technology evolves faster than analog, for instance, so there may be an advantage in separating signal conditioning from digitization steps.
“In our signal generator design, we have a synthesizer module and an RF output module with frequency multipliers,” said Aeroflex’s Carey. “We segmented it that way so that as requirements shift, only one part of the system changes. Customers upgrading from 3 GHz to 6 GHz, for instance, only needed to replace one module.” Too much segmentation can become a drawback, however. “We could have separated out the baseband, too, but then size, cost, and calibration became issues,” Carey said.
To help future-proof software, developers should ensure that applications code behavior is independent of processor changes. “If you use good programming practices, such as no hard timing loops, then moving to a faster processor is not a problem,” said Dewey. “You need to use development tools that help keep you independent of the processor speed.”
Developers might also want to structure code to take advantage of parallel processing. Processor technology has stopped evolving toward higher clock speeds for increased performance and is now moving to multiple cores on chip. The move to multicore can provide faster computing and can also change the way a test runs from sequential steps to simultaneous operations in parallel, but only if the code structure supports parallelization.
Moving away from the Windows OS family is also a possibility. For many applications, a real-time OS might be appropriate, especially if deterministic or mission-critical operation is important in the testing. For others, a switch to Linux might help prevent OS obsolescence from forcing legacy systems to change.
Finally, developers should consider getting expert advice. “Work with your vendor to find the correct components for your system that will have long working lifetimes,” said NI’s Friedman. “They will help address specific needs and recommend choices.” These may end up being vendor-specific, but with the right provider, the risk is small and the potential benefits great if your PXI system design is to have a long and productive life.
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