Just as the “one size fits all” approach never really worked in clothing, no single test instrumentation approach fits all applications. To create systems that employ the best available instrument for each test procedure, some engineers are building hybrid systems that combine instruments from platforms such as PXI, LXI, GPIB, and VXI. Hybrid systems let engineers choose the best approach for each application, but unless such systems are crafted carefully, they may not achieve the performance the developer expects.

Hybrid systems combine various architectures, allowing developers to blend legacy and optimized equipment. Courtesy of National Instruments.

The design flexibility that a hybrid architecture allows is also an increasing draw for developers. “Test systems must adjust to new needs quickly,” said McDonell, “so to get more flexibility, developers are looking to hybrid systems.” He explained that the hybrid approach lets developers strike a balance between the optimized functionality of dedicated instruments and the ability of virtual instruments to quickly address new test requirements. McDonell noted that “the trend toward hybrid systems has evolved from being budget-driven to being driven by the need to keep up.”

Hybrid systems also free developers from the constraints of a specific architecture, especially when the needed functionality is not available in that architecture or does not meet the performance requirements. Keithley Instruments' multi-applications products marketing director Chuck Cimino said, “Optimization of a test system may dictate the specific format for a system element. High power or extreme precision may need a GPIB or LXI instrument, for instance, while high-speed digitization may need PXI.” Instrument makers acknowledge that no one instrument architecture is superior at everything.

While hybrid systems may be the answer for many applications, they can be difficult to optimize. Instrument vendors report that one of the main stumbling blocks to successful hybrid system development is the creation of artificial performance bottlenecks. NI's McDonell said, “People get into trouble when they fail to think about the performance inherent in the various buses. They can end up trying to do things like sending high-speed data from a PXI digitizer over GPIB to an arbitrary waveform generator.”

Chris Armstrong, Keithley's associate marketer for data-acquisition products, added, “It's almost always possible to get a hybrid system running, but the trick is in optimizing it. The synchronization you need to tweak the system may not be there unless you think ahead and plan it out. It's not a hacker's world.”

The issue of synchronization is a close second to performance as a stumbling block. Three methods for synchronizing test events are available: direct-signal, backplane-based, and time-based synchronization.

Direct-signal synchronization involves cabling directly from one instrument to another to provide trigger and clocking signals and is available on most instruments. Backplane-based synchronization uses a shared set of lines in the backplane for such signals, as in PXI and VXI systems. Time-based synchronization depends on each instrument having access to a time-of-day clock so users can initiate events based on a schedule; clock sources for test instruments include the IEEE 1588 network-based synchronization for instruments with Ethernet connections and clocks derived from the Global Positioning System (GPS).

Because not all architectures support all three types of synchronization, developers must consider synchronization as carefully as bus bandwidth when linking instruments in a hybrid design. Otherwise, they can end up with convoluted system cabling or will have to coordinate timed events with triggered ones by inserting delays. Both results lead to added complexity and reduced performance.

An additional hybrid challenge involves the system software. “Almost all test instruments need a driver or the like for remote operation,” said Agilent's Van Woerkom, “so you have to worry about versions and compatibility with operating systems, especially when you are mixing older and newer devices. It's not a slam dunk that they will all work together.”

Fortunately, developers of hybrid systems can take advantage of existing standards to simplify development and maintenance. On the software front, Woerkom pointed to the Interchangeable Virtual Instrument (IVI) drivers as one technology that makes test instruments more “plug and play.” NI's McDonell cited the Virtual Instrumentation System Architecture (VISA) specification as also helping to simplify software development.

Both IVI and VISA make it possible for programmers to write drivers that refer to a generic instrument (such as an oscilloscope) rather than to a specific model. Such abstraction not only simplifies test software development but also eases system upgrades by permitting the software to remain virtually unchanged when an instrument is replaced with a different model.

Vendors also provide resources that can help you build a hybrid system. NI, for example, maintains a library of drivers for more than 5000 instruments. Agilent offers an I/O Libraries Suite, which includes the Agilent Connection Expert tool that identifies all the ports in a system and loads the appropriate drivers. Agilent's I/O Monitor tool also helps identify when communications errors occur in a multivendor system. Keithley has developed the Test Script Processor language to help developers synchronize and automate test system behavior.

For ease of assembly, vendors have developed bridges and adapters that make it simpler to connect instrument architectures. Robert Rennard, president of the LXI Consortium, noted that there are now a variety of LXI adapters available for making GPIB, PXI, and USB connections in a way that makes the secondary architecture appear to behave like an LXI node. In addition, GPIB and Ethernet interface cards are available for PCs and PXI systems as well as for VXI, M-module, and other bus bridges.

Though the assembly and programming of hybrid systems has gotten easier, developers must still plan out their system design. Vendors recommend that developers begin by learning the basic performance characteristics and tradeoffs of each architecture they intend to use so they will be less likely to create artificial bottlenecks in the data flow. To that end, manufacturers have published application notes, white papers, and Web pages that give overviews of the various architectures (see "Resources" list at the end of this file).

The next step is to decide on the overall system architecture, including data flow and synchronization patterns. Keithley's Cimino recommended that developers start by defining the measurement requirements and then identifying the instruments, regardless of interface, that can meet those requirements. With that list of candidates in hand, Cimino says, developers can more accurately identify a system architecture that will meet their needs.

McDonell of NI—the company that initially developed the PXI specification—suggested that developers consider using PXI as the system architecture's foundation, adding other instrument buses as needed in a layered approach. He pointed out that PXI offers all three synchronization types, making it a logical center for coordinating test activity. The LXI Consortium's Rennard commented that in large systems a single controller may get overloaded, and so he recommended that such systems adopt a LAN backbone and segment their operation into multiple control zones, taking advantage of peer-to-peer communications to exchange information.

Whichever backbone structure is adopted, however, developers should try to keep things as simple as possible. “It's probably not a good idea to mix all the test architectures together in a single system,” said Agilent's Van Woerkom. “The more different types you add, the more unstable it becomes and the harder to manage.”

Despite the challenges, hybrid systems are rapidly becoming the best approach for many test requirements. They help leverage existing equipment and extend the lifetime of systems by supporting replacement of instruments without the need for reprogramming. Hybrid designs also open opportunities for embracing new capabilities as they become available.

“Companies are working to make sure that the different instrument architectures will work together as seamlessly as possible,” said Van Woerkom. “You don't have to get rid of your old equipment, or be afraid to try the new stuff.” 

Resources

Pickering Test
Comparison of VXI and PXI Modular Test Platforms
Comparison of SCXI and PXI Modular Test Platforms
www.pickeringtest.com/appnotes.html

IEEE
Integrating PXI with VXI, GPIB, USB, and LXI instrumentation

Adlink Technology
GPIB: Challenges and Potentials

Keithley
The Right Bus for the Job

Agilent 
Test System Development Guide
 
National Instruments
“Instrument Control Fundamentals” Web page (www.ni.com/icfundamentals)