This month is the 10th anniversary of the VXI platform, 10 years since the formative VXIbus Consortium meetings that spawned the VXI concept. As we pass this milestone, it’s time to reflect on how VXI got where it is today, and how it will address the test needs of the future.
A Platform With a Calling
VXI was developed to answer a call for tools that were efficient, compact, and designed to capitalize on the growing power of the personal computer. Did the platform succeed in meeting these demands? At this writing, the answer is a decisive yes, but the path to today’s implementation was not a direct one.
Like most new technologies, VXI did not spring fully formed from the drawing boards. Both hardware and software experienced evolutionary changes over the years. But there were some key turning points that transformed the platform, building it into a more robust, versatile architecture. At the same time, these changes broadened VXI’s appeal, and made it the widely used platform it is today.
First-Generation VXI Took Off
With a body of newly minted VXI standards on the books in 1987, VXIbus Consortium members faced the challenge of getting actual products out the door. Many of these vendors were well-established instrumentation companies with broad lines of GPIB and stand-alone benchtop instruments. So the natural solution was to port existing GPIB instruments to VXI—to repackage the circuitry into the new modular format, retaining as much functionality as possible.
This strategy served both users and vendors well:
It satisfied the demand for users in the aerospace and laboratory research markets.
It equipped aerospace and laboratory research users with proven tools. The new VXI scopes, meters, and signal sources offered the reliability and measurement quality of their benchtop predecessors.
The ported VXI instruments helped vendors understand the needs of their markets: where VXI instruments were used, how they performed, and what improvements were needed.
There were some shortcomings. VXI’s high-speed backplane was underutilized. And many a ported VXI product still included the architectural features that made for a good hands-on instrument but not the best possible computer-controlled system component. As a result, some important potential areas of throughput improvement were not embraced by repackaged benchtop instruments.
Dedicated Modular Platform
The next step was to develop a generation of designed-for-VXI instruments that delivered the full advantage of the computer-based architecture. After all, VXI was conceived as a high-throughput manufacturing platform. By the early ‘90s, such second-generation tools were appearing in quantity.
The second generation of VXI tools benefited from the lessons learned from ported instruments. Backplane architecture matured, hardware configuration jumpers were replaced by autoconfiguration hardware, and innovations like Fast Data Channel brought much-improved throughput over the earlier byte-at-a-time GPIB scheme. Some of the best of these new VXI instruments still shared technology with their benchtop cousins, but discarded the GPIB and manual-control baggage and operated at computer bus speeds.
These second-generation instruments greatly improved VXI performance. Feeds and speeds increased. Digitizers, a benchmark of VXI achievement, reached the 1-GHz realm, and signal generators, DMMs, and counters showed proportionate improvements. Dedicated instruments, such as SONET test sets and avionics bus testers, emerged to address application-specific market needs.
VXI instruments became significantly smarter. Individual instruments carried out signal and data processing locally. These smarter VXI instruments also were designed to be aware of each other, sharing information about triggers, interrupts, and test results. Both of these improvements diverted time-consuming tasks from the system controller, reducing its workload.
The latest VXI hardware development is not so much a new generation as a logical evolution of the VXI concept: a continued reduction in size combined with improvements in performance. A new class of multifunction VXI instruments has appeared. Each of these incorporates several instruments into one VXI module. Some are niched into narrow applications, while others are more general-purpose in nature.
For example, the Tektronix VX4101 integrates a counter, a DMM, and a switch controller in one package. Figure 1 depicts a C-size, single-width multifunction module.
Software Is Key to Ease of Use
Market pressure keeps everything dynamic, and VXI is no exception. In the early ‘90s, with the VXI platform stabilized and accepted, users moved their wish list to the next priority—ease of use.
Market studies exposed particular concerns with interoperability and the complications of integrating the software and programming VXI systems. The original VXIbus Specifications had concentrated on the hardware, treating software requirements with a bit more latitude.
As a result, the nonproprietary mandate of the VXI architecture became diluted. Consequently, software from vendor A might not talk to a controller from vendor B, which might not work correctly with an application package from vendor C, and so on.
The solution once again came from the cooperative efforts of the VXI community. The new VXIplug&play Alliance was formed in September 1993, by VXIbus Consortium members, other interested vendors, and users. This new organization’s charter was nothing less than full resolution of VXI software interoperability issues.
The result of the alliance’s effort was a complete restructuring of standards governing the VXI software environment. The VXIplug&play standard lays out guidelines for every important aspect of VXI software and hardware integration.
VXIplug&play‘s Virtual Instrument Software Architecture (VISA) provides a foundation for instrument, controller, and test-program interaction. Internally, VISA is the communication I/O layer of software between programming tools and hardware. From the user’s perspective, VISA provides a standard set of function calls and data structures compatible with every instrument’s software. As the underpinning for standardized soft front panels and instrument drivers, VISA is a key element of VXIplug&play interoperability.
VXIplug&play also defines operating system and programming frameworks. These give every popular operating system a place in the VXI domain. The choice of frameworks includes the Windows 3.1-based WIN framework as well as its WIN95 and WINNT descendants plus the HP-UX and SUN frameworks for Hewlett-Packard and Sun workstations, respectively.
The cornerstone of program development within the frameworks is a host of C source code-compatible languages; including Microsoft Visual C++, Borland C++, and LabWindows/CVI from National Instruments. The frameworks also support dynamic link libraries-compatible languages, specifically Visual Basic, HP VEE and NI LabVIEW.
In the past, VXI users were limited to the operating systems their instruments were designed for. Now, frameworks provide programming language interoperability, irrespective of the origins of the instruments.
For most VXI users, VXIplug&play’s most engaging traits are its standardized soft front panels and instrument drivers. Together, these provide a fast and reliable means of getting an instrument up and running in a system. Where system integrators once had to concoct their own instrument checkout routines (and often create a lot of low-level code to do so), soft front panels put up an on-screen representation of the instrument.
Given this, it is easy to exercise instruments interactively. In the past, access was not nearly as straightforward, sometimes turning routine programming procedures into lengthy troubleshooting sessions.
Instrument drivers also have matured along with other aspects of the VXI architecture. In the now-standardized realm of VXIplug&play, drivers are key differentiators among VXI vendors these days.
Much as the makers of benchtop instruments boast of measurement features, easy-to-use controls, and displays, drivers make a VXI instrument’s reputation. VXI manufacturers strive to create drivers that will set their products apart from those of the competition. This is an area where the user benefits from the leapfrog game of technological advances.
Perhaps most importantly, drivers can grow and evolve over time, enhancing instrument performance and capabilities. Figure 2 shows a representative VXIplug&play soft front panel.
Today, even the Internet plays a role in VXI. Many VXI vendors have established web sites from which users can download the latest instrument drivers, catalogs, and documentation at no cost. No longer must users wait for a software update from the manufacturer. The alert VXI user can monitor web sites and keep pace with new driver versions, bug fixes, software updates, and the latest product information.
What About the Cost of VXI?
When VXI first came to market, users were willing to pay a premium for the compactness and performance that the new platform offered. But instrumentation users are a cost-conscious lot. Before long, market pressure mounted to reduce the cost of acquiring and using VXI.
All of the historic trends have worked in concert to resolve the VXI cost issue. Today’s VXI is a far more cost-effective platform than that which emerged in 1987.
On the hardware side, many of the new VXI instruments introduced over the last couple of years offer higher performance at much lower cost than their predecessors. In some cases, the VXI instrument is the least expensive tool in its performance class—even when benchtop instruments are included. Recent introductions in Slot-0 modules, mainframes, switches, and other functions also have boosted performance while cutting prices.
Multifunction modules offer another dimension of cost savings. Thanks to economies in packaging, interfacing, and device integration, some multifunction module configurations actually may cost less than the equivalent functions bought separately. Others are priced at about the same level as separate modules, but they save precious mainframe space and integration time.
In the software realm, VXIplug&play has had a profound impact on system cost—not because it reduces the cost of the hardware, but because it dramatically decreases the effort required to integrate, use, and program VXI systems. This reduces labor costs for integration and programming, and minimizes project slippages.
Conclusion
VXI has come a long way in 10 years. Today’s VXI instrument performance keeps pace with, and in some cases, surpasses the best benchtop tools. With more than 80 vendors and 1,000 products in the marketplace, VXI offers a solution for almost every computer-controlled test need. And with the powerful, open VXIplug&play software environment, VXI has become the flexible, easy-to-use tool that its creators envisioned.
About the Author
David Haworth is a member of the Tektronix VXI Product Line Marketing group and has been with the company since 1980. He is one of the founders of the VXIbus Consortium, SCPI Consortium and the VXIplug&play Systems Alliance, and represents Tektronix on the VXIbus Consortium Board of Directors and the VXIplug&play Systems Alliance. Mr. Haworth also is serving his seventh term as the VXIbus Consortium president and chairman of the Board. Tektronix, P.O. Box 500, M/S 39-122, Beaverton, OR 97077-0001, (503) 627-3127.
Copyright 1997 Nelson Publishing Inc.
June 1997