Choosing the Right Test Platform

Scalable architecture spanning platforms unifies test strategy.

As engineers face expanding test challenges and shrinking budgets, the choice of test platform becomes increasingly important. Competitive corporations must invest in technologies that allow them to build better products faster, and at a lower cost.

Test platform decisions today affect the cost not only of current systems but also of future ones as fewer dollars flow to new development. A test platform must be right for both today and tomorrow in terms of performance, capability, cost and size.

For years, the choice of test platform was simple: GPIB. Now there are two more choices: VXI and PC-based data acquisition (PC-DAQ) systems. With VXI and products based on the VXIplug&play standards, a new, easy-to-use computer-based platform is available for test. Going in a different direction, PC-DAQ uses the internal PC expansion bus to connect instruments to the computer. PC-DAQ systems have become more popular as PCs and their peripheral expansion buses have become more powerful without increasing costs.

Each of these platforms—GPIB, VXI or PC-DAQ—has its merits and can solve a wide range of applications. Because test systems differ in size, capability and performance, you should choose platforms that meet your needs and are cost effective. Make intelligent decisions up-front so that when the short-term needs are fulfilled, the system and the development effort can be leveraged to future projects.

The Building Blocks

Before delving into a technical comparison of each platform, let’s look at the key building blocks of a test system: the controlling computer, the software and the instruments. Broken down further into functional elements, a test system consists of acquisition/stimulus, analysis and presentation capabilities (Figure 1). Every instrumentation platform supports this model; however, each platform distributes these building blocks differently between the computer and the instruments (Figure 2).

Optionally operated stand-alone, GPIB instruments completely integrate or embed acquisition/stimulus capability, intelligent processing for data analysis, and display and control buttons for the user interface.

VXI and PC-DAQ instruments cannot be controlled alone; they must work together with the computer to realize instrument functionality. In this architectural scheme, instruments and computer share system resources such as system memory, DMA and interrupts for acquisition/stimulus functions; the microprocessor and system memory for data analysis; and the monitor and graphics capability for user interaction. This symbiotic connection between instrument and computer creates a virtual instrument architecture for VXI and PC-DAQ platforms.

GPIB systems emphasize the instruments in a test system, whereas VXI and PC-DAQ depend heavily on the controlling computer and the software. In a virtual instrumentation system, the faster the data can be transferred and the more powerful the microprocessor to process and display it, the more test system capability can be transferred from the instrument to the computer.

All three test alternatives take advantage of innovations in the computer industry, resulting in higher performing test systems at the same or lower cost. But, because of the tight coupling of instruments and computer with VXI and PC-DAQ, these platforms derive greater benefit from these advances than do systems based entirely on GPIB. As computer performance increases, VXI and PC-DAQ test systems become inherently faster, increasing the capability of certain applications and also reducing test time.

Software Is Key

Software is a pivotal component in any test system. For systems such as VXI and PC-DAQ that rely heavily on the computer, software defines many of the features and capabilities of the entire system. The more capable and powerful the software, the better the test system.

Test software built upon industry standards ensures that your test development lives beyond a single system. Standard software tools and components provide consistent programming interfaces for each stage of current and future development.

Not only can the software be reused from one system to the next, but also the learning curves associated with new programming tools and application programmer interfaces (APIs) are shorter. Software standards protect your development investment so it can be reused on new systems.

Now that we have defined a framework for evaluation, let’s take a look at each test platform in more detail.

Functional Differences

Performance is one of the most important elements to consider when comparing test platforms. Test systems that can perform a job faster not only increase the range of possible applications, but also increase capacity, saving money. Although raw data throughput numbers are often quoted for each platform, overall system performance may be affected by factors other than the data rate of the connecting bus.

For GPIB, classic throughput rates exceed 1 MB/s, while HS488-capable GPIB controller interfaces and instruments top 7 MB/s. Using a string-based model, commands and data are passed between the instrument and the controller. The controller sends a string command to an instrument, and the instrument responds after it has performed the desired function. Most GPIB instruments use embedded intelligence to parse and interpret the incoming command strings. The GPIB architecture does not facilitate direct register manipulation of the instrument, so the connection between instruments and controllers remains tangential.

You can directly manipulate the registers of VXI and PC-DAQ instruments, yielding high-speed instrument control, centralized system intelligence and shared system resources. The advent of the PCI bus has boosted VXI and PC-DAQ performance to unprecedented levels. PCI-based VXI controllers or PCI-based computers with PC-DAQ boards capitalize on the high bandwidth PCI bus to eliminate the peripheral expansion bus as a potential bottleneck in a system.

VXI and PC-DAQ solutions based on the older 16-bit ISAbus were often hampered by its data throughput. The PCI bus erases the performance boundaries for these platforms with an overall throughput rate of 132 MB/s, with 32-bit bi-directional transfers.

While PCI and PC-DAQ systems based on the PCI bus do have greater burst rate numbers (80 MB/s for VXI vs 132 MB/s for PCI), the VXI specification defines signals explicitly for high-speed instrument control. These include backplane triggering, timing and prioritized interrupts. Special VXI local bus implementations for data transfer also provide a parallel data path that off-loads the VXI bus, increasing data throughput for the entire system.

VXI controllers and instruments that use DMA can efficiently transfer large blocks of data to conserve valuable bus bandwidth as the DMA controller—not the computer’s microprocessor—transfers the data. VXI controllers that take advantage of the PCI bus, the additional VXI instrument signals and DMA combine high-performance elements of the PC platform with extensions for timing, triggering and data transfer to create the highest performance instrument control platform.

System Size

PC-DAQ systems offer the smallest physical size for test systems because most, if not all, system components fit inside the computer. The smallest possible test configuration uses PCMCIA DAQ cards and laptop computers to further reduce test system size and make portable test systems viable. While offering the smallest size, PC-DAQ-equipped computers typically offer a limited number of expansion slots, which in turn limits the number of boards that can be plugged into a system.

Because of its instrument-on-a-card standard, VXI packs more instruments in a smaller area than GPIB rack-and-stack systems. Although much smaller than GPIB systems, VXI is typically larger than most computer-bound PC-DAQ systems. Most VXI mainframes house up to 12 instruments (with one slot used for the VXI controller). The VXI specification defines a standardized expansion mechanism such as MXI that allows you to add up to eight VXI mainframes without any loss in performance. The expandability and instrument density position VXI systems more toward larger test systems.

The Instruments

It is no secret that GPIB offers the widest selection and availability of instruments of the three test platforms. GPIB has had over 30 years of product development with more than 10,000 GPIB instruments to choose from. GPIB-equipped instruments range from general-purpose instruments such as digital voltmeters to the more sophisticated instruments designed for vertical markets such as the telecommunications industry. In fact, GPIB digitizers outpace VXI and PC-DAQ in regard to the higher-end A/D instruments with finer resolutions and faster sampling rates (Figure 3).

PC-DAQ also has many instruments available, but instruments solving specific or narrow applications are generally not the rule. Most PC-DAQ systems have a limited number of available expansion slots, so most PC-DAQ board vendors have produced multifunction boards for a wide range of applications. Software customizes the instrument capabilities to the task at hand.

Because data can be transferred to and from PC-DAQ boards at high speeds, PC-DAQ boards provide generic instrumentation functions, and the software processes the data to perform FFTs, peak detection, digital filtering, scaling or unit conversion. This flexibility allows sophisticated instruments to be built in software instead of hardware, saving money. Because of the diversity of GPIB instruments, however, there are likely to be applications that require a special type of instrument not available for PC-DAQ.

Combining elements of both GPIB and PC-DAQ, VXI instruments vary greatly in cost, capability and scope. With cost playing a greater role than ever before, as competitive pressures force companies to reduce the cost of test, VXI instrument vendors are compacting their instrument designs by leveraging off the virtual instrument architecture. They are packing more signal I/O capability into a single instrument module, delivering multifunction instruments that can be used in a wider range of applications.

Examples of such multifunction instruments are VXI Technology’s VMIP Series and National Instruments’ VXI-MIO Series, which provide analog I/O and digital I/O on a single C-size VXI module. These instruments follow the PC-DAQ model, but cost rather than the number of available expansion slots is driving this trend.

Software Development

Software development is responsible for a majority of the costs and development time for test. Most test engineers today save an enormous amount of time using software libraries called instrument drivers to control their instruments. Instrument drivers are software modules with intuitive high-level functions that manage instrument communication and control. All three test platforms take advantage of instrument drivers, but in different ways.

A first step in choosing instruments for a test system—whether the system is GPIB, VXI or PC-DAQ—is to determine if an instrument driver exists for a desired instrument. While instrument drivers lower development costs, they are not a mandatory component of the instrument. Some instrument drivers are supplied by the instrument vendor, while others are developed by third parties.

Although there are no multivendor formal standards for GPIB instrument drivers, there is an inherent, uniform structure that most instrument drivers follow.

In VXI, the VXIplug&play System Alliance has formally defined standards for instrument drivers. However, while a DMM from Company A may have the same capabilities as a DMM from Company B, the instrument driver may implement a completely different API. The lack of a consistent API across instrument drivers means that instruments cannot be interchanged without rewriting software.

PC-DAQ employs a more generic instrument driver approach derived from the concept of multifunction measurement and control devices. PC-DAQ software drivers, such as NI-DAQ^® from National Instruments, perform the same functions as traditional instrument drivers. But the NI-DAQ software works with all types of instruments, including those for PCMCIA, ISA, PCI and VXI, to deliver a software solution that spans these platforms. This level of standardization allows test engineers to interchange test components, facilitating systems that work with both VXI and PC-DAQ, thus creating a truly scalable test architecture.

Scalable Test

For years, end users sought reuse among software and hardware components for their test systems. De facto and formal software standards have facilitated reuse on a single platform, whether GPIB, PC-DAQ or VXI. However, as other groups within a company chose to standardize on different platforms, such as VXI for production test or the PC for design verification, corporate-wide reusability went out the door. To lower the costs for an entire company, a scalable test solution was clearly needed.

Since PC-DAQ and VXI components are now interchangeable, scalable test is a reality (Figure 4). You can develop test systems using National Instruments’ PC-based E Series instrumentation class boards and move this same software based on NI-DAQ to the VXI platform using the VXI-MIO Series without any software rewrite.

Because the software remains consistent between PC and VXI platforms, you can deploy a scalable test architecture spanning functional boundaries within an organization. The R&D engineer can test initial designs using a cost-effective PC-based system, the test engineer can reuse this software with a VXI system for production, and the service technician can reuse this same software with a laptop computer and a PCMCIA PC-DAQ interface.

Conclusion

From a short-term perspective, the choice of test platform ultimately depends on the application. Smaller size or portable applications may benefit from the compactness of PC-DAQ. Higher performance or larger systems can use VXI. GPIB offers the largest selection of instruments, which is important for systems that require specialized capabilities not found in either VXI or PC-DAQ.

Your test platform choice need not be mutually exclusive: GPIB, VXI or PC-DAQ can be used in the same system to build hybrid systems that combine the best of all worlds.

Because of the long-term implications of your current test platform choice, testing strategies must leverage industry standard technologies and facilitate hardware and software reuse, minimizing costs both today and tomorrow. As the entire test industry moves toward a virtual instrument architecture that takes advantage of new computer technologies, VXI and PC-DAQ will become the prevalent test platforms as cost, size and performance drive system requirements.

By choosing a scalable test architecture that spans test platforms, you invest in a set of tools and technologies that can unify a testing strategy throughout an organization, leveraging the work cost-effectively between groups and from one system to the next.

About the Author

James Kimery is the VXI Product Manager with National Instruments. He received his B.S. in electrical engineering from Texas A&M University in 1986 and an M.B.A. from the University of Texas at Austin in 1992. He is a member of the IEEE. National Instruments, 6504 Bridge Point Parkway, Austin, TX 78730-5039, (512) 794-0100.

	GPIB	PC-DAQ	VXI
Transfer Width	8	8, 16, 32 (expandable to 64)	8, 16, 32 (expandable to 64)
Throughput	1 MB/s+ (3-wire) 8 MB/s (HS488)	1-2 MB/s (ISA) 132 MB/s (PCI)	40 MB/s 80 MB/s (VME64)
Timing and Control Capabilities	None	None	8 TTL Trigger Lines 2 ECL Trigger Lines 10-MHz Clock
Instrument Availability	> 10,000	> 1,000	> 1,000
Expandability	Built-in using multiple interface cards	Available from third parties	Standardized using MXI
Size	Large	Small	Medium