Using Hybrid Test Systems
Utilizing a layered architecture is key to optimization
A common test system management challenge is adding new technologies while balancing longevity and scal-ability. Developing a system that uses PC
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FIGURE 1 : Hybrid systems maximize test equipment through software
technology as the core gives you the ability to optimize your system by incorporating multiple buses to form a hybrid multi-platform sys-tem. Matching the right software and modular hardware is key to building successful hybrid test systems. With a well-defined modular architecture, hybrid systems help you integrate newer technologies with existing hard-ware, so you can upgrade system com-ponents as needed to maximize your investment.
Hybrid systems combine test and measurement components from modu-lar instrumentation platforms such as PXI and VXI and stand-alone instru-ments that connect externally across GPIB, USB and LAN. Each of these buses provides unique benefits depend-ing on your application requirements and the connectivity options available on your instrument. For more infor-mation about these instrumentation buses, refer to Table 1
Integrating multiple buses to form hybrid systems
Figure 1 shows one example of a hybrid system, but any number of combinations is possible. In the dia-gram, the nerve center of the system is a PC — either a standard PC or an embedded PXI controller. The instru-ments that connect externally across ments themselves include PXI and PCI plug-in devices, as well as traditional instruments connecting to the PC over GPIB, USB and LAN.
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TABLE 1 : Overview of instrument communication buses
The key to creating and maintaining a hybrid system is implementing a sys-tem architecture that natively accom-modates multiple bus technologies and uses open, multivendor hardware platforms. With the proper computer platform, driver, application software and test system management software, you can combine the strengths of many types of instruments, including legacy equipment and specialized devices.
In a hybrid system, it is the PC and software that pull together the various instrumentation hardware components into a single system. You can incorporate your existing/legacy test routines into the new system with minimal rework, introduce future test routines into the system without a complete system redesign, and sim-plify replacement of individual instru-ments and I/O devices.
Developing a hybrid system architecture
Using a layered architecture with modularized hardware and software increases the longevity of your system and helps you to make changes with minimal effort. Figure 2 shows an example of a five-layer architecture, with well-defined industry-standard buses and platforms separated from common application programming interfaces. This simplifies the integra-tion of multiple platforms and eases maintenance and upgrades by requir-ing only minor changes in specific lay-ers, as opposed to redesigning the sys-tem to accommodate new components.
The architecture starts with the device I/O layer, which contains the individual instruments based on multiple instrumentation buses. The computing layer includes the embed-ded and remote controllers, which connect to various instrumentation buses for modular instrument control. The measurement and control services layer features the hardware and instru-ment drivers that bridge the hardware to software. This layer is often over-looked, yet it is critical in providing productivity and longevity,
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FIGURE 2 : Five-layer system architecture for developing test systems
because it supplies the essential link between hardware and software. For example, the NI Instrument Driver Network contains the industry’s largest source of instrument drivers for more than 5,000 instruments from over 200 different vendors for NI LabVIEW, LabWindows/CVI, and Measurement Studio software.
The fourth layer, the application layer, is composed of the individual test routines written in programs such as the LabVIEW graphical develop-ment environment. The highest level is the system management layer, which provides a framework to call the test programs as well as log data, generate reports and manage users. For system longevity, it is important that this layer provide flexibility to connect to many different tools and existing sys-tems. For example, with NI TestStand, you can develop, manage and execute test sequences written in any program-ming language.
The software framework, the top three layers in the five-layer architec-ture discussed above, simplifies the integration, configuration and pro-gramming of multiple buses. By follow-ing this type of architecture, you can take advantage of specific bus strengths to meet all of your application needs.
Choosing the right bus
When considering the technical merits of different instrumentation buses, bandwidth and latency are two of the most important bus character-istics. Bandwidth measures the rate at which data is sent across the bus, typi-cally in megabytes per second. A bus with high bandwidth is able to trans-mit more data in a given period than a bus with low bandwidth. Bandwidth is important in applications such as complex waveform generation and acquisition, as well as RF and commu-nications applications.
Latency measures the delay in data transmission across the bus. By anal-ogy, if you compared an instrumen-tation bus to a highway, bandwidth would correspond to the number of lanes and the speed of travel, while latency would correspond to the delay introduced at the on and off ramps and stoplights. A bus with low (mean-ing good) latency would introduce less delay between the time data was transmitted on one end and processed on the other end. Latency, while less observable than bandwidth, has a direct impact on applications where a quick succession of short, choppy commands are sent across the bus, such as in handshaking between a digital multimeter and switch, and in instrument configuration.
However, according to the article “The Myth of the ‘Ideal Bus,’” pub-lished in the November 28, 2005, issue of Industrial Control DesignLine, there is not one bus technology that is ideal for all applications. Each bus technol-ogy has its own strengths, weaknesses and appropriate uses. Table 2 compares the features of several different buses.
Case study: High-channel-count hybrid test system
An excellent example of employing the strengths of different bus tech-nologies in a hybrid system is Boeing’s Quiet Technology Demonstrator (QTD) project. Boeing’s objective was
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TABLE 2 : Bus Technology Feature Comparison
to reduce the noise of commercial jetliners during takeoff, landing and sustained flight. The project required a distributed system architecture that was low-cost and scalable to 1,000 channels or more while still maintaining tight timing and synchronization between channels. During stage one of the QTD project, Boeing used VXI systems to accomplish its task. The VXI system limitations were channel count and bandwidth, timing and synchroniza-tion, cost per channel, and significant time required for data retrieval. For the second stage of the project, Boeing decided to deploy a new system that would address these limitations.
Boeing engineers created a high-end, low-cost system that distributed the data acquisition across multiple PXI chassis while tightly synchroniz-ing all channels. They connected each PXI chassis through gigabit Ethernet to a central host computer. They also had the ability to expand their system to an unlimited number of channels. According to Boeing Technical Fellow James Underbrink, “With this new sys-tem, not only were we able to improve the capabilities of the individual acquisition channels, but we also achieved a five-to-one reduction in the amount of cable required and cut the cost of microphone systems by 30-to-one for flyover test applications.”
By taking advantage of hybrid sys-tems, you can integrate new buses into an existing test system to help balance design considerations, lever-age the various technologies available, and extend the life of your system. Choosing the right software and modular hardware core and develop-ing your system based on a layered architecture are key components in optimizing your test system.
Jennifer Schwartz is a product marketing engineer at National Instruments. She may be reached at editor@ScientificComputing.com.
Hybrid System Resources
Boeing’s QTD project www.ni.com/solutions
Instrument Driver Network www.ni.com/idnet
LXI Consortium www.lxistandard.org
PXI Systems Alliance www.pxisa.org
VXI Consortium www.vxibus.org
ATE Automatic Test Equipment | CVI C (language) Virtual Instrument | DAQ Data Acquisition | GPIB General Purpose Interface Bus | I/O Input/Output | IVI Interchangeable Virtual Instruments | LAN Local Area Network | MXI Multisystem Extension Interface | NI Natiopnal Instruments | PCI Peripheral Component Interconnect | PCIe PCI Express | PXI PCI Extensions for Instrumentation | QTD Quadruple Terminal Digits | RF Radio Frequency | USB Universal Serial Bus | VISA Virtual Instrument Software Architecture | VXI VME Extensions for Instrumentation