Connecting Multiple Platforms in a Hybrid Test System

Hybrid systems combine a variety of instrumentation buses and platforms into one system. Through hybrid systems, you achieve the flexibility to combine the instruments regardless of the bus, allowing you to choose the instruments best suited for your application needs. In addition to enhanced flexibility, hybrid systems provide extended longevity for the test system with the ability to use existing hardware and software, but still incorporate new technologies with enhanced performance and lower cost. A common question when designing a hybrid system is how to physically connect the components of the system to maximize flexibility and performance.

Instrumentation Buses

With a variety of instrumentation buses available, people often focus on one instrumentation bus or platform upon which to build the system. With a hybrid system, however, you can integrate various buses to take advantage of the strengths of each. A large number of instrumentation buses and platforms are available including PXI, VXI, GPIB, USB, and LAN. The proven industry standard GPIB has served the industry for over thirty years by providing a breadth of instrumentation. USB has grown in popularity for test and measurement applications for its easy- to- use plug and play capability, bandwidth, and widespread availability. Used for many years in test applications, LAN is well suited for distributed applications or remote monitoring due to its ability to support larger distance needs. VXI, VME Extensions for Instrumentation, is an open standard with flexible, modular packaging that has seen adoption in the military/aerospace sector. PXI, PCI extensions for Instrumentation, is a high performance rugged, open system for modular instrumentation and combines PCI electrical-bus features with modular Eurocard packaging of CompactPCI. By building on open PC technology, PXI is a high performance, low-cost platform. Oftentimes, the bus used in a system is determined by whether the available instruments provide the required measurement functionality, but the properties of the bus, including bandwidth and latency, are important factors to consider.

Bandwidth is the rate at which data is transmitted, and latency is the delay in the transmission of the data. Each of these is an important factor in determining how well a particular bus can perform the measurements needed for the system. For instance, bandwidth is important in applications that require data streaming like waveform acquisition or generation while latency impacts applications that involve functions like digital multimeter measurements and switching. Figure 1 shows a comparison of various instrumentation buses with respect to bandwidth and latency.

With a hybrid system, you can use the different buses to create a system that fits your needs. You can also lower system development time and cost by combining existing instruments with multiple buses to prevent the need for new or specialized equipment.

Software Integration Enables Hybrid Systems

No matter which buses and instruments are used in the system, the software is the key to successful integration of the system. The configuration manager, Measurement and Automation Explorer (MAX), allows for easy configuration of multiple buses into one test environment. This software, which is provided with National Instruments device drivers, allows for configuration and testing of the devices as well as creating device aliases and tasks for use in the system software. NI LabVIEW, LabWindows/CVI, and Measurement Studio can utilize the configurations created in MAX to easily integrate the various device buses in one easy-to-use programming environment. As shown in Figure 2, MAX provides a single environment to identify and configure all the NI hardware in the system. These configurations can then be called programmatically by the system software in the test application.

In this example, MAX provides an integrated view of a PXI and VXI system including chassis and instrument modules, as well as other plug-in devices, ports, and remote systems available on the system. Through this unified interface, you can view, configure and test all hardware in the system. LabVIEW, the application development environment used, then builds on top of these configurations for seamless integration of multiple buses. The flexibility of LabVIEW and MAX allows virtually any hardware bus to be integrated into a versatile hybrid system.
While the software tools enable the integration of multiple buses, the system topology – that is, how you physically connect the separate buses and instrumentation into one system - is still important. The system topology will depend upon a variety of factors including performance, distance, connectivity, and timing and synchronization. These will dictate how you assemble a hybrid system. The first question for determining the system topology is deciding what the computing center of the system ought to be. You can choose to control the system from a PC or an embedded controller in a modular hardware platform like PXI or VXI. When deciding between an external PC and an embedded controller, you should consider performance impact, distance needs, and available connectivity to other buses. Typically within a PXI or VXI system, you achieve the best performance by using an embedded controller since you are not limited by the bandwidth of the link used for remote control. Remote controllers, however, might be appropriate if the performance is not pushing the limits of the link or if you need to meet distance requirements, for instance when you want the host interface located separately from the test equipment. Another consideration when is the connectivity available to other buses like GPIB, USB, and LAN. Both PCs and embedded controllers provide USB and LAN connections. For GPIB, PCs require an additional controller interface such as a PCI card while many embedded controllers for modular buses include an integrated GPIB interface. If you require connectivity to other buses you should evaluate the available methods for connection from the system center.

A second consideration when determining the system topology is ensuring that you can support timing and synchronization needs. Both PXI and VXI provide timing and synchronization features for instruments contained in the chassis through the backplane. With multiple buses, however, you might need to synchronize instruments across different buses. For instance, if you have a PXI and a VXI chassis, each has its own reference clock, but the application might require that the two chassis be synchronized. You can accomplish this by taking advantage of the clock and trigger connectors available on many controllers, chassis, and modules. Figure 3 provides an example topology in which PXI serves as the system center and connects to a VXI system and a stand-alone instrument. By using the PXI-8105 embedded controller as the center of the system, you can take advantage of the built-in GPIB connectivity. You can control the VXI portion with a MXI-2 remote controller with a PXI-6652 timing and synchronization board to share triggers and a clock.

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Figure 3. PXI to VXI and GPIB hybrid system uses PXI-6652 and MXI-2 to share triggers and clock.

PXI is often an appropriate choice as the computing center due to its timing and synchronization capabilities and low latency to measurement devices. In addition, it also has a high Windows native slot count, which allows you to add buses and access them from the PC without the need for special drivers. If demands in your system require a different topology, you have other options to synchronize clocks and trigger across different buses. In Figure 4, you can see a list of some of the NI PXI and VXI products that provide clock and trigger capabilities.

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Figure 4. Timing and triggering connectors on PXI and VXI products enable synchronization between buses.

When designing a hybrid system, choose a topology that will best meet the system needs. Because hybrid systems enable the mixing and matching of buses, you can integrate multiple instruments of different buses according to your needs. Many connectivity options are available from a wide variety of instrumentation buses, so that the topology options are nearly limitless. You should carefully evaluate factors to determine what the computing center should be and whether you can support the timing and synchronization needs when deciding the system topology. The integrated software framework then enables the successful integration of these physical components into one environment. Hybrid systems provide enhanced flexibility and extended longevity for the test system at a lower cost by allowing you to combine existing hardware and software with new technologies.

Relevant NI Products and Whitepapers

National Instruments, a leader in automated test, is committed to providing the hardware and software products engineers need to create these next generation test systems.

Software:

• NI TestStand Test Management Framework
• LabVIEW Graphical Programming Environment
• Signal Express Interactive Measurement Software
  

Hardware:

• Modular Instruments (Oscilloscopes, Multimeters, RF, Switching, and more)
• Multi-function Data Acquisition
• PXI System Components (Chassis and Controllers)
• Instrument Control (GPIB, USB, and LAN)
  

Whitepapers
NI offers a Designing Next Generation Test Systems Developers Guide. This guide is collection of whitepapers designed to help you develop test systems that lower your cost, increase your test throughput, and can scale with future requirements. To read the entire developers guide, you can: Download the PDF (90+ page) version or view the web-version of the Designing Next Generation Test Systems Developers Guide.

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