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The drive towards hybrid test07 October 2009Today, there are a multitude of different platforms and bus technologies around which automated test systems can be architected. PCI, PXI, LXI, GPIB, USB and VXI are the most widely used, but when compared, there are clear advantages and drawbacks to each. So how do you decide which is the best architecture for your application? Does a decision have to be made at all? Why should test engineers be limited to a single vendor or architecture? The ideal solution would surely be to incorporate the best features from each bus in a hybrid approach, where multiple instruments on a variety of buses are combined into a single system. In this way, a test system designer could, for example, benefit from the high throughput and low latency of PXI express, while still using GPIB, USB or LAN/LXI for connecting to specialist instrumentation.
Software and instrument drivers are at the core of making the hybrid system a reality, while the rapid advancement of PC technology is propelling hybrid systems to soaring levels of performance. Advanced software drivers such as IVI (Interchangeable Virtual Instrument) enable a high degree of flexibility in test systems. This protects investment in hardware and software when accommodating changing specifications, future technologies and ongoing maintenance. Modern multithreaded desktop operating systems and high level multicore-aware application development environments (ADEs) such as National Instruments LabVIEW provide desktop supercomputing for data analysis. The use of high quality driver and test software is therefore equally as important as using a quality piece of hardware to conduct the physical measurements needed.
The creation of a hardware abstraction layer (HAL) through robust instrument drivers allows you to add or replace measurement hardware without the need to re-architect the whole system. Instrument drivers abstract the specific instrument command set away from the developer, thus simplifying and shortening test program development time. The most advanced drivers, such as those compliant to IVI foundation specifications, form a layer that separates the test routines from the hardware; this is the HAL. By implementing a HAL, the flexibility and longevity of a system are increased, as it allows instrument interchangeability, enabling the replacement of end-of-life components and new technologies to be integrated without modifying the test software.
National Instruments now provides more than 7,000 freely downloadable instrument drivers (www.ni.com/idnet) for a variety of buses and development environments, including NI LabVIEW, LabWindows/CVI and Microsoft Visual Studio. At the lowest level, most driver architectures use VISA (Virtual Instrumentation Software Architecture) to give bus and platform independence. This independence allows the user to send commands such as “*idn?” to identify an instrument regardless of the physical bus that the instrument is using. Plug-and-play drivers take this a stage further abstracting some of this low level complexity by grouping common command changes together. This allows developers to use commands like “read”, “write” or “initialise” that may consist of several low-level VISA commands. IVI drivers, as mentioned above, are more sophisticated and feature increased flexibility for more complex applications that require interchangeability, state-caching or instrument simulation. The ability to allow instruments to be interchanged in a system without modifying the test software is valuable, as it enables a more flexible, maintainable system that can be upgraded to incorporate the latest technologies and replace end-of-life components. The IVI foundation specified eight distinct classes of instrument: DC Power Supplies; DMMs; Function Generators; Oscilloscopes/Digitisers; Power Meters; RF Signal Generators; Spectrum Analysers and Switches. An instrument that has a driver conforming to one of these classes may be substituted with another instrument of the same class, regardless of manufacturer or bus connection. State-caching functionality registers and stores the current state of each instrument, eliminating redundant communication transmissions that may be sent to the instruments within the system. This reduction in instrument communication can provide significant performance improvements, especially for higher latency buses such as GPIB and LAN/LXI. IVI drivers can also be configured to run in simulation mode, where the actual instrument and signal it acquires or generates is simulated in software. This is a great aid in system development, as it allows engineers to develop the test software without access to instrumentation hardware.
To maximise performance, software programs must have the ability to benefit from the multicore processor architectures ubiquitous in modern computers. The traditional approach of using text-based programming tools is a limiting factor to accessing this advancement in test software, as implementing multithreading is far from trivial. By moving to a high level graphical programming language such as NI LabVIEW, where parallelism is an intuitive programming process and multithreading is automatic, new possibilities are opened up for concurrency and test time reduction. By implementing parallel threads, either multiple devices or multiple subsystems can be tested simultaneously. Throughput can further be improved by incorporating parallel programming techniques like pipelining, to minimise system idle time and optimise performance.
This hardware architecture was proven to be a success by Francois Charreire, working at the Test Means Department, Alstom Transport when developing a flexible and scalable test bench to meet the manufacturing test needs of electronic products at the Alstom Transport site in Villeurbanne, France. By relying on a hybrid architecture with PC and PXI instrumentation, as well as NI TestStand and LabVIEW software, they were able increase test throughput and integrate a hardware/software abstraction layer to increase durability. Charreire stated: “We achieved our goals of the project and developed a generic tester, which can meet most of our current and future testing needs, by replacing the rack that interfaces to the product under test. The instrument rack interchangeability makes maintenance easier and the tester available, allowing for manufacturing optimization throughput according to our needs. As far as customers are concerned, this generic rack is a substantial way for our customers to save money and increase product life. The new test bench also presents a time-to-market advantage because now we can quickly test any new product and speed up its time to market.”
The recognition that no single bus is optimised for every test system, that there is no “silver bullet”, means that hybrid is becoming the archetypal test architecture. By adopting a software-centric hybrid architecture, test engineers can accommodate flexibility and scalability on any bus they choose. Combine this with the processing increases realised from running pipelined test routines on multiple processor cores, the performance potential of future hybrid test systems is limited only by the innovation of their designers.
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