USB for Automated Test

USB has been a hot topic in the test and measurement world for both users and manufacturers. A recent online survey conducted by Sensors magazine showed that engineers are more likely to choose USB over other buses for their data acquisition applications. But as with any new bus, there is a great degree of interest from the scientific and engineering community about how it compares with other buses and how it can be used.

USB Evolution

Figure 1. Bandwidth Progression for Various Buses

USB 2.0 delivers faster performance than 100 Mb/s Ethernet, which is currently found on most PCs, laptops, and network routers. With more than 2 billion ports around the world, it is one of the fastest-growing bus technologies in the computing industry. USB has evolved from a simple, low-speed peripheral bus for accessories such as mice and keyboards to a bus with a theoretical rate of 480 Mb/s for more demanding applications such as streaming multimedia.  With throughput rates in excess of 400 MB/s, the USB 3.0 specification is poised to push USB into even more demanding applications.

Bus Comparison

Figure 2. Bus Comparison Chart for Bandwidth and Latency

Figure 2 plots various buses based on their bandwidths and latencies, which are two important bus attributes in automated test. Bandwidth is defined as the amount of data that can be transferred within a prescribed amount of time, and latency (which is also known as responsiveness) is defined as the time delay between the initiation of a request for data and the beginning of actual data transfer. Although USB is typically faster than 100 Mb/s Ethernet and slower than internal buses such as PCI, its latency is about 100 times longer than that of its PC bus counterparts. Buses typically used for instrument control such as local area networks (LANs), GPIB, and serial are listed toward the bottom and left sides of the chart.

NI USB Products for Automated Test

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Figure 3. NI USB Devices for Measurement and Automation

Figure 3 shows a variety of options for automated test using USB measurement hardware.  NI offers:

• Drivers for Stand-Alone USB Instruments - NI provides thousands of instrument drivers to control USB-based stand-alone instruments on its Instrument Driver Network. Program instruments with NI LabVIEW, LabWindows™/CVI, Microsoft Visual Studio, and more.
• USB Instrument Control - To simplify connectivity to stand-alone instruments that are based on GPIB or serial, NI offers GPIB-to-USB and serial-to-USB converters.
• NI USB Instruments - NI provides bus-powered, portable USB instruments including a USB digital multimeter (DMM), USB digitizers, and a USB RF power meter.
• NI USB Data Acquisition
  • Low-Cost, Bus-Powered USB Devices - The low-cost, bus-powered NI USB-6008 and USB-6009 offer basic analog input and digital I/O for simple data-logging applications and academic laboratory environments.
  • USB NI M Series - For higher-performance requirements, consider both bus-powered and wall-powered M Series data acquisition devices.  
  • NI CompactDAQ System - For a modular system to perform sensor and electrical measurements, consider NI CompactDAQ, which provides USB connectivity to a wide variety of compact measurement modules.
• USB OEM Options - NI offers board-only versions of USB products for OEM applications, including OEM USB data acquisition and OEM USB digitizers.

Hybrid Test Systems

Although USB is beginning to address a larger portion of the industry's test requirements, no single bus can satisfy all needs and applications. To achieve greater flexibility and extend system longevity without redesigning entire systems in an effort to fight obsolescence, more and more engineers are now choosing a hybrid system approach.

Figure 4. Diagram of a Hybrid Test System

Because modular instruments have built-in timing and synchronization that may not be possible between stand-alone instruments, engineers commonly use PXI at the cores of their hybrid systems. In such a configuration, you can choose PXI modular instruments for your test system components that require the tightest timing and synchronization, and connect other instruments as peripherals. You also can treat a PXI system as a peripheral by cabling it to a PC as if it were a stand-alone instrument. In addition, you can use hybrid systems to integrate long-established technologies such as VXI.

The key to creating and maintaining a hybrid system is implementing a software architecture that transparently supports multiple bus technologies.

Summary

During the design of a test and measurement system, it is important to remember that a single bus technology cannot meet all application needs. For example, GPIB is best-suited for connecting to stand-alone instruments and LAN is well-suited for distributed or remote applications. If your design requirements mandate a hybrid system, be sure to choose a software architecture that can integrate the pieces seamlessly.

USB, on the other hand, is extremely portable, fast, and easy to use. New innovations allow USB data acquisition devices to complement an existing test system as USB reaches capabilities never thought possible on a widespread commercial external bus. 

For more information on NI USB products for automated test, refer to the following documents:

USB Multifunction Data Acquisition
NI CompactDAQ USB System
USB Instruments: DMM and Digitizer
USB Instrument Control
USB Switches
NI Signal Streaming: Sustaining High-Speed Data Streams on USB

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