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 next 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 could be used.

USB products are ideal additions to test systems requiring hot-swappable, conditioned and specialized I/O. This paper covers the USB position relative to other buses available for automated test; USB advantages and disadvantages; and finally USB benefits and requirements for a modern, high-performance hybrid automated test system.

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 Hi-Speed USB theoretical rate of 480 Mb/s for more demanding applications such as streaming multimedia.

Bus Comparison

Figure 2. Bus Comparison Chart for Bandwidth and Latency

Figure 2 plots various buses based on their bandwidths and latencies, 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.

Challenges with USB Throughput

The USB bus was not originally designed for high-speed measurements, limiting its functionality in automated test. In the test and measurement industry, USB has traditionally been used as a replacement for slower serial and GPIB buses on stand-alone instruments with their own displays and processors. Due to its ubiquity on PCs, USB has become a popular communication bus for stand-alone instrumentation as well as other applications including low-cost, low-speed, and low-accuracy measurements, sometimes with embedded sensors.

On the other hand, devices based on the PCI bus have generally been associated with high-throughput automated test. With typical high-performance PCI and PCI Express data acquisition boards running analog inputs at more than 1 MS/s along with other functions, high data throughput rates and low latencies are essential in automated test. Although the USB protocol provides high data transfer rates for peripherals such as external hard drives when a single stream of data flows in a single direction, multiple streams and bidirectional data transfers can be extremely slow over USB. High throughput and low latency are necessary for automated test systems to decrease test times and increase efficiency; this prevented USB from being more than a simple instrument control bus in most current test systems. To overcome throughput challenges and extend the use of USB in test and measurement instrumentation, National Instruments has created an innovative technology called NI signal streaming.

NI Signal Streaming

With NI signal streaming, National Instruments pushes the limits of USB as a high-performance bus with throughput and latency, two important bus attributes for automated test.

National Instruments implemented three main innovations in NI signal streaming to offer higher throughput and reduce latency. First is the ability to quickly transfer large data sets through USB. NI achieved this with a custom application-specific integrated circuit (ASIC) that manages data flow between the data acquisition front end and USB. Second is an optimized message-based communication between the USB controller and the device. This allows for a single message to replace dozens of low-level register command transfers on the USB bus. Finally, additional device-side intelligence enables devices to choose the size of the data transfers depending on pending requests, making the device more responsive.

This custom design, combined with the two other NI signal streaming innovations, delivers a performance increase of up to 1,600 percent for single-point analog input and up to 250 percent for single-point analog output.

More information on NI signal streaming can be found here.

USB Products for Automated Test

While NI signal streaming provides a solution for higher throughput, other application requirements still challenge potential users of USB instrumentation. Among them, the need for a more rugged form factor and connectivity is prevalent in industrial situations, and the need for higher-performance measurements is common for cutting-edge design and research.

To partially address these needs, National Instruments and several third-party manufacturers are providing more and more rugged industrial application solutions equipped with strain relief, military-grade USB connectors, hazardous voltage isolation, and more. Further, NI has expanded its USB offerings in terms of performance with a new specialized instruments for automated test: a 6½-digit digital multimeter (DMM) and a 100 MS/s digitizer.

Figure 3. USB Devices Available for Measurement and Automation

The top of Figure 4 includes PC-based USB instruments, USB instrument control devices for GPIB and RS232 instruments, and stand-alone instruments that can connect to USB. In addition, a wide range of data acquisition devices, from low-cost to high-performance, are available. Some are bus-powered while others are wall-powered. Data acquisition systems such as NI CompactDAQ allow for integrated signal conditioning, medium-channel counts, and hot-swappable modules. A large variety of OEM products are also available to help shorten time to market by providing commercial off-the-shelf data acquisition for custom integration. Most National Instruments USB data acquisition products feature NI signal streaming technology.

These USB products are ideal additions to test systems requiring hot-swappable modules, small- to medium-channel-count signal conditioning, and custom/specialized I/O. At the heart of this flexibility is a hybrid test system.

Hybrid Test Systems

As seen earlier, no single bus can satisfy all needs and applications. Regardless of the performance of any bus or platform, it may not be possible to build a test system based on a single technology. To achieve greater flexibility and extend system longevity without redesigning entire systems 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. Hybrid systems also enable the integration of 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, visit:
USB Multifunction Data Acquisition
USB 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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