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The Expanding Environment of USB Data Acquisition

Wed, 10/24/2007 - 11:40am
The Universal Serial Bus has become the standard connection specification for most current electronic products, with FireWire and wireless still real competitors.
Designed more than 10 years ago as a common interface for personal computers, the Universal Serial Bus (USB) has become the de facto standard for many consumer electronics devices (except for Apple components, which use the FireWire version of the IEEE 1394 interface). With more than 2 billion USB-enabled interfaces now being used, it is also now moving into the industrial and research areas as well. And its use is likely to grow even more with work on new specifications recently announced.

USB standards
NI’s USB-9152, USB-6255, and USB-6259 devices address high-channel-count, portable, and benchtop data acquisition applications. Image: National Instruments
USB has replaced the serial and parallel ports on a PC, with at least four USB ports standard on every PC now manufactured. USB has become the current standard connection for most computer peripherals including mouse devices, keyboards, scanners, printers, game controllers, and those very convenient flash drives. USB was designed to be hot-swapped or connected and disconnected without rebooting the computer. There are several configurations of USB, including the rectangular Type A that most of us use, along with the square Type B. Mini-A and Mini-B USB connectors are used on smaller portable products, like cell phones, digital cameras, and PDAs.

USB 1.0 has a maximum bandwidth of 12 Mb/sec, while USB 2.0—finalized in 2001—has a maximum bandwidth of 480 Mb/sec. FireWire has a maximum bandwidth of 800 Mb/sec, making it preferable for video data transfers—the High-Definition Audio-Video Network Alliance (HANA, www.hanaalliance.org) has made the IEEE 1394 bus its standard connection interface for AV component communication and control.

The keynote speech at the Intel Developer Forum (IDF) in September in San Francisco, Calif., was given by Pat Gelsinger, SVP and General Manager of Intel’s Digital Enterprise Group, Santa Clara, Calif. In his keynote, Gelsinger, who was also Intel’s first CTO, announced that Intel is working with Hewlett-Packard, Microsoft, NEC, NXP, and Texas Instruments on a USB 3.0 specification that could push its bandwidth beyond 4 Gb/sec, 10 times its current rate. While still in the prototype stage—Gelsinger did demonstrate the technology at the IDF—USB 3.0 would require fiber-optic cabling to handle this speed, although it would be backward compatible to USB 2.0 and USB 1.0 as well.

Gelsinger indicated that the final version of specifications would be completed in the first half of 2008, with USB 3.0 peripherals appearing on the shelves in 2009 or 2010. While exceeding the current FireWire bandwidth, the USB 3.0 would allow improved connections to external hard drives, flash readers, and video devices. The IEEE 1394b specification does support optical connections up to 100 m in length and data rates up to 3.2 Gb/sec. The 1394 Trade Association (www.1394ta.com) also is considering specification upgrade to 10 Gb/sec, since the cables and connectors defined for the 1394b specification have already been rated for those speeds.

While there is increasing usage and acceptance of USB 2.0 in many consumer products, the current higher-speed rating for the 1394/1394b specs will continue to give it the edge until the first USB 3.0 products become available. And even then, there are potential cabling-length limitations that might reduce the 5-m limit for USB 2.0 to less than half that for the USB 3.0 spec. On the other side, the FireWire Trade Association is looking into the use of FireWire up to 100 m in length on various cable configurations, including coaxial cables. FireWire proponents also question the reality of being able to put the USB 3.0 specification into actual hardware products (i.e., silicon) in the time frames currently being proposed.

Also in the fray of data transmission systems is ultrawide bandwidth (UWB) wireless, which after many years of missed schedules, is just now becoming available in products in the form of Wireless USB 1.0. Several laptop computers—Dell, Lenovo, Toshiba, and others—are now available that use UWB wireless technologies to connect to their docking stations, with other peripheral-based connections in the wings. UWB is faster than Bluetooth (3 Mb/sec) and uses less power.

The performance of the Wireless USB 1.0 standard is targeted at 480 Mb/sec at 3 m and 110 Mb/sec at 10 m, still considerably short of the current FireWire and the proposed USB 3.0 capabilities. Wireless USB 1.0, however, is the first high-speed wireless personal interconnect technology to meet the needs of multimedia consumer electronics, PC peripherals, and mobile devices. It also preserves the functionality of wired USB while unwiring the cable connection and providing enhanced support for streaming media Windows CE devices and peripherals.

USB data acquisition
Getting back to currently available technologies, National Instruments (NI), Austin, Texas, recently introduced the data acquisition (DAQ) industry’s first 80-channel USB 2.0 data acquisition devices, the new USB M-Series devices. At the same time, NI introduced a new USB-9237 C Series module. These introductions bring NI’s total USB DAQ product offering to more than 45 products with single-channel sampling rates of up to 1.25 MS/sec, analog generation of up to 2.86 MHz, and correlated digital input/output rates of more than 1 MHz for low-, mid-, and high-channel-count applications.

The 80-channel USB M-Series devices are the test and measurement industry’s highest channel-count USB multifunction DAQ devices. The BNC USB M series offers direct BNC connectivity, while the USB C-Series modules offer less expensive and more portable hardware.

Using the 80-channel USB M-Series devices, engineers can now collect more data with less equipment because the devices provide high-channel count application solutions in a single device. The USB-6225 has 80 analog 16-bit inputs (250 kS/sec) with two 16-bit analog outputs (833 kS/sec); 24 digital I/O (8 clocked) and two 32-bit, 800-MHz counters/timers. The Model 6225 is ideal for applications such as high-channel-count data logging and for sensor measurements when used with signal-conditioning devices.

The USB-6255 also has 80 analog 16-bit inputs, but with 1.25 MS/sec single-channel capabilities (750 kS/sec aggregate). It also has two 16-bit analog outputs (2.8 MS/sec), 24 digital I/O (8 clocked), and two 32-bit, 80-MHz counters/timers. The Model 6255 is ideal for applications such as dynamic signal acquisition and also for sensor measurements with signal conditioning.

Both are designed specifically for mobile or space-constrained applications, where the plug-and-play USB installation minimizes the configuration and setup times. Direct screw-terminal connections on these devices helps minimize costs and simplifies the signal connections.

These devices also feature NI’s recently introduced proprietary streaming technology, which gives the user DMA (direct memory access)-like bi-directional, high-speed streaming of data across USB. The devices are approximately 27 cm x 17 cm x 4.5 cm in size. They operate on Windows Vista/Vista x64/XP/2000 and NI’s LabVIEW Real-Time operating system.

NI introduced four other M-Series DAQ systems (USB-6251, USB-6259, USB-6221, and USB-6229) with 16 to 32 mass-termination-type analog inputs and 250 kS/sec to 1.25 MS/sec sampling rates. All of NI’s M-Series DAQ systems have 16-bit resolution, +/-10 V range, and two 32-bit 80-MHz counter/timers.

These four models are also available with integrated BNC (bayonet connectors for coaxial cables) connectivity with 8 to 16 BNC-based analog inputs. All other specifications of these BNC-based USB devices are similar to their mass termination versions.

Pushing the limits of USB
To optimize the use of the USB and deliver high-performance DAQ for its customers, NI engineers created several key technologies to push the limits of USB throughput and latencies. NI’s proprietary signal streaming combines three innovative hardware- and software-level design elements—fast data transfers, message-based instructions, and device-side communications intelligence—to enable sustained high-speed and bi-directional data streams over the USB.For fast data transfers, NI’s signal streaming works directly with the system-timing controller to deliver four onboard, high-speed DMA channels directly into four USB endpoints. This frees resources for the processor to work on other tasks, such as message translation. Onboard DMA transfers ensure that data is always ready to be read or written at the USB endpoints.

NI signal streaming also provides message-based communications to avoid lengthy transmissions—the host needs to send only a single, high-level message such as “acquire.commit” through the USB. The device processor then converts this message into the dozens of register-level commands that are necessary to set up the system-timing controller.

And while these two technologies contribute to higher data throughputs, an additional device-side intelligence combined with message-based instructions contributes to lower latencies. These all contribute to a single-point performance increase of up to 1,600% for analog inputs and up to 250% for analog outputs.

Also included in NI’s USB introduction is their USB-9237 Bridge and Strain Measurement Module, having four simultaneously sampled analog inputs designed for sensors, such as strain gauges, pressure transducers, load cells, and other bridge-based measurements. No external power or excitation is needed on the USB-9237 because it’s bus-powered and has a built-in excitation of up to 10 V for the connected sensors. Standard RJ50 connectors on the device are used for mass terminations. Crimping the RJ50 connector onto a sensor—similar to crimping a standard network cable—provides a quick connection of the sensor to the DAQ module.

—Tim Studt

Resources
1394 Trade Association,
Southlake, Texas, 817-416-2200, www.1394ta.com
Intel,
Santa Clara, Calif., 408-765-8080, www.intel.com
National Instruments,
Austin, Texas, 800-531-5066, www.ni.com
Universal Serial Bus,
Beaverton, Ore., 503-619-0426, www.usb.org
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