While the bulk of data acquisition devices on the market today use the PCI or PXI bus, the USB bus is quickly gaining popularity.
A 2003 Venture Development Corporation study titled 'Data acquisition product requirements and usage trends' showed that almost half of the respondents planned to buy a USB data acquisition device by 2007. Although PCI and PXI data acquisition devices tend to have higher performance specifications, you now can combine USB data acquisition with other technologies to take measurements at the highest limits of simplicity, portability, and distance. National Instruments recently released five new USB 2.0 full-speed (12 MBps) DAQ products that deliver these USB characteristics.
The first in a series of new NI USB products released last year, the NI SCXI-1600 digitiser module gives the entire SCXI signal conditioning product line a facelift by making all existing SCXI signal conditioning and switching modules USB-enabled. In addition to USB connectivity, the SCXI-1600 provides 16-bit analog inputs at 200 Ksps. The user simply plugs the module into any SCXI chassis slot, plug the chassis into their computer using the USB cable, turn the chassis on, and most SCXI modules are automatically detected and ready to run within seconds.
National Instruments also added to the high-performance USB DAQ product family by designing two upgrades for the company's older USB DAQ product, the NI DAQPad-6020E. The new NI DAQPad-6015 and NI DAQPad-6016 deliver 16-bit, 200 Ksps analog inputs with up to 32 digital I/O lines. For a price comparable to the DAQPad-6020E, users get improved performance as well as integrated signal connectivity. Both new USB-based DAQPads include screw terminal connectors for superior portability compared to the DAQPad-6020E, which requires an extra terminal block and cable.
NI also introduced the USB-9200 series, a new line of bus-powered products with integrated signal conditioning. NI marked the launch of the USB-9200 series with two new products, the NI USB-9215 and NI USB-9211. Both modules provide 250 V rms of channel-to-ground isolation and removable screw terminal connectors. The USB-9215 is a 16-bit simultaneous-sampling analog input device with four channels, while the USB-9211 offers four channels of thermocouple inputs with 24-bit analog-to-digital conversion, integrated cold junction compensation, and auto zeroing.
In May 2004, NI launched a Sensors Plug&Play initiative, creating a line of DAQ products based on the IEEE 1451.4 standard for smart Transducer Electronic Data Sheet (TEDS) sensors. By using Sensors Plug&Play technology with the new USB DAQ devices, engineers and scientists can create a complete plug-and-play data acquisition system from sensor to software. Numerous vendors now offer sensors pretested to work seamlessly with NI Sensors Plug&Play hardware, and developers can use NI LabVIEW, Microsoft Visual Basic, and C to create software applications to quickly and easily take measurements. To develop a plug-and-play DAQ system, combine the USB-based SCXI-1600 module with the SCXI-1520 strain input module and the SCXI-1314T TEDS terminal block for bridge-based smart TEDS sensors. For legacy sensors that do not include TEDS technology, download Virtual TEDS, available at ni.com/sensors, for use with any of the NI USB DAQ modules.
Sensors Plug&Play combined with USB offers not only the rewards of lower set-up and configuration costs, but also reduced programming costs. Compare the process of setting up and configuring a data acquisition system without this technology to the same process combining Sensors Plug&Play and USB.
The reduction in set-up time can reach 50%. Based on a National Instruments survey of more than 350 engineers last year, decreasing the time - and therefore cost - of each set-up and software development stage by 50% could result in an overall development cost reduction of 26,5%.
Extending the 5 m distance limit
Distance may not be an attribute typically associated with USB technology because the USB specification limits the distance between host and device, hub and device, or host and hub to 5 m. It also limits the number of layers to seven, which means you can place a USB device a maximum of 30 m away from the host by using the maximum of five hubs in between. However, several vendors have developed new technologies that extend the distance between host and device to up to 2 km while still remaining true to the USB specification. Two primary methods exist for extending the distance:
* CAT5 (Ethernet) cable (100 ft to 100 m): These types of extenders provide two units - a local (host-side) unit and a remote (device-side) unit - connected with CAT5 cable through RJ45 connectors. The device-side unit acts as a remote hub and often has multiple ports. This is a less-expensive solution for USB extension. (Vendors: Network Technologies, Icron Technologies.)
* Fibre-optic cable (30 ft to 2 km): This method uses the same concept as CAT5 extenders, with a local unit and a remote unit. However, in this case, they are connected by fibre-optic cable, which has a longer transmission range. This solution is more expensive than CAT5. (Vendors: Icron Technologies, Opticis.)
For portable or simple measurements, USB data acquisition is emerging as the new preferred choice. As USB data acquisition devices continue to shrink in size and require less power, they also become more portable and affordable. NI USB DAQ devices go beyond what they cannot achieve alone through compatibility with Sensors Plug&Play and various bus extension technologies. With new technologies on the horizon like USB On-The-Go and Wireless USB, future applications for USB in measurement and automation would appear limitless. Visit ni.com/usb - or call the local toll-free number, 0800 203 199.
For more information contact Michael Hutton, National Instruments SA, 011 805 8197.
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