By Alex Mendelsohn, eeProductCenter Senior Technical Editor
Legacy PC users may miss them, but most new PCs are devoid of parallel printer ports, and in many cases even the venerable serial I/O connector. Universal Serial Bus (USB) is taking center stage, as Centronix and RS-232 conventions fade into PC history.
One attraction is USB's plug-and-play ease of use. Others include USB's hot swappability, and its ability to create mini-networks of signal processing and control peripherals.
Data acquisition equipment makers, aware of the ascendance of USB, are busy rolling out products that underscore USB's utility in configuring and connecting PC hardware. Using USB compatible instruments, you can use your PC for both single-ended and differential analog signal capture, custom signal generation, and digital I/O handling.
Just as RS-232 connectivity served virtually every serial I/O task imaginable in the past, USB peripherals now tackle a variety of tasks to ease data acquisition across a range of complexity and price points. Some of these peripherals cost as little as $75, although USB's utility is apparent in both high-end and low-end products.
Data Translation's (Marlboro, Mass.) DATAX measurement system, for example, is priced at about $5500. At the other end of the spectrum are moderate-speed data acquisition and control pods such as LabJack Corp.'s (Lakewood, Colo.) Model U12 ($119) and Measurement Computing Corp.'s (Marlboro, Mass.) competing PMD-1208LS and PMD-1024LS pods.
Out Of The Box
Essentially a USB-based smart cardcage, the Data Translation DATAX extends signal conditioning and data acquisition out of the PC into an external crate. In this environment, the ruggedized DATAX gives you modular connectivity and data acquisition. The system accommodates low-bandwidth sensor signals such as those from RTDs, thermistors, strain gauges, and thermocouples.
With 16 slots to house combinations of the company's existing DAQ and PAD plug-in modules, the DATAX can comprise an application-specific USB-connected adjunct. As such, it can lend itself to a range of automotive, industrial, and manufacturing tasks, as well as general R&D and lab work. Data Translation's product is also shipped with ready-to-run software.
A USB-based smart cardcage, Data Translation's DATAX extends signal conditioning and data acquisition out of a PC into an external crate. The ruggedized offers both modular connectivity and data acquisition, accommodating low-bandwidth sensor signals from RTDs, thermistors, strain gauges, and thermocouples.
The system's multi-channel plug-in modules tout 1-kV rms isolation, and you can invoke a variety of programmable range and filter settings. To get you started, a DATAX chassis comes preconfigured with voltage-amplifier, high voltage, bridge, and thermocouple modules.
Modules can also be remotely programmed over RS-232 or RS-485 lines, or can be front-panel and/or software controlled. There's also an oscilloscope emulation package that lets you monitor data online and capture it to disk, either manually or automatically on appropriate triggering. You can also capture data and stream it to more than one file simultaneously, even at different sample rates, in DATAX's chart recorder and oscilloscope modes.
Much lower cost data-acq and control pods are horses of a different color, but they're still significant ponies in the USB stable. LabJack's and Measurement Computing's pods are USB Human Interface Devices, or HIDs.
Under the USB definitions, HIDs support connection using four-wire cables, and they use Microsoft USB HID class drivers, thus no third-party device drivers are needed. You can also connect and power multiple pods (they're all HID peripherals) using a USB hub. All USB connections automatically support bi-directional data flows between PCs and pods.
In LabJack's Model U12, the USB cable provides all the DC power to operate the data-acq pod, as well as communications with the host PC. No power supply is required. Furthermore, thanks to USB's inherent plug-and-play hooks, the PC "automagically" recognizes a U12 when it's connected, installing required drivers automatically. If you then run an application from LabJack's CD-ROM collection, you'll be able to almost immediately start I/O monitoring as well as control.
The LabJack U12, sampling at 1.2 ksamples/s, handles both software- and hardware-timed acquisition. When using software-timed acquisition, the host PC sends a command to the LabJack, and it responds with any data. This mode can acquire four channels at up to 50 samples/s/channel, or eight channels at up to 25 samples/s/channel.
LabJack's Model U12 pod uses the USB cable to provide DC operating power as well as communications with a host PC.
With hardware-timed acquisition, the PC sends a command to the LabJack telling it to start a burst-mode or stream-mode acquisition. In burst mode, up to 4096 samples can be acquired and stored at up to 8192 samples/s. After the acquisition is complete, the data are transferred to your PC.
LabJack also offers a board-level version of the pod product. Dubbed the LabJack U12-PH, it sells for as little as $64 a pop in OEM quantities. The board-only version of the LabJack U12 is equipped with pin-headers instead of screw-terminals.
For its part, Measurement Computing offers pods that are markedly similar to LabJack's. In fact, LabJack claims that Measurement's PMD-1208LS and PMD-1024LS products are clones of its wares. "Measurement Computing's specs are exactly like ours," contends LabJack spokesman Toby Stensland. "We published conservatively over- and under-rated specs, yet Measurement subsequently came up with the exact same numbers."
Measurement Computing's data-acq pods are similar to LabJack's. They're compatible with USB v1.1 as well as v2.0, operate under Windows 98SE, Windows ME, Windows 2000, or Windows XP, and are configurable for either eight 11-bit single-ended analog inputs, or four 12-bit differential inputs.
Whether that's true or not is beyond my purview, but having two companies with markedly similar data-acq merchandise gives you a choice. You can deal with a small company (LabJack) that may be very responsive to your support needs, or perhaps deal with a larger one (Measurement Computing) that may offer more support tools and software.
Adapting Good Ideas
Indeed, Measurement Computing's marketing manager Bob Judd counters LabJack's claims, noting that his firm sometimes adapts a good idea that it sees on the market, and then supports it with a broader base of tools. "Our PMD-1208LS and PMD-1024LS pods can be used with our own SoftWire graphical configuration software," he notes, "but we also support them under National Instruments' LabVIEW and Agilent's VEE."
Like LabJack's pods, Measurement's PMDs are compatible with USB v1.1 as well as USB v2.0, and operate in Windows 98SE, Windows ME, Windows 2000, or Windows XP environments. The PMD-1208LS's analog inputs are configurable for either eight 11-bit single-ended inputs, or four 12-bit differential inputs.
Like the LabJack U12, the PMD-1208LS offers a sample rate up to 1.2-ksamples/s in eight software selectable input ranges. In addition to the analog inputs, the unit gives you two channels of 10-bit analog output, as well a 32-bit counter and 16 lines of digital I/O. These units sell for about $109 each, with USB cables. Quantities of 25 or greater cut the cost to less than $80 a pop.
For its part, the PMD-1024LS adds a 24-bit digital I/O interface to your USB port. Using an industry standard 82C55 chip, 24 lines are divided into two 8-bit ports and two 4-bit ports, each of which may be independently programmed as I/O. PMD-1024LS single piece pricing is less than $100 each; pricing for 25 or more is about $75.
Moving up the USB data-acq scale, another company offering USB instrumentation is PerkinElmer. Its product also lets you set up mini-networks of signal processing gear under Windows. The company's $600 Model 3830 Multiplexer lets you interconnect up to six different sources as well as measuring devices. The Model 3820 is a universal counter. It's priced at about $900. Either model draws about 500 mA from a PC's USB port or USB hub.
The Model 3830 lets you establish interconnections between various sources and instruments. Connections are set up using your mouse and an icon-based Windows user interface. The box is equipped with a half dozen BNC connectors for I/O.
Internally, it uses DPST reed relays for connection and disconnection of both signal and ground conductors for each of its six I/O ports. The reed relays are high-rel types made by Hamlin, and each can switch up to 200 mA, handling 5 W, or 50 V. Contact resistance is only about 150 milliohms, so you can be assured that the MUX won't affect your readings or signals to any great degree. The switches are also reasonably fast, switching in less than 1 ms.
For its part, the Model 3820 Universal Counter acquires, displays, and saves data to disk. This counter accommodates signals up to about 125 MHz, but may be useful out to 160 MHz. Like the MUX, it's plug-and-play, so all you need to get it up-and-running is connect it to a USB port, load the software, and connect your probes (PerkinElmer offers optional probes) to the box's high-Z inputs. Like the MUX, you can move the counter from one PC to another quickly. Like the MUX, it too draws about 500 mA through the USB connection.
PerkinElmer's box establishes mini-networks of signal processing under Windows. This universal counter module draws about 500-mA from a PC's USB port or USB hub.
Signals in the range of +/-500-mV can be measured using a standard 1X scope probe, or +/-5-V can be measured with a 10X probe. The unit's discriminator thresholds are adjustable in 10 mV increments from -0.2 V to +0.5 V with a 1X probe connected, and from -2 V to +5 V with a 10X probe. An AUTO function automatically sets the threshold at the mean of the maximum and minimum signal amplitudes, and the system's event counter triggers at threshold crossing on a positive slope.
No discussion of USB data-acq would be complete without mentioning handhelds. An example is the Nicolet Instrument Technologies (Madison, Wis.) Phazer. This endearing little spectral analyzer is designed for impact testing, modal analysis, vibration testing, and the like. Weighing in at just eight ounces, the Phazer fits in the palm of your hand, is compliant with USB v1.0 and v1.1, and works under Windows 98, Windows 2000, or Windows ME. The unit's hardware includes 18-bit A/D converters per channel, and a 24-bit DAC output.
Nicolet Instrument's Phazer spectral analyzer fits in the palm of your hand, is compliant with USB v1.0 and v1.1, and works under Windows. It includes 18-bit A/D converters and a 24-bit DAC.
Like many USB instruments, the Phazer uses USB for DC power (it draws 500 mA) and connectivity, with the host PC handling its display. Nicolet's Prism Windows-based software displays gathered data as information in both time and frequency domains.
In the former, a Phazer can time-capture events, perform auto- and cross-correlation, and do orbit plots. In the frequency domain, the Phazer/Prism combo performs spectral analysis, with auto-spectrum, power-spectrum, and cross-spectrum features. It can also perform FFTs (fast Fourier transforms) and do impulse processing using frame sizes ranging from 256 samples to 8,192 samples. A Phazer system can do a 1,024-point FFT in less than a millisecond, dishing up nice-looking waterfall displays.
For more information, contact:
Data Translation, Inc., 100 Locke Dr., Marlboro, Mass. 01752-1192. Phone: (800) 525-8528 (From the US and Canada only) or (508) 481-3700. Fax: (508) 481-8620.
LabJack Corp., 13701 W Jewell Ave, Suite 284, Lakewood, Colo. 80228-4173. Phone: (303) 942-0228. Fax: (303) 716-0101.
Measurement Computing Corp., 16 Commerce Blvd., Middleboro, Mass. 02346. Phone: (508) 946-5100. Fax: (508) 946-9500
PerkinElmer, 45 William St., Wellesley, Mass. 02481-4078. Phone: (781) 237-5100.
Nicolet Instrument Technologies, Inc., 5225-4 Verona Rd., Madison, Wis. 53711. Phone: (608) 276-5600. Fax: (608) 327-6700.