USB reaches mainstream status

What do you get for $150? Web Exclusive: Compare the specs of a sampling of USB data-acquisition products from several manufacturers. (Microsoft Excel format)

Read other articles from this issue: Table of contents, June 2005 FEATURES China on the air Keep your eye on the metal bumps USB reaches mainstream status Video goes mobile Distributed data

National Instruments' DAQPad pioneered USB data-acquisition in 1998; both were slow-speed, high-resolution instruments. Data Translation was the first to offer acquisition rates comparable to plug-in cards (100 ksamples/s) when it introduced the DT9801 in 1999.

When those first USB instruments appeared, Windows support for USB was still in flux. With Windows 98 Second Edition, Microsoft stabilized USB support, although not until Windows 2000 did USB take off. Today, many USB data-acquisition products require Windows 2000 or Windows XP.

Figure 1.  Fluke’s Model 189 DMM now connects to PCs though their USB ports. Courtesy of Fluke.

Figure 2.  IOtech’s PersonalDaq/54 is a descendant of the PersonalDaq/56, one of the first USB data-acquisition modules. Courtesy of IOtech.

Not everyone wants to write custom application software, though. To aid users who want to collect data without programming, manufacturers of USB data-acquisition modules provide datalogging software for collecting and storing data. IOtech adds offline data-analysis and plotting software. As a general rule, the more you pay, the more you get for stand-alone software, although Dataq Instruments supplies a flexible datalogging program even with its $50 module.

Over the last year, Data Translation, Measurement Computing, and National Instruments have introduced USB data-acquisition modules priced just under $150. To get a feel for how these units work and what you get, I tried each of them. "What do you get for $150?" below explains my impressions of the modules and their software. (I chose not to evaluate the Dataq $50 instrument in order to keep the playing field level; thus, I limited my evaluations to the three $150 models.)

Some companies provide application-development packages with their USB modules. Data Translation, for example, supplies a fully functional version of DT Measure Foundry that runs for 14 days. In that time, you can develop run-time-only applications that don't expire. Measurement Computing supplies a fully functional, free copy of SoftWire, an application-development program for Visual Studio .NET.

USB isn't the perfect I/O bus for all applications, though. Some engineers question its usefulness in production environments, and with good reason. The USB cable's connectors can easily come out. In addition, the USB cables themselves lack the robustness of IEEE 488 cables, and they lack the length of Ethernet cables—because of signal reflections, the USB specification limits cable length to 5 m (Ref. 1).

The bits that travel along USB cables can move at "full speed" (12 Mbps) on USB 1.1-compliant modules and computers or at "high speed" (480 Mbps), provided that the host PC, the instrument, and all hubs in between comply with the USB 2.0 specification. But be aware of products that claim to be "USB 2.0 compatible." Such products may communicate with a USB 2.0-compliant port, but at full speed, not high speed. For example, National Instruments' Anderson said that at present, all of his company's USB data-acquisition products run at USB full speed but are compatible with USB 2.0-compliant ports. Check an instrument's data sheet if to see if will run at high speed.

Although USB moves the analog circuits away from the PC and closer to the measurement source, you can still get into trouble if you don't properly connect your signals. Interference caused by ground loops can produce measurement errors. If you use a USB instrument with a laptop PC running on battery power, you won't introduce a ground loop though the computer. If you use a PC connected to AC mains power, however, you can introduce ground loops through the power cord. "Isolate or denigrate" highlights how ground loops can cause errors and how USB instruments isolate measurement signals from PC grounds (Ref. 2). Keithley Instruments has also published a paper on making measurements with USB data-acquisition instruments (Ref. 3).

The 5 V of power that a USB cable supplies to a peripheral device may not provide enough power for the devices that a data-acquisition module controls. For example, multifunction data-acquisition modules provide digital I/O lines, and digital I/O modules provide as many as 96 I/O lines.

Because of the limited power of the USB, you may not be able to use digital outputs to source current. Or, the current you get may not be sufficient for your application. The best way to get adequate current is to use the digital outputs to sink current supplied by an external supply. Check the specification for a module's digital outputs to see what you need to drive your circuits.

With USB product offerings now wide enough to cover many applications, the bus is ready to take its place among the major choices of instrument buses. All that remains is to see how USB instruments from different manufacturers work together in automated test systems.

1. "USB Frequently Asked Questions," USB Implementer's Forum, Portland, OR. www.usb.org/developers/usbfaq.

2. Rowe, Martin, "Isolate or denigrate," Test & Measurement World, March 2005. p. 21.

3. Tucker, Jonathan, "Understanding the Benefits and Potential Hazards of Using USB-Based Data Acquisition Solutions for Test & Measurement Applications." Keithley Instruments. www.keithley.com/data?asset=50071.