Redefining The Workstation

PCs AND CARD CAGES
Shawlee and other engineers have noted that some PCbased instruments (those that plug directly into a PC's PCI slot) are limited in usefulness. While they're inexpensive, they often emit or are susceptible to noise. Also, ground loop problems are common. According to NI director of measurement technology Ken Reindel, if these cards are designed properly in the first place, these issues become non-issues—whether the card lives in a PC or in a box.

"The problems that need to be solved to put precision, low-noise instrumentation in a PC environment are similar to those of building a traditional box product, because today's box products contain at least one microcomputer, display, power supply, and user interface much like a computer does," says Reindel. "Either way, the sensitive circuitry must be properly shielded, power supplies must be properly bypassed, and layout must be optimized to avoid interference-induced errors."

VXI (a VME-based card system) instruments represent a high-end solution to these PC issues, but they tend to be quite expensive. These industrial-strength, high-speed instruments-on-a-card work well in teststand applications for aerospace, military, automotive, and telecom applications but would be too expensive to be considered a workstation for the average design engineer.

UPGRADING FROM GPIB
GPIB, the venerable communications link between test equipment and host computers, hasn't necessarily lost its effectiveness. However, as many industrial automation users are discovering, equipment based on readily available off-the-shelf (OTS) PC communication technology can be less expensive—and it may be even easier to install and configure.

Harpaz notes that a significant portion of his company's test-instrumentation development effort was directed toward the actual means of communication with various metrology equipment, including an old laser interferometer that uses IEEE-488. "Whatever other devices may be available in the future, I would like all instrument makers to equip them with some easy communication links such as USB or Ethernet," he says.

Harpaz's wish is slowly coming to fruition. In 2004, a consortium cofounded by Agilent and VXI Technology Inc. introduced LAN eXtensions for Instrumentation (LXI, www.lxistandard.org). LXI combines features from VXI and GPIB. But it removes some of the more expensive items, such as the card cage, slot 0, and expensive PC-to-instrument communications link (typically $100/m). Ethernet was chosen as the communication link.

Chuck Cimino, the business development manager at Keithley Instruments and an LXI Consortium member, says that USB also was considered, but its deficiencies were disconcerting. With PC architectures changing so quickly, it's unknown how long USB will be supported. Ethernet has a 30-year track record with faster and faster technology that's always backward-compatible.

While LXI instruments can stand alone, the specification defines various module sizes for rack mounting and allows for tying existing VXI and PXI card cages into the system with a network adapter (Fig. 4). Cimino notes that signals enter and exit modules from the front while LAN, power, and trigger connections are located on the rear.

In addition, LXI divides instruments into three classes. Class C (lowest level) provides basic discovery and Web content. (The instrument must be able to serve up a Web page.) Class B includes the IEEE 1588 precision time protocol. And, Class A adds event triggering through an external triggering bus called LXI Trigger.

The LXI Consortium released LXI Revision 1 last September. Agilent, Data Translation, Fluke, Keithley Instruments, Kepco, Lambda, Racal, Teradyne, Yokogawa, and other companies support LXI. Instrument makers like Fluke, says Hammond, are monitoring the activities of the LXI Consortium because LXI could help define Fluke's next-generation products.

LXI is scalable. Engineers can begin with a host computer and one or two instruments and then add devices to run a large test stand. LXI is all about compatibility. Cimino makes an interesting comparison. "Think about what PXI Express will do to your PXI investment. Then think about what 10-Gbit Ethernet will do to your LXI investment. PXI Express instruments won't plug into a PXI mainframe," he says. So, can your old 10-Mbit Ethernet node talk to your 1-Gbit node? Yes, it can!

Unless you're equipping your "workstation" to test the latest silicon chips or the fastest computer buses, you won't need the highest-end instruments available. But you will expect reliability, accuracy, repeatability, and quick responses to signals you're trying to observe. Even more so than today, software and communications in the future will redefine the scope of your instrumentation—your workstation—and communicate your data to wherever it may be needed in the enterprise.

See Figure 5