Instrument Ethernet links get standardized
Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 3/1/2005
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| The peer-to-peer communications that the LXI standard allows permits functional blocks to combine into synthetic instruments. Courtesy of Agilent Technologies. |
The utility of tying bench instruments together to automate test and measurement has long been proven. The venerable GPIB (general purpose instrumentation bus) has been used for more than 30 years. The interface is showing its age, however, and is being challenged by the industry's need for increasing bandwidth, faster data rates, and lower cost in its small- and medium-sized systems.
A number of potential replacements have come forward, including USB, FireWire, and the CANbus, but none has gained widespread support in the industry. This has left a void that Agilent Technologies and VXI Technology sought to fill by creating both the LXI (LAN extensions for instrumentation) standard and the LXI Consortium (www.lxistandard.org) to administer it. Many major instrument companies, including Keithley Instruments and Measurement Computing Corp., have joined the consortium since its creation in September 2004.
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| Representative LXI instruments such as this temperature measurement module are already appearing, based on draft versions of the standard. Courtesy of VXI Technology. |
The new standard also uses Ethernet for its low cost and future potential. "LXI can leverage ever-increasing Ethernet throughput, making it ideal for next-generation automated test system challenges," said Pat Byrne, VP and GM of Agilent's Wireless Business Unit. "It is not limited by bandwidth, software, or computer architectures, and it provides a basis for long life-cycle instrumentation."
The use of Ethernet to connect test instruments is not new, of course, but the current command structure for instruments and the data-communications formats are different for each instrument family. The LXI standard would unify the interactions among instruments. "We're not turning the world on its ear with this," said Bob Vogel, VP of marketing at consortium member Aeroflex Test Solutions. "We're just trying to come up with a standard approach that allows common system calls and software reutilization to make life easier."
Dropping the user interfaceThe LXI standard has both mechanical and functional elements. On the mechanical side, the standard calls for instrument modules to be rack-mounted and self-contained, with their own power supply, processor, and LAN connections. Modules can be either one- or two-rack units in full- or half-width sizes in order to make them easy to mix-and-match within the rack. Because the modules are controlled by a computer and are linked to the user through the network, they do not require displays, buttons, or dials. Instead, the user interacts with the modules through standard Web browsers and COM drivers to simplify system control and integration. The mechanical standard also calls for modules to have signal I/O ports on the front panel and have LAN and AC power connections on the rear.
The functional side is more complex to define, and the consortium is working with all its members to prepare detailed technical specifications addressing issues such as triggering, interrupt handling, discovery, synchronization of multiple devices, and software interfaces. Some of the most challenging aspects of LXI implementation are timing and instrument synchronization. The intent is to address these issues using existing standards. Triggering, for example, will use a Trigger Bus that will be similar to that of GPIB instruments and will be compatible with VXIbus devices. The Trigger Bus will use a high-speed differential LVDS interface.
For timing, LXI will implement the IEEE 1588 Precision Timing Protocol (ieee1588.nist.gov/). This protocol allows users to simplify cabling by triggering instruments over the LAN instead of using the Trigger Bus. The standard was developed for submicrosecond synchronization of LAN-based devices regardless of their separation, and work is underway to achieve low-nanosecond values.
High performance is only one of the goals for LXI instrumentation, however. The consortium expects that LXI will help lower the cost of test instrumentation as well as simplify the implementation of manufacturing test efforts once a design is complete. A move to implement synthetic instrumentation is also expected to come with LXI adoption.
LXI helps to reduce test instrumentation cost by eliminating the redundant user interfaces in a test system. The modules share the user interface provided by the host computer, saving the cost of displays, dials, and the like for each instrument. In addition, eliminating these elements allows the instrument to be packaged in a smaller housing with less cooling required.
Standard interfaces support software reuseSimplifying the manufacturing test effort relies on an expected move by LXI vendors to create LXI modules that use the same boards and software as their bench instruments, minus the user interface hardware. Such hardware reuse further helps to lower cost, but more importantly, it facilitates software reuse. As development engineers create software and procedures for system test using their bench instruments, their efforts can instantly translate to the equivalent LXI modules for use in manufacturing, because the bench and LXI instruments are essentially identical.
This feature of LXI may be of special interest to communications engineers. LXI will be able to provide them with the latest technology in a form useful both on the bench and in the factory. Then, by quickly converting development test software into manufacturing test software, LXI can help communications developers meet critical time-to-market deadlines.
Communications engineers may also be interested in the synthetic instrument possibilities offered by LXI. Synthetic instruments are the result of breaking traditional instruments into their fundamental building blocks, such as digitizers, upconverters, and the like, and then using software to combine these blocks to synthesize the desired instrument. This approach helps eliminate the redundancy found in a collection of stand-alone instruments, thus lowering cost. It also helps ensure consistency in measurements because the same functional modules are used in each test. The peer-to-peer capabilities of LXI give it the ability to implement such synthetic instruments.
The LXI Consortium expects to have its specifications available by mid-2005. Member companies will then begin implementing LXI interfaces for key products, although no specific timing has been announced. Even so, there are some prototype products available. VXI Technology is showcasing its EX1048 and EX1629 temperature and strain-gauge instruments as examples of LXI implementations. In addition, Agilent has created a prototype set of modules for creating synthetic RF equipment.
Ultimately, LXI represents a natural evolution in instrumentation. With LAN connectivity getting less expensive and more capable, it was inevitable that electronic instruments would adopt it as an easy-to-use standard interface for user control. The LXI Consortium hopes to ensure that configuring the instruments to share data and work in concert becomes just as easy.
