LXI development continues
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 4/25/2006 2:21:00 PM
LAN eXtensions for Instrumentation (LXI) is an Ethernet-based standard for instrument communication and control. Its proponents claim that it lets Ethernet replace IEEE 488 as the first choice for instrument communications. While that transition will take years, adoption of LXI has begun. To learn more about the progress of LXI, I spoke with Bob Rennard, program manager at Agilent Technologies and president of the LXI Consortium (www.lxistandard.org).
Formed in 2004 by Agilent Technologies and VXI Technology and quickly joined by Keithley Instruments, the LXI Consortium currently has over 40 member companies, including test-equipment makers, system integrators, and military/aerospace companies. "We formed the LXI consortium when we saw equipment makers building Ethernet into their instruments," said Rennard. "We didn't want individual companies to develop their own interoperability standards."
| For more information
"IEEE 1588 in Test and Measurement Applications as Specified in LXI Standard v1.0," a presentation by Bob Rennard "IEEE 1588 Keeps Time In Sync," by Martin Rowe. "Learn to apply LXI instrumentation," by Rick Nelson. The article summarizes a Webcast produced by T&MW and sponsored by Agilent Technologies, VXI Technology, and Keithley Instruments. LXI product announcements covered by T&MW: VXI Technology's EX2500 controller: Agilent's LXI-compliant DMMs: Elgar's DCS and DLM lines of programmable DC power supplies: Keithley's Model 2910 vector signal generator: Pickering Interfaces' 60-100 chassis: |
At around the same time, the LXI consortium expects to release version 1.1 of the specification. "Version 1.1 is a minor release," Rennard said. "It clarifies some ambiguities in version 1.0." During our discussion, Rennard pointed to the advantages of a LAN-based communication bus, citing that every PC comes with an Ethernet port, which eliminates the need for an IEEE 488 interface card and the bulky cables that IEEE 488 requires. (Editor's note: Some engineers prefer the bulky IEEE 488 cables because you can bolt them in place and they're stronger than Ethernet cables.) With Ethernet, a PC can automatically identify instruments and their addresses.
Ethernet also provides for peer-to-peer communications, something you don't get with IEEE 488. For example, an overvoltage condition on a power supply can initiate messages to other instruments for them to begin a shutdown sequence.
With Ethernet, you're also not limited by the number of network nodes or by distance. Rennard cited an application that used over 100 Elgar power supplies controlled by a single PC. With Ethernet, the system used standard routers and switches to connect the power supplies to a PC.
Because Ethernet sends data in packets, triggers and interrupts can contain instrument-specific information. For example, when an instrument requests service from a system controller, the controller will know which instrument needs service. With IEEE 488, instruments can send interrupts, but the system controller must check each instrument to find the one needing service.
LXI also offers precision timing. Instruments that meet LXI Class A and Class B requirements take advantage of IEEE 1588 (ieee1588.nist.gov), a network-timing standard, to synchronize and time stamp measurements. "Data-acquisition users like the IEEE 1588 timing, particularly in large-channel-count system such as those used in jet-engine test," said Rennard.
The LXI standard requires that instruments include a Web server. Thus, you can configure and control instruments by simply connecting them to a PC and using your browser. You can automate a Web interface to let you get model number, serial number, and calibration data from instruments in a test rack.
LXI-compliant instruments can also run scripts stored in their memories. Thus, you can create a distributed network for test. Each instrument has a set of commands, and you can group them together into scripts that you download from a PC. Then, the PC just gives an instrument a command to run a script. "Ethernet works best in a 'chunky' environment as opposed to a 'chatty' one," commented Rennard. By keeping the scripts in the instruments, you minimize the number of small data transfers.
A common application of a script is for a function generator or arbitrary waveform generator. You download a script containing instructions for generating a waveform and run it whenever it's needed. Because the scripts still reside in a main program, you can download them into a replacement instrument. Keep in mind, though, that scripts are manufacturer specific. Therefore, you'll likely need a new script if you change manufacturers.
Whenenver you connect an instrument to a company LAN, you intrude on the space administered by your information technology (IT) department. They worry about network security because test instruments are unknown to IT people.
"The military people have the best security for their Ethernet-based test systems," said Rennard. "They don't connect instruments to the network. They connect Ethernet-based instruments to a PC that doesn't connect to the corporate network." Alternatively, you can connect your test-system computer and instruments to a subnet, which isolates your test system from a corporate network.
Because it's an industry standard, LXI has the potential to replace IEEE 488 in many test applications. The number of available of LXI-compliant instruments, which Rennard expects to top 75 by year's end, is still small compared to IEEE 488. At first, engineers will begin to use LXI-compliant instruments in conjunction with IEEE 488 instruments. Some systems will use LAN-to-IEEE 488 converters to keep instruments in use. There are just too many IEEE 488-based instruments in use and on the market today for Ethernet to quickly replace the bus.
