LXI (LAN extensions for Instrumentation) is the standard for Ethernet control of intelligent instrumentation. LXI provides a powerful, low-cost bus alternative to GPIB for communication with instruments and integrates seamlessly into existing test systems using GPIB or other instrumentation standards such as VXI, PXI, PXI Express, or AXIe.
LXI has seen unprecedented growth in the test-and-measurement (T&M) industry since its introduction in 2005. Currently, there are over 55 of the top T&M companies worldwide sponsoring and developing this technology, and there are more than 1740 products in over 220 different product families (Figure 1).
Since LXI is based upon Ethernet, it provides a simplified infrastructure, readily accessible cabling, and a very stable platform that has consistently evolved over the last 30 years while maintaining backward compatibility. Gigabit Ethernet provides bandwidth that rivals the VXI (VXIBus) and PXI (PCIBus) platforms.
In fact, LXI instruments are not that different from traditional, modular, rack-and-stack instrumentation such as VXI or PXI, but they do offer some advantages. From a measurement perspective, there is really no difference. The main difference is in the way that instruments are controlled and communicate with one another and are physically located in the test system. Rack-and-stack instruments share a common chassis, backplane, and processor. Ethernet serves as the backplane in LXI. That is one of the distinct advantages of LXI that makes it ideally suited for applications where the instrumentation hardware is distributed throughout a test system, lab, or separated by a significant distance from the host computer and connected to a company’s WAN or the Internet.
LXI Core and Extended Functions
The LXI specification is defined as a group of “Core” features and optional extended functions. In order to be certified as LXI-compliant, devices must comply with the “Core” features defined in the LXI Device Specification and can optionally comply with one or more of the extended features (Figure 2). LXI extended functions replace the previous Class model in earlier versions of the specification.
The LXI Core features can be summarized as follows:
- Open industry standards such as TCP/IP Ethernet, IPv4/IPv6, Web browsers, and IVI drivers
- High-speed Ethernet I/O that support data rates of 100 Mb/s to 10 Gb/s and even faster in the future
- Built-in Standardized Web Interface to easily configure and operate the instrument, collect and analyze data without software programming, and operate instruments remotely
- Interchangeable Virtual Instrument (IVI) drivers for programmatic control that works reliably in a variety of programming environments, provides high performance interfaces, and eases program development and maintenance.
Timing and Clock Synchronization
In distributed test systems, it is extremely important for all devices to be synchronized in order to correlate measurement data. Since rack-and-stack instruments share a common backplane, and therefore reference the same clock, all modules are very tightly synchronized.
Similarly, the LXI clock-synchronization extended function inherits IEEE 1588-2008, a protocol used to synchronize real-time clocks with submicrosecond accuracy in devices of a networked distributed system. This allows common timer events to be tied to absolute times for very precise triggering and synchronization and also enables the correlation between instruments in order to aid monitoring and debugging.
In addition, the LXI time-stamping extended function enables all measurements to include an IEEE-1588 timestamp, indicating the point in time that a particular LAN event such as triggering, measuring, or connecting channels occurred.
The LXI Wired Trigger Bus (WTB) extended function is a hard-wired means of connecting LXI devices together that allows them to receive hardware trigger signals from one another or from a system controller on a dedicated bus interface. The WTB provides eight independent trigger channels, not dissimilar to the 8-line hardware trigger buses that are commonly used in VXI and PXI. In ATE systems, where test instruments are usually close to one another, the trigger sent over the WTB is very precise and corresponds to that found in back-plane systems.
As an alternative to the WTB, the LXI event-messaging extended function enables LXI instruments to signal each other over the Ethernet cable based on events or time without computer intervention. The accuracy of triggering times with the WTB is in the order of nanoseconds, compared to that of LAN-based triggering that is typically in the millisecond range, depending on the distance the trigger needs to travel to reach the device.
Power and Cooling
In backplane-based test systems, all modules share the same power supply and cooling. This is often inefficient and costly since the power supply is one of the most expensive components and is most often not used to capacity. On the contrary, in LXI instruments, the power supply and cooling are designed for precise requirements specific to the instrument itself and therefore reduce overhead and cost.
Since LXI instruments communicate via Ethernet, they are ideal to be used in distributed or remote test systems and can be closely located to the UUT reducing costly cable lengths and increasing the precision of the measurements. In addition, the Web interface available on all LXI instruments enables them to be controlled remotely from anywhere in the world at anytime. This is ideal for troubleshooting and system monitoring and is simply not possible with rack-based test systems such as PXI and VXI, where test engineers must be physically present in front of the test system to communicate with it.
With the LXI event-log extended function, it is possible to see what is happening in an instrument or test system at anytime. The LXI event log records LAN events such as setting a trigger or taking a measurement.
Ready for the Future
The LXI standard continues to evolve and keep pace with advancements in technology. Along with such advancements also comes the retirement of tried-and-true protocols such as VXI-11 and IPv4. Two of the most recently approved extended functions address these issues.
The HiSLIP extended function leverages the HiSLIP standard created by the IVI Foundation to create a fast control interface and extend the features of VXI-11, as well as emulate the capabilities of GPIB devices. The IPv6 extended function ensures that vendors of LXI instruments approach IPv6 in a consistent way and comply with government standards before its widespread use in test systems.
Hybrid Test Systems
Often, it is cost-prohibitive to create a completely new test system. Engineers need to reinvent their existing systems to fit test requirements by integrating new technologies with older hardware and software. It is not uncommon for a single test system to include VXI, PXI, GPIB, AXIe, USB, and LXI instruments. Hybrid test systems enable test engineers to maximize their existing investments while incorporating the latest technologies such as LXI instruments. In some instances, LXI features such as IVI drivers make it relatively simple to migrate code from a different platform to LXI as its replacement. This strategy can be an interim solution prior to migrating to a fully LXI-based system.
Facing increased cost pressures, test system engineers need to design their systems to last longer, be more flexible than ever, and take advantage of readily available commercial-off-the-shelf (COTS) equipment. Since LXI is the standard for controlling instruments over Ethernet, it provides a powerful, efficient and cost-effective alternative to GPIB and rack-based instrumentation.
LXI uses standard I/O and off-the-shelf, inexpensive LAN cables and routers or switches. Nearly every computer today includes an integrated Ethernet interface, and there is no need for special hardware such as GPIB interface cards, chassis, or cables. All LXI instruments include standard drivers for easier test programming and a consistent, built-in Web interface for faster configuration, operation, and maintenance.
LXI enables new applications by overcoming the challenges of distributed systems and remote monitoring, previously not possible with rack-based instruments. LXI integrates seamlessly within existing test systems using GPIB or modular architectures such as PXI, PXI Express, VXI, or AXIe, enabling test engineers to start taking advantage of LXI technology today and migrate to a complete LXI test system when they are ready.
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