Frequently Asked Questions

  1. What is LXI?
  2. Why do we need LXI?
  3. Why Ethernet instruments?
  4. Is any instrument with a LAN port an LXI instrument?
  5. Can anybody develop LXI modules?
  6. What benefit does LXI bring to test developers?
  7. How does LXI differ from rack and stack instruments?
  8. You talk about system-ready instruments. How is LXI different?
  9. You talk about synthetic instruments. How are they different from LXI?
  10. What is the LXI consortium?
  11. PXI has over 100 suppliers and LXI has only a few. How can you claim that LXI is the wave of the future?
  12. Is VXI dead?
  13. VXI and PXI have a trigger backplane. How will LXI trigger its modules?
  14. LXI, USB, and IEEE1492 have long first word latency. Is there anything defined in the LXI Standard to circumvent delays from the TCP/IP stack or packet collisions?
  15. Does LXI replace PXI?
  16. Where can I get a copy of the LXI specifications?

1. What is LXI?

LXI is an instrumentation platform based on industry standard Ethernet technology designed to provide modularity, flexibility and performance to small- and medium-sized systems. LXI’s compact, flexible package, high-speed I/O and reliable measurements meet the needs of R&D and manufacturing engineers delivering electronics for the aerospace/defense, automotive, industrial, medical, and consumer electronics markets.

The LXI standard defines small, modular instruments using low-cost, open-standard LAN (Ethernet) as the system backbone. LXI was developed to offer the size and integration advantages of modular instruments without the constraints and cost and constraints of card-cage architectures. The standard will evolve to take advantage of current and future LAN capabilities that go well beyond legacy T&M connectivity capabilities.

LXI modules nominally have no front panel or display, using the host PC and Ethernet connections to present setup and results. DUT connectivity is reserved for the front of the module, with line power and Ethernet IO (RJ-45) on the rear panel. LXI modules feature self-contained power supplies to improve reliability, lower costs, and to enable widely distributed system architectures. Measurement IP may reside on the module, in the PC, or both and the LXI standard encourages seamless software migration between rack and stack instruments and LXI modules, where applicable, to preserve user software investments.

Customer reaction to LXI, particularly from systems integrators, has been overwhelmingly positive.

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2. Why do we need LXI?

Extensive customer research showed customers want to reduce set-up and integration time by connecting directly to the standard ports on their PC. They don’t like specialized cards and cables, they hate expensive Slot 0 controllers, and they expect software and drivers that simplify test system set-up. They prefer small footprints for deployed systems, yet they like full-featured instruments for development and troubleshooting. They told us they want the capability of their favorite instruments packaged in a small, easy to integrate format.

Military customers asked for synthetic instruments that feature state-of-the-art microwave performance. Some previous implementations simply did not have the board space to create high performance instruments, forcing integrators to use both card-cage and stand-alone architectures in their systems, eliminating expected size savings.

In response to requests for PC-standard IO, several T&M suppliers began requiring LAN and USB ports on all new instruments. This proved enormously popular. The LXI standard takes industry-standard IO one step further by removing front panels, displays, and expansion cards to shrink the physical size for deployed systems. Now, customers with limited floor space or complex logistics needs can get state-of-the-art performance in small packages. LXI offers a significantly smaller form factor, simple low-cost PC-standard IO, and easy rack mounting with no card-cage, yet maintains linkages with full-featured instruments for development and troubleshooting.

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3. Why Ethernet instruments?

Ethernet is the logical choice. PC-standard technologies offer clear advantages to instrument applications, as the PC industry employs far more engineers than the test industry to develop, improve, and cut costs on PC-standard technologies. It simply makes sense to stick to PC-standard technologies where possible and focus test engineering resources on what they do best, make great measurements.

Ethernet is by far the most widely accepted communications interface in use today; nearly every computer is manufactured with an integrated Ethernet interface and networking hardware is becoming increasingly inexpensive. Ethernet speeds have increased three orders of magnitude in the past fifteen years, yet backward compatibility is preserved; GPIB and MXI have not matched this feat. Further, many of the attributes that make Ethernet so popular to the computer industry are also attractive to the instrumentation community, yet instrument IO standards like MXI and GPIB have not kept pace. Rather than replicate what the Ethernet community has developed, it makes far more sense to leverage Ethernet engineering.

Technical advantages such as TCP/IP error checking and fault detection, long inter-device connectivity, and peer-to-peer communications clearly exceed the limitations of parallel bus and other serial based interfaces. The TCP/IP stack provides error detection and correction that will typically not interfere with throughput rates, especially when a dedicated test system network is used. Furthermore, Ethernet connections can span 100 meters point-to-point, encompass a radius of 200 meters with the use of a hub, switch, or router, or extend to thousands of kilometers if fiber interfaces are used.

Some advantages of Ethernet include:

  • UPnP, SNMP, DHCP, etc. enable automatic discovery, addressing, asset management, and network management
  • Serve up web pages, UI, manuals, and support. Local or server based—it is invisible to the user. New use models, including remote desktop, instrument sharing, and multi-site collaboration are possible
  • Unlimited range, unlimited number of nodes.
  • Multiple media choices, from CAT5 cable to wireless to glass, giving integrators unprecedented connectivity and isolation options
  • LAN cables cost pennies per meter, compared to GPIB and MXI cables that cost $100 per meter or more
  • All PCs have a standard LAN port. No special cards are needed.
  • Speed scales with processor
  • Simple linkage to enterprise software and peripherals.
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4. Is any instrument with a LAN port an LXI instrument?

The short answer is no. To ensure interoperability among suppliers, standards must define what LXI is and is not. Failure to adhere to the specifications will result in poor interoperability and user frustration.

The LXI standard strives for simplicity. It relies heavily on industry-standard definitions such as 802.3, IVI, IEEE 1588, etc. While these go a long way toward ensuring interoperability, there are many instrument-specific requirements that must be addressed before Ethernet based devices can be accepted as the next generation platform for modular instruments. Some of these include:

  • Cooling
  • Triggering
  • Interrupt Handling
  • Mechanical Interfaces
  • Discovery
  • Multiple Device Synchronization
  • Software Interfaces
  • Network Routing, Switching
  • EMI/RFI

The most challenging aspects of LXI implementation involve instrument synchronization and timing, test network architecture, and software interoperability. The LXI specification outlines different implementation approaches to address these issues.

The LXI Consortium will develop detailed technical specifications over the coming months.

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5. Can anybody develop LXI modules?

The LXI Consortium is open to any company interested in expanding LAN-based instrument capabilities. We have several levels of participation, so most companies should find a level that fits their interests and budget.

To ensure interoperability and a consistent user experience, LXI is a trademarked brand that may only be used by Consortium members.

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6. What benefit does LXI bring to test developers?

  • LXI is based on standards--Ethernet (IEEE 802.3), TCP/IP protocols, LAN cables, web browsers, IVI-COM drivers, and standard rack sizes.
  • LXI offers seamless migration from product development through manufacturing. Engineers can use traditional instruments during the R&D phases, when a user interface is important. When moving to manufacturing, they can migrate to small, faceless LXI modules that use the same software and test routines developed on the standard instruments.
  • LXI delivers flexibility with a modular approach to instrumentation. Purchase the instrument modules desired without the need for a cardcage, slot 0, or proprietary link. In the case of RF instruments, LXI brings the benefits of synthetic instruments (modules that can be mixed and matched to form traditional instruments).
  • LAN offers new capabilities. The LAN industry spends hundreds of millions of dollars each year employing thousands of engineers to improve the speed, performance, and capabilities of LAN. There are more engineers working on LAN alone than in the entire test and measurement industry. Today, LAN offers capabilities that extend far beyond the crude point-to-point connectivity offered by MXI, GPIB, and other test-industry standards. Peer-to-peer communications, remote operation, synthetic instruments and simultaneous operation are a few examples.
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7. How does LXI differ from rack and stack instruments?

From a measurement perspective, LXI and rack and stack instruments are identical. They use the same measurement science, boast the same specs, and offer the same features, and run the same software.

The difference is in how it is packaged. Standard rack and stack instruments are designed to be used standalone (without a PC) from their user-friendly front panels. By removing handles, bumpers and feet these same instruments can be mounted into a rack and run with a PC. LXI modules, on the other hand, are controlled with a PC and mounted into a test system. They do not have a front panel so they have all the signal connections on the front and all the power and communications signals on the back. They are optimized to run over high-speed LAN.

LXI offers some advantages not found in rack-and-stack instruments. By using LAN, LXI supports peer-to-peer operation, which opens the door for synthetic instruments and simultaneous operation, speeding measurements, reducing redundancy, and driving cost savings.

The test industry learned a few things from VXI, PXI, and MMS, and we applied these lessons to LXI. Since modular instruments represent a subset of the overall test equipment market, making specialized modular instruments can be expensive and inefficient. Inevitably, the industry wants the capabilities and price of high volume instruments, yet low volumes make that impossible. Rigid footprints often require custom-engineered instruments that have little in common with rack-and-stack counterparts, leaving little room for leverage. Development costs are spread across a small base and unique supply chain footprints fail to generate scale economies. We saw this with VXI scopes that cost many times more than their rack and stack counterparts because the VXI products were essentially custom engineered products, sharing few parts with their rack/stack counterparts. In the case of LXI, the boards and software are identical to the rack/stack versions, giving customers the same performance, software, and costs they get in high-volume rack/stack versions. The result is greater product availability, improved compatibility with full-featured bench instruments, and generally lower costs.

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8. You talk about system-ready instruments. How is LXI different?

LXI complements system-ready instruments. System-ready instruments are designed to be open -- used either on the bench with their user-friendly front panel or in the test system via its standard GPIB, LAN, and USB communication ports.

LXI products are small faceless modules for use in an automated test system, with the PC providing the user interface, typically through a web interface. They provide the same performance and may be a subset of the functionality found in full-featured system-ready instruments. Some LXI modules will be lower cost than system-ready instruments, because of less functionality, but in some cases, like synthetic instruments, the modules may be more expensive because of the benefits and capabilities they provide over traditional instruments.

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9. You talk about synthetic instruments. How are they different from LXI?

Since LXI is an implementation vehicle for building synthetic instruments, we view synthetic instruments as a subset of LXI.

Synthetic instruments are traditional instruments broken into fundamental building blocks (digitizer, arb, up-converter, down-converter, etc.) and PC software to aggregate and “synthesize” different measurement tasks. The blocks can be used in different combinations to make different kinds of measurements, where the blocks are often dynamically re-configured to perform different measurement tasks at different times. An example may be an RF downconverter that is used for spectral measurements for one test sequence then gets redeployed to make network analyzer measurements in another test sequence; rather than having similar downconverters in stand-alone instruments, where one sits idle most of the time, the synthetic downconverter sees double-duty, saving cost, size, and logistics footprint.

Synthetic instrument building blocks can be any modular format. LXI uses low-cost LAN as the backplane on which to build and connect these functional blocks. Some LXI modules will be self-contained instruments while others will be functional building blocks, depending upon the application. Therefore, synthetic instruments may be in the form of LXI modules but not all LXI modules will be synthetic instruments—some may be complete instruments. LXI offers this flexibility.

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10. What is the LXI consortium?

The LXI Consortium is a not-for-profit (501c3) corporation made up of leading companies in the Test and Measurement Industry. The group’s goal is to ensure interoperability and a consistent user experience by developing, supporting, and promoting the LXI standard. Since announcement in September 2004, many companies have expressed support for a LAN-based T&M standard. Please see the Company Directory for the latest list of members.

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11. PXI has over 100 suppliers and LXI has only a few. How can you claim that LXI is the wave of the future?

LXI is based on LAN and LAN has 1000’s of suppliers and millions of users. Unlike the PC that keeps changing -- ISA to EISA to PCI to cPCI to PCI-X to PCI Express-- LAN continues to get better, increasing its bandwidth three orders of magnitude from 10 Mb/s to 10Gb/s.

Within test and measurement, many leading suppliers already provide LAN-based instruments. Customers are very excited about LAN, and they seek seamless interoperability between vendors. LXI is the next logical step to provide a more modular format and software standards for the test system.

Many LAN-based products available today are based upon vanilla Ethernet implementations, often simply replacing GPIB with CAT5. They do not address instrumentation-specific issues such as synchronization and timing, nor do they bring many of the latest Ethernet capabilities such as DHCP, SNMP, UPnP, etc. to the instrumentation world. LXI will support IEEE 1588, specifically designed to ensure accurate device triggering and phase relationships.

LXI will complement PXI to support the quality measurements needed for electronic test. We anticipate LXI frames that will accommodate PXI modules, making all PXI modules available in LXI and allowing users to build systems one module at a time, free from card-cage constraints.

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12. Is VXI dead?

VXI continues to get investment for applications that require the high-speed and high-density benefits this architecture delivers. We’ve found VXI works well in aerospace-defense, automotive, high-speed digital, and telecom applications where tight inter-module linkages and high channel count are critical. But as noted above, this capability comes at a steep price premium. For general-purpose instruments, and in small to medium size applications, most customers prefer to use rack & stack because they offer a manual user interface and display, they are lower cost for equivalent capability, and they offer the flexibility to use the same instrument on both the bench and in the rack.

LXI leverages and builds upon some of VXI’s most attractive features. It brings the advantages of computer independence and a modular architecture (small form factor and high-speed communications) without the overhead of a cardcage and proprietary link to the PC. With LXI, each module is self-contained, requires no start-up decisions, and can be added without penalty as needed.

LXI allows suppliers to leverage designs from system ready instruments into a form factor that better suits the needs of the test system engineer. With VXI, suppliers often developed products twice—once with a GPIB interface and once with a VXI interface. LXI allows them to develop a product once with a standard LAN-based interface, allowing suppliers to leverage hundreds of $M in development dollars each year

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13. VXI and PXI have a trigger backplane. How will LXI trigger its modules?

LXI will be triggered like traditional GPIB instruments, where users can send a trigger over the Trigger Bus to start a measurement from an external event. The LXI Trigger Bus interface follows traditional triggering paradigms, allowing LXI to seamlessly synchronize with other architectures such as VXIbus devices. The LXI Trigger Bus implementation is defined around a high-speed differential LVDS interface.

LXI will also implement IEEE 1588 Precision Timing Protocol (PTP), giving users triggering directly over the LAN, simplifying cabling. This standard was developed for sub-microsecond synchronization of LAN-based devices anywhere in the world and work is well underway to achieve very low nano-second timing between modules. PTP is ideally suited to large channel count, distributed measurement applications like jet engines, generators, antenna ranges, or process plants. See the NIST IEEE 1588 web site for more information.

Additionally, the LXI specification will adopt the Trigger Bus interface to address the seamless synchronization of LXI instruments with other architectures such as VXIbus devices. The LXI Trigger Bus implementation is defined around a high-speed differential LVDS interface.

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14. LXI, USB, and IEEE 1492 have long first word latency. Is there anything defined in the LXI Standard to circumvent delays from the TCP/IP stack or packet collisions?

As with any LAN environment, traffic management can impact timing. Collisions are not an issue when using switches, although without priority, you can still get stuck in queues; for example, a message may get stuck behind a very large message and even with priority, it will continue to trail the long message in subsequent queues.

The question assumes that all actions are triggered by receipt of a message. If test sequences or other actions can be defined ahead of time and stored as scripts in the executing device, then IEEE 1588 PTP allows these scripts to execute without message delays and retain sync across multiple instruments. The scripts contain action/execution time specifications either absolute or relative to some 'start' time which is then the only message that needs to go over the LAN at run time. Time based execution reduces control traffic on the communication media by replacing much of the run time traffic with configuration traffic prior to execution when the latency issue is not present.

Even if a 'start' message or its equivalent needs to go over the LAN, latency can be overcome with circular time stamped buffers in all acquisition measurements (like a distributed logic analyzer). It is only when an action changes, as opposed to measures, the DUT that latency becomes a concern, and then only if the execution time is truly impossible to pre-compute or bound. Even here there will be a fraction of the cases where the action can be rolled back if it is later determined that it was inappropriate.

Further, 'arming' and 'triggering' functions can avoid much of the OS latency. Of course you would also need to control the switch environment. Also, there are new technologies like RDMA (requires special NIC HW) that are designed to "short circuit" the TCP stack latency and yield latencies around 40us or less. RDMA is just emerging and to this point has been focused on tasks like SCSI transport.

We have measured "raw socket round trip" latencies between two 3GHz class PCs with Gb NIC HW known to have good latency characteristics (but no RDMA support) below 70us. We expect, over time, Moore's Law will help here as fast processors and GB LAN become more common.

Some leading edge work underway has demonstrated low ns synchronization latencies.

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15. Does LXI replace PXI?

The LXI Consortium includes some major PXI players. LXI will complement PXI for customers that need more accuracy, resolution, bandwidth, and functionality than is possible in PXI. Since LXI leverages technology from rack/stack, it offers cutting edge performance not normally found in modular formats. LXI is a solution aimed at customers performing automated tests on electronic devices, while PXI is a better solution for many lower-end data acquisition applications.

The LXI Consortium looked at the ubiquity and low cost of LAN. LXI does not require special cards or cables since every PC now comes standard with LAN. LAN-based LXI supports synthetic instruments and peer-to-peer networking, enabling some unique capabilities not presently available to the test engineer in any format.

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16. Where can I get a copy of the LXI specifications?

The most recent version of the LXI Standard is LXI Device Specification 2011, Rev. 1.4 (Released May 18, 2011), and it is available for view/download  at http://lxistandard.org/Specifications/Specifications.aspx. Older versions of the specifications are located at http://lxistandard.org/Specifications/OlderSpecifications.aspx.

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