LAN eXtensions for Instrumentation (LXI) is a standard developed by the LXI Consortium, an industry consortium that maintains the LXI specification and promotes the LXI Standard. The LXI standard defines the communication protocols for instrumentation and data acquisition systems using Ethernet. Ethernet is a ubiquitous communication standard providing a versatile interface, the LXI standard describes how to use those standards in instrumentation application in a way that promotes simple interoperability between instruments.
The LXI Consortium provides the structure that ensures instrumentation developed by various vendors works effectively as long as the instruments are compliant. The LXI Consortium ensures that the LXI standard complements test and measurement systems, such as existing GPIB and PXI systems.
Overview
Proposed in 2005 by Keysight (formerly called Agilent Technologies) and VTI Instruments (formerly called VXI Technology), the LXI standard adapts the Ethernet with the ubiquity of the World Wide Web and applies them to test and measurement applications. The standard defines how existing standards should be used in instrumentation applications to provide a consistent feel and ensure compatibility between vendors equipment.The LXI standard does not define a mechanical format, allowing LXI solutions to take any physical format deemed suitable for products in their intended market. LXI products can be modular, rack mounted, bench mounted or take any other physical form.
LXI supports synthetic instruments and peer-to-peer networking, which provides number of unique capabilities to the test engineer. Utilizing Ethernet communications, permits flexible packaging, high-speed I/O, and standardized use of LAN connectivity addresses a broad range of commercial, industrial, aerospace, and military applications.
LXI products may have no front panel or display or they may include embedded keyboards and displays. Connections to the DUT are permitted to be on the front or the rear to suit market demand, most devices provide front panel connectivity to allow Ethernet and power connections to be provided to the rear panel.
Use of Ethernet allows the simple construction of systems requiring distributed instrumentation systems and control and monitoring systems over large distances.
The inclusion of an optional Extended Function based on IEEE 1588 Precision Timing Protocol allows instruments to communicate on a time basis, initiating events at specified times or intervals and time stamping events to indicate when these events occurred in a system.
Interoperability
LXI devices can coexist with Ethernet devices that are not themselves LXI compliant. They can also be present in test systems which include products based on the GPIB, VXI, and PXI standards.
The standard mandates that every LXI instrument must have an Interchangeable Virtual Instrument (IVI) driver. The IVI Foundation defines a standard driver application programming interface (API) for programmable instruments. IVI driver formats can be IVI-COM for working with COM-based development environments and IVI-C for working in traditional programming languages or IVI.NET.
Most LXI instruments can be programmed with methods other than IVI, so it is not mandatory to work with an IVI driver. Developers can use other driver technologies or work directly with SCPI commands.
Standardization
The LXI Standard has three major elements:
- A standardized LAN interface that provides a framework for web based interfacing and programmatic control. The LAN interface can include wireless connectivity, as well as physically connected interfaces. The interface supports peer-to-peer operation, as well as master/slave operation.
- A trigger facility based on the IEEE 1588 Precision Timing Protocol that enables modules to have a sense of time, which allows modules to time stamp actions and initiate triggered events over the LAN interface. More information on the IEEE 1588 Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems is available from NIST.
- A physical wired trigger system based on an Multipoint Low-Voltage Differential Signaling (M-LVDS) electrical interface that tightly synchronizes the operation of multiple LXI instruments.
The specification is organized into a set of documents which describe:
- The LXI Device Core Specification which contains the requirements for the LAN interface which all LXI Devices must adhere to
- A set of optional Extended Functions which LXI devices can adhere to. If a device claims conformance it must have been tested under the LXI Consortium Conformance regime. As of March 2016 there are 7 Extended Functions specified
- HiSLIP
- IPv6
- LXI Wired Trigger Bus
- LXI Event Messaging
- LXI Clock Synchronization (based on IEEE1588)
- LXI Time Stamped Data
- LXI Event Log
LXI Consortium
The LXI Consortium is a not-for-profit (501c3) corporation made up of test and measurement companies. The Consortium’s primary purpose is to create, maintain, develop and promote the adoption of the LXI Standard. The LXI Consortium is open to all test and measurement companies, and participation by industry professionals, systems integrators, and government representatives is encouraged. The first Consortium meeting was held November 17–18, 2004. Membership is divided into four levels: Strategic (Keysight Technologies, Pickering Interfaces and Rohde & Schwarz), Participating, Advisory, and Informational.
Consortium members meet several times a year at PlugFests held around the world where conversations regarding the LXI Standard are discussed face-to-face meetings in working groups. The public is invited to attend tutorials intended for users and manufacturers interested in joining the LXI ranks. PlugFests keep LXI instrument manufacturers current with changes in the specification, test implementations. It also provides an opportunity for to certify new products as LXI conformant via an independent testing lab.
The Consortium’s standard development efforts are performed by volunteers working through a number of committees and technical working groups (WGs), Work progression is managed by use of Statement of Work (SoW) documents that set out the reasons and objectives for new work items.
Specification History
In September 2005, the LXI Consortium released Version 1.0 of the LXI Standard. Just one year later, Version 1.1 followed with minor corrections and clarifications. In October 2007, the Consortium adopted Version 1.2; its major focus was discovery mechanisms. A discovery mechanism allows the test system to recognize and register a new instrument plugged into the system so the user and other instruments can work with it. Specifically, LXI 1.2 included enhancements to support mDNS discovery of LXI devices. Version 1.3 incorporates the 2008 version of IEEE 1588 for synchronizing time among instruments,
All the revisions of the LXI standard provide backward compatibility and systems can be created which contain any of the versions of the standard.
The latest version (and older version) of the standard is available on the LXI Consortium Specification page of its website.As of March 2016 the standard is at Revision 1.4 and work is progressing on Revision 1.5.
Conformance Testing
The LXI Consortium is unique amongst test and measurements standards in requiring LXI Devices to be tested to the standard. The compliance requirements ensure that at the point of test devices are fully conformant to the standard giving users confiendce that there will be no compatibility issues between vendors products
To support this compliance regime a test suite is available. After a vendor joins the LXI Consortium it can gain access to the Consortium’s Conformance Test Suite software, which they can use as a pre-test before submitting the product to the Consortium for compliance testing. Once a product is ready to submit, a company can choose to have its product tested at a PlugFest or an approved test house. A Technical Justification route allows vendors to certify compliance of derivative products on submitting test results to the Consortium to show that the device has been tested on the test suite.
The LXI Consortium offers detailed instructions on how to get an LXI product certified. The process by which Plugfest testing is perfumed is described here.Dates of upcoming PlugFests are available on the Consortium’s website.
Conformant Instruments
The number of LXI-compliant instruments has grown dramatically, starting from a handful of products from just two vendors in December 2005. This expansion in instrument availability is seen as likely to speed widespread acceptance of the LXI platform and encourage migration to LXI from older instrument platforms. As of March 2016, the Consortium had certified over 2700 instruments as being compliant with the Standard. The latest information on compliant instruments is available on the LXI Consortium’s website.
LXI Reference Design
Recognising that the investment of vendors into LXI interfaces can be significant the LXI Consortium has created an LXI Reference Design, a software package which simplifies the vendor creation of LXI conformant interfaces. The Reference Design has been made portable so that it can be applied to a wide variety of hardware platforms and operating systems (primarily Linux and Windows). Version 1 of the Reference Design was released in March 2016 and is made available with no licence fee to all members of the LXI Consortium. The LXI Consortium plans to continue to enhance the Reference Design and incorporate more Extended Functions in future work packages.
LXI Discovery Tool
The LXI Consortium has also created a Discovery Tool that uses mDNS and/or VXI-11 protocols to disover LXI Devices on a network. This tool is available form the LXI website for anyone to use and has no requirements for chargeable proprietary software tools to be installed.
LXI Guides
The LXI Consortium has also created a series of guides on use and advice about of LXI.
References
- Test & Measurement World magazine
- Interchangeable Virtual Instrument (IVI) Foundation website
- All-about-Test Test & Measurement News, Glossary, and Links