The LXI Standard
LAN eXtensions for Instrumentation - or LXI for short - is an industry standard for communication with measurement instruments and within test systems. The LXI standard expands the Ethernet specifications by functions and capabilities which facilitate its use in the measurement technology environment. The standard is mainly of interest due to its large available data rates, good interoperability between instruments by different manufacturers and low costs for cabling, switches and other network components. It lends itself as a replacement for IEEE488 (GPIB).
LXI Basic Functions (Core)
The LXI standard defines a number of basic functions which must be implemented by every device which conforms to LXI.
- A web page which is hosted by the instrument itself and which supports an interactive diagnosis and optionally the operation of the instrument
- Standardized status LEDs which simplify the diagnosis of connectivity problems
- The implementation of specific protocol functions for searching for LXI devices in the network. As an example, these make it possible for VISA Utilities of different manufacturers to identify all measurement instruments which are available in the network
- Availability of an IVI driver for the device
LXI Extended Functions
In addition to the basic functionality, from version 1.4 onwards, the standard defines so-called "extended functions". These allow an elegant implementation of complex functionality, which requires the cooperation of several devices. Some examples for extended functions are:
- The "LXI wired Trigger Bus" with trigger routing via LVDS signals
- LXI Event Messaging
- LXI timing synchronization based on IEEE 1588-2008 - a protocol for synchronizing real-time clocks in Ethernet networks
- LXI time stamps for data and event logs
- VXI-11 (from version 1.5 onwards; previous versions: basic functionality)
The implementation of extended functions is optional for device manufacturers.
The LXI Consortium
The LXI standard is pursued and supported by the LXI Consortium, a non-profit organization with more than 50 member companies. The interoperability of LXI devices is ensured by a defined certification procedure. The standard was defined in 2005; today, more than 50 measurement instrument manufacturers offer more than 2500 conforming instruments with LXI interfaces.
Excerpt from the member list of the LXI Consortiums:
- Keysight Technologies
- Rohde & Schwarz
- National Instruments
- Pickering Interfaces
- VTI Instruments
- Elgar / Sorensen
History of the LXI Standard
In 2004, Agilent Technologies (today: Keysight Technologies) and VXI Technology (today: VTI Instruments) introduced the LXI standard. The goal of this standard was the definition of a common interface for networking measurement instruments, meant to combine the advantages of GPIB instruments and the functionality of modular systems, like e. g. VXI. Besides the ability to program the instruments, the availability of trigger and synchronization functions was a key issue.
From the beginning, the standard was intended as an open industry standard, and soon, further manufacturers joined the consortium.
The LXI standard is continually promoted by the members of the consortium; for that purpose, members regularly meet at the so-called Plug Fests and in work groups. The Plug Fests also offer an opportunity for the certification of new LXI devices.
LXI Version Level | Publication Date
Vs. 1.0 | Sept 2005
Vs. 1.1 | April 2006
Vs. 1.2 | Aug./Sept. 2007
Vs. 1.3 | Sept./Oct. 2008
Vs. 1.4 | May 2011
Vs. 1.5 | Nov. 2016
Up to version 1.4. of the standard, the function definitions were based on hierarchical classes.
Class C defined the Basic Functions.
Class B supplemented the time synchronization in accordance with the IEEE1588 Precision Time Protocol and brought many improvements, especially for distributed systems.
Class A enhanced the standard by a fast hardware trigger bus (M-LVDS) for local systems.
Separation of "Core" and "Extended Functions"
The old LXI class model turned out to be inconvenient for manufacturers who implemented specific measurement technologies for certain application fields. As an example, a manufacturer of fast subsystems with hardware trigger functions also had to implement the IEEE1588 standard, even though there was no real need for it in the target application.
From 2011 onwards, version 1.4 took account of this by dissolving all the old classes, except for the basic class. The additional functions may now be combined as desired; each manufacturer only implements the extensions that are considered useful by means of "Extended Functions".
In November 2016, version 1.5 brought several structural enhancements. However, the most noticeable change in this version was the downgrade of VXI-11 support, which had hitherto been compulsory, to an Extended Function.