LXI Standard

LAN (eXtensions for Instrumentation) is a standard developed by the LXI Consortium, an industry consortium that maintains the LXI specification, promotes the LXI Standard, and ensures interoperability. The LXI standard defines the communications protocols for instrumentation and data acquisition systems using Ethernet. Ethernet is an accessible and versatile interface, that must be implemented against a standard for instrumentation to communicate effectively. 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. [1]

LXI is the next generation of test systems combining state-of-the-art measurements in a small package at a cost-effective price. LXI modules are full-rack width, 1U tall or half-rack width, 1U or 2U tall. Signals enter and exit the module fr om its front panel while LAN (IEEE 802.3), power, and trigger cables are found on the back of the module. All modules are designed to be easily mounted in a standard 19" rack or stacked on the bench. [2]

Example of an LXI system including conformant LXI Devices and non-LXI instruments

The LXI Standard defines devices using open-standard LAN (Ethernet) for system inter-device communication. The standard will evolve to take advantage of current and future LAN capabilities. It provides capabilities that go well beyond the capability of other test and measurement connectivity solutions. It will provide users with solutions that are denser, smaller, faster, and cheaper than other solutions. [3]

Interoperability

LXI devices can communicate with devices that are not themselves LXI compliant, as well as instruments that employ GPIB, VXI, and PXI, into heterogeneous configurations. In order to simplify communication with non-LXI instruments, 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. There are currently two IVI driver formats: IVI-COM for working with COM-based development environments and IVI-C for working in traditional programming languages. Most LXI instruments can be programmed with methods other than IVI, so it's not mandatory to work with an IVI driver; developers can use other driver technologies or work directly with SCPI commands. [1]

Industry consortium

The LXI Consortium is a not-for-profit corporation made up of test and measurement companies. The Consortium's primary purpose is to promote the development and adoption of the LXI Standard, an open, accessible standard identifying specifications and solutions relating to the functional test, measurement, and data acquisition industry. The Consortium is open to all test and measurement companies, and participation by industry professionals, systems integrators, and government representatives. The first Consortium meeting was held November 17-18, 2004. Membership is divided into four levels: Strategic, 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 at 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 certifying 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), which include Compliance WG, LAN and Web WG, Physical WG, Programmatic Interface WG, Resource Management WG, Specification Revisions, Technical Committee, and Timing and Synchronization. [1]

History of LXI

• In 1972 Hewlett-Packard engineers invented the Hewlett-Packard Interface Bus (HP-IB) as an open standard communications bus (IEEE-488) fr om instruments to the computer. It later became as GPIB (general-purpose interface bus) and was found on nearly every instrument. For 30 years GPIB instruments (also known as rack-and-stack instruments) were the preferred architecture for test systems. State-of-the-art measurements and excellent price/performance solutions are found in GPIB instruments.
• In 1985, Hewlett-Packard, Tektronix, Wavetek, Racal-Dana, and Colorado Data Systems introduced VXI (VME eXtensions for Instruments); a modular instrument standard for the U.S. military. These modular instruments (instruments-on-a-card) became very popular in Aerospace/Defense industry and manufacturing test applications where size and throughput were important.
•  In 2004, Agilent Technologies (formerly Hewlett-Packard) and VXI Technology, Inc. introduced LXI (LAN-based eXtensions for Instrumentation); combining the best of GPIB instruments and VXI modules. With LXI you get the reduced size of VXI, high throughput of LAN, and the high performance measurements of GPIB. No cardcage, no slot 0 and no expensive PC to instrument communications link are required in LXI. [2]
• 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 a user and other instruments can work with it. Specifically, LXI 1.2 included enhancements to support plug-and-play identification of LXI devices.
• Version 1.3 incorporates the 2008 version of IEEE 1588 for synchronizing time among instruments, so systems using LXI Class A and Class B devices will synchronize with each other basing on the 1588-2008 Precision Time Protocol (PTP) standard. [1]

In addition to XML-based discovery, Version 2.0 of the Standard will incorporate even more significant changes. This functionality include the extension of LXI device Web pages to support instrument configuration and interactive testing of different LXI trigger capabilities such as LAN peer-to-peer messages, IEEE 1588 time events, or the wired trigger bus of LXI Class A devices. Logging of all the events in LXI devices will improve as will the ease of troubleshooting LXI-based test systems. Resource Management is another major extension that enables management and allocation of LXI devices in a network with more than one controller accessing instruments. Yet another working group is dealing with the standardization of script downloads into LXI devices. Execution of custom scripts can be done without the system controller, simplifying the development of test software and increasing the throughput of test systems. The most current version of the standard (Version 1.3) is available on the LXI Consortium's website). [1]

Standardization

The LXI Standard has three key functional attributes:

• A standardized LAN interface that provides a framework for web based interfacing and programmatic control. The interface supports peer-to-peer operation as well as master slave operation. The LAN can also support the exchange of trigger signals.
• A facility based on IEEE 1588 that enables modules to have a sense of time that allows modules to timestamp actions and initiate triggered events over the LAN interface.
• A physical wired trigger system based on an M-LVDS electrical interface that allows modules to be connected together by a wired interface using twisted pair transmission lines. [3]

Overview of device classes

Class C is the baseline with LAN capabilities, a Web interface, and IVI drivers. Class B adds expanded triggering, such as multicast and peer-to-peer communications between instruments and time-based trigger events. The time-based triggering is made possible by implementing the PTP, which can distribute a precision timing source across many Class A and B devices over a LAN. Class A devices build upon Class B devices by adding a wired trigger bus for precision triggering. [1]

LXI Functional Class Models

The Functional classes do not imply any particular physical size for an LXI Device.

Functional Class C

Class C LXI Devices provide a standardized LAN and web browser interface that is conformant with the LXI Standard. This class is particularly suited to applications where non-LXI products have been adapted to the standard, but it is also well suited to applications where there is no necessity to offer triggered or timed functionality. This class may also include physically small products, such as sensors, that use battery power or PoE (Power over Ethernet) wh ere simple device architecture, low cost and small size are key attributes.

Functional Class B

In addition to Class C features, Class B LXI Devices provide a standardized LXI Event interface, synchronization API and support the IEEE 1588 timing aspects. The IEEE 1588 interface allows devices to execute triggered functions equivalent to those available over GPIB and with similar or better timing accuracy.

Functional Class A

In addition to Class B features, Class A LXI Devices provide a wired trigger bus interface. The LXI Trigger Bus provides a standardized capability of supporting trigger events between devices whose timing accuracy is limited by the physical lim itations of cables and LXI Device hardware. The trigger functionality is broadly equivalent to the backplane triggers of modular instruments in card cages, though cable lengths may typically be longer than backplane trigger lengths. [3]

LXI Device Class Hierarchy [9]

LXI Interfaces

Adopting any new standard requires some level of change and an associated learning curve, but LXI has the advantage of using the most prolific communications standard in the world as the underlying foundation.

The key features of LXI that make smart instruments possible include the following:

• A broad range of form factors ranging from multi-U, IEC rack mount units to small, portable devices.
• A LAN interface featuring a defined standard network configuration, a Web page for network configuration, and a discovery mechanism for locating LXI devices on a network.
• An IVI-compliant instrument driver.
• A Web interface consisting of a set of standard Web pages that provides information about the instrument, a standard way to configure the LAN interface, and other features.
• Timing and synchronization based on IEEE 1588 that synchronizes real-time clocks in each device and can be used to timestamp data or initiate time-based triggers. Peer-to-peer messaging from one LXI device to another that implements LAN triggering and supports a short data payload. A uniform trigger model and event structure. [4]

1) Physical interface

Physical interface

The flexibility of the standard allows instrument suppliers to adopt IEC 60297 for 19-in. rack-mount units or alternatively a half-rack configuration. This latter opportunity gives implementers the opportunity to adopt a defacto standard or use the LXI-unit standard as defined in the specification, providing a large amount of flexibility when establishing a mechanical package size and configuration. [4]
LXI accommodates standard instruments with front-panel displays and keyboards as well as instrument modules without front panels in one test-system architecture. LXI systems built with faceless modules have several advantages over faceless modules in VXI and PXI. First, LXI modules do not need an expensive card cage with a multilayer backplane, high-speed fans, a high-performance power supply, a Slot-0 controller, or a proprietary communications link between the card cage and the PC. Second, LXI modules fit nicely in the rack alongside other LXI instruments and existing GPIB instruments. Finally, LXI modules can be sized to match performance unlike card-cage products that may compromise performance to match size restrictions. [8]

Cooling

LXI modules provide their own cooling. The devices take air in from the sides and exhaust it out the rear. Half-rack modules are designed so adequate cooling is maintained even if one side of the module is completely blocked by an instrument sitting next to it in the rack. [8]

Electrical

LXI modules must adhere to the CSA, EN, UL, and IEC standards of the region or market they serve. Modules are powered from standard single-phase sources of 100 to 240 V AC at 47 to 66 Hz. The specification makes provisions for devices to be powered via DC voltages or Power over Ethernet (POE). [8]

Switches and Indicators

The LXI specification standardizes the type and location of switches, cables, and indicators. The Ethernet connection (RJ-45) is located on the left side of the rear panel while the power connector is located on the right side. The power switch is next to the power connector in the lower right corner of the rear panel. The trigger bus connectors are on the right side of the rear panel next to the power. Each LXI module without a front panel must have a LAN configuration initialize (LCI) button, preferably on the rear panel. The button should be labeled LAN RST or LAN RESET and protected with mechanical protection or a time delay to prevent inadvertent operation. The LCI must put the module into a known state in case the module loses communications with the PC.

The signal connections to an LXI module are located on the front panel. When the LXI device does not have a front-panel display, it must have three indicator lights in the lower left corner:

• The lower light is the power indicator, which is green when power is applied.
• The middle light is the LAN indicator, which is solid green for normal operations, flashing green when the device is being identified, and solid red if a LAN fault occurs.
• The third light is the IEEE 1588 indicator that is off when the LXI device is not synchronized, solid green when it is synchronized as an IEEE 1588 slave, blinking every second when it is the IEEE 1588 master, and blinking every two seconds if it is the grand master in the system. A solid red light indicates a fault. [8]

1) LAN interface

LXI adds some additional features to standalone LAN instruments, such as a standard HTML configuration page, and several best practices for implementing LAN instruments. LXI also adds optional timing and synchronization features including IEEE-1588 Precision Time Protocol and a bussed Hardware Trigger (These features are required in certain classes of LXI instruments).

Using specialized LAN hardware, IEEE-1588 devices are capable of achieving synchronization in time of within +-100 ns. This capability makes IEEE-1588 attractive for applications with relatively low acquisition rates (below 1 MS/s) that require synchronization over large distances.

Most LXI instruments will look very similar to existing LAN implementations, in fact, a majority of current LXI devices are updated versions of existing products. LXI devices that implement the optional synchronization capabilities are well-suited to applications that require instruments to be distributed over large distances. [7]

A Comparison of LAN and LXI instrument features.

Feature	Existing LAN Instruments	LXI Instruments
LAN interface	Required	Required
Trigger inputs / outputs	Optional	Optional
Web Configuration Panel	Optional / Typical	Required
IVI-compliant Instrument Driver	Optional / Typical	Required
Bussed hardware trigger	Optional	(Required for Class A)
IEEE-1588	Optional	(Required for Class A,B)

Every LXI device must implement IEEE 802.3. The network must support TCP/IP with IPv4 at a minimum. The spec recommends Gigabit (1000Base-T) Ethernet that negotiates down to 100Base-T and that users should build their network with CAT 5 cable and infrastructure to support 100Base-T at a minimum. Although 10Base-T is permitted, it is expected that nearly all vendors will provide 100Base-T at a minimum. At 100-Mb/s, LXI is roughly 12 times faster than GPIB. [8]

The maximum cable length for any segment of a LAN is 100 meters with 10/100Base-T hubs extending this distance to approximately 1,600 meters. With routers, switches, bridges, and repeaters, a LAN segment can essentially have unlim ited length, a clear advantage over a GPIB network topology. [4]

LAN features:

• All device classes use Ethernet as their communications interface and are designed to comply with IEEE 802.3 for both physical and function compliance. Required LAN features for all three classes include the following:
• A LAN configuration initalize (LCI) mechanism that provides the means to configure the LXI device to default network settings as well as some type of protective mechanism (time delay or mechanical) that prevents inadvertent operation of the LCI.
• A LAN activity indicator that can be a one- or two-color LED. The indicator, which identifies devices and LAN fault conditions, is located on the front panel of the device.
• Ethernet connection monitoring via Microsoft' s Media Sense implementation or via some other means.
• Capability to adapt to networks operating at an equal or lower speed than that of the LXI device with autonegotiation enabled as a default configuration.
• Support for the user datagram protocol (UDP), IPv4, and TCP/IP network protocols.
• Support for the Internet Control Message Protocol (ICMP).
• Support for three LAN configuration protocols: dynamic host configuration protocol (DHCP), dynamically configured link local addressing, and manual. Also, LXI devices must support either an automatic or manual configuration or separate configuration settings for each of the three LAN configuration protocols.
• Capability to detect a duplicate IP address and subsequently disconnect.
• Use of the VXI-11 discovery protocol and support for the stand-ard commands for programmable instrumentation (SCPI) query "*integrated digital network (IDN)" that allows all module classes to respond with manufacturer, model, serial number, and firmware revision number information. [9]

2) Software interface

All LXI-compliant instrumentation is required to furnish an IVI driver: IVI-C or IVI-COM. This provides the flexibility to choose the development environment best suited to the application. These drivers can be used in a number of application programming environments such as C/C++, MatLAB, Visual Basic, VEE, and LabVIEW/LabWindows CVI. It also is possible to port instrument functionality into non-Windows based environments when source code is provided by the manufacturer. [10]

3) Web-Interface

Imagine being able to monitor a test system from the comfort of your office when the instrument might be 100 feet away, 100 miles, or halfway around the world. That' s one key benefit of LXI instruments. The built-in web server gives users an easy way to configure, learn, access remotely, and even assist in programming the product. Virtually no software installation is required to start using the instrument. If your computer has a LAN port, a LAN cable, and virtually any web browser, you can connect to the instrument.

The IP address is all you need to connect a web browser to the instrume.

Manufacturers use different color schemes and templates to give a unique look and feel to their family of instruments, but the information presented on the home page adheres to the LXI spec. It includes manufacturer, product description, serial number, host name, VISA programming string, Telnet port, socket port, MAC address, IP address. [11]

Web Interfaces for Agilent N6700A (Low-Profile MPS Mainfraim) and 34980A (Multifunction Switch Measure Unit)

The LXI standard requires that all LXI devices serve up a Web page with key information about the product. It must be viewable from a standard W3C Web browser and conform to HTTP 1.1 with hypertext markup language (HTML) pages of version 4.01 or later. The model number, firmware revision, manufacturer, IP address, and basic configuration information are required. In some cases, module manufacturers will build a complete user interface that can configure the instrument and view results using a standard Web browser.

While the home page, the associated communications configuration, and help pages all must be HTML based, the Browser Web Control pages can be Java scripts, Java applets, and flash programs. Java applets and flash programs are executed by browser plug-ins, which means you might have to load some additional software on the PC before controlling instruments using such programs. [11]

4) Synchronization

The LXI standard has three classes of products based mainly on the types of triggers they implement:

• Class C, the base class, contains all the detailed physical, electrical, Ethernet, and Web pages but leaves it up to the individual suppliers of the products to implement the most appropriate triggers.
• Class B requires IEEE 1588 triggering in addition to the Class C requirements.
• Class A adds the trigger bus to the Class C and Class B requirements. [8] 

LXI Triggering

The IEEE 1588 (time synchronization of networked devices) standard is based on technology developed by Agilent Labs over the past 10 years. The protocol designates one device as the master clock and proceeds to synchronize the clocks of other devices on the network. [8]

Synchronization 

1. Devices sel ect the most accurate clock (master) in the system.
2. Master clock sends out a synch pulse and its time— all slaves note this time.
3. Each slave sends a message with its time to the master— the master calculates the offset between slave time and reception time (offset time).
4. The master sends the offset time to each slave; each slave adjusts its clock to compensate for the offset. Master and slave are now in synch.
5. Periodic synch pulses keep the slaves synchronized to master.

Over the course of a few seconds, the master clock and the slaves are synchronized to 100-ns accuracy or less, depending on the accuracies of the clocks. Triggering of devices is performed by telling each device when to start its measurement or signal output activity. LAN overhead or latency has no bearing on the devices being triggered because they all start their activity based on time, not on when they receive a command over LAN.

With IEEE 1588, the need for trigger lines is eliminated because triggering is set via time. This method opens up the possibility of synchronizing measurements across instruments that are not physically next to each other and provides a very precise means to correlate measurements across multiple instruments. It may very well become the defacto standard for applications that require precise correlation of signals and measurements.[8]

LXI Features

LXI Devices are designed to provide a dense and compact solution for test systems. Many LXI Devices will provide only minimal manual user interfaces to reduce device complexity and space. Many LXI Devices are expected to be 1U to 4U high and half rack width, but devices occupying a full rack width are expected to be implemented for many applications, particularly more complex functions. Some LXI Devices, such as sensors, may be much smaller and have mechanical dimensions not intended for rack mounting.

User connections to LXI Devices are recommended to be on the front while LAN, trigger and power supply connections are on the rear. Indicator lights on the front panel show the presence of power and have a LAN status indicator to ensure users can quickly spot simple functional or connectivity problems. A recommended indicator shows the operating condition of the IEEE 1588 clock system.

The LXI Device power can be provided by a standard AC power supply with automatic voltage selection or by a DC power source. Alternatively an isolated DC input connection can be provided (or a Power Over Ethernet source for lower power LXI Devices) for applications wh ere AC supply operation is not desirable. A 48 V isolated supply is recommended for these applications to align with the standards for Power Over Ethernet, but other voltages are permitted.

LXI Devices are expected to conform to the relevant standards applicable for intended markets for safety and environmental standards. [3]

Compliant 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 2009, the Consortium had certified 1,117 instruments fr om 22 manufacturers as compliant with the Standard. [1]

• LXI Spectrum Analyzers
• LXI Function/AWGs
• LXI Power Meters
• LXI Power Supplies
• LXI Switching
• LXI Multifunction Mainframes
• LXI Digital Multimeters
• LXI Signal Generators
• LXI SI Function/Arbitrary Waveform Generators
• LXI SI RF/Microwave Downconverters
• LXI SI RF/Microwave Upconverters
• LXI SI IF Digitizers
• LXI Thermocouple Instruments
• LXI WTB Accessories
• LXI Bridges
• LXI Signal Analyzers
• LXI Oscilloscopes
• LXI Power Analyzers
• LXI System Source Meters
• LXI Vector Network Analyzers
• LXI Audio Analyzers [5]

LXI benefits

• Wide acceptance of LAN and its low cost compared to other control interfaces.
• Minimal setup and debug time is required to make the system operational.
• The ubiquity of LANs and the low cost of Ethernet are features that make LXI a very attractive alternative to other control bus implementations such as GPIB, RS-232, FireWire, or USB.
• All LXI-compliant instrumentation is required to furnish an IVI driver: IVI-C or IVI-COM. This provides the flexibility to choose the development environment best suited to the application. These drivers can be used in a number of application programming environments such as C/C++, MatLAB, Visual Basic, VEE, and LabVIEW/LabWindows CVI. It also is possible to port instrument functionality into non-Windows based environments when source code is provided by the manufacturer.
• Using a dedicated LAN connection or subnet helps ensure optimal system performance and reliability since this communications interface will not be subjected to superfluous intranet traffic or other non-runtime tasks such as datalogging.
• Controlling or programming GPIB or LXI devices relies upon VISA, an industry standard software I/O component that makes the transition fr om a GPIB instrument to an LXI instrument very easy. [4]

Member Companies [6]

Strategic

• Agilent Technologies
• Pickering Interfaces Ltd
• Rohde & Schwarz GmbH & Co KG

Participating

• Aeroflex, Inc.
• AMETEK Programmable Power
• Bruel & Kjaer S & V
• C & H Technologies, Inc.
• EADS North America Defense
• Giga-tronics Incorporated
• Keithley Instruments, Inc.
• National Instruments Corporation
• Proft InFocus, LLC
• Tektronix
• The MathWorks, Inc.
• VTI Instruments Corporation
• Wheelwright Enterprises

Advisory

• GOEPEL Electronic GmbH
• TDK-Lambda Americas Inc.
• Pacific MindWorks
• ZTEC Instruments

Informational

• Acery Technologies Co. Ltd.
• ARC Technology Solutions
• Beijing Aerospace Measurement & Control Corp.
• Beijing Control Industrial Computer Corporation
• Bustec
• Chroma ATE Inc.
• Circuit Assembly Corp.
• COM DEV Ltd.
• Data Patterns (India) PVT., LTD.
• Data Physics Corporation
• Data Translation
• DowKey Microwave
• Good Will Instrument Co., Ltd.
• Hitech Group International, Ltd
• Holding Informtest
• JDS Uniphase Corporation
• Kepco, Inc.
• Kikusui Electronics Corporation
• LeCroy
• LXinstruments GmbH
• Magna-Power Electronics Inc.
• NH Research
• NPP MERA
• Pacific Power Source, Inc.
• Picotest Corp.
• Rigol Technologies, Inc.
• AAI Corporation
• TEGAM
• Teradyne
• TTi Ltd.
• Universal Switching Corporation
• Yokogawa Electric Corporation

University

• Harbin Institute of Technology

Links

1. en.wikipedia.org/wiki/LAN_eXtensions_for_Instrumentation/
2. www.lxistandard.org
3. www.ni.com/white-paper/2922/en/

Author(s):  Afonskiy, Alexander, Novikov, Andrey