LXI Standard Version 1.4 Changes

Pickering Interfaces and LXI

Pickering Interfaces products have migrated from compliance to Version 1.3 of the LXI Standard to Version 1.4 for the standard with the completion of formal testing for the 60-102 LXI Modular Chassis to Version 1.4.

Pickering Interfaces has a policy of migrating with the evolution of the standard to ensure that users get the very best experience possible of LXI. We have always aimed to complete adoption within one year of a migration of the standard

LXI Standard

The LXI specification has gone through changes and continues to “evolve” as the market demand for Ethernet(LAN) controlled instruments grows. Specification evolution is a healthy response as it ensures that specifications continue to relevant and the customer can get a better product for their needs. In this article we discuss the recent changes to the LXI Specification and what drove the LXI Consortium to invest in, and then to adopt, the changes.

Products were first introduced conforming to the LXI standard in December 2005 which complied to the V1.1 of the standard. Since that time more than 1800 products have been certified from over 30 different vendors covering a wide range of products types and capabilities.
In that intervening time there have been two major revisions of the standard, Version 1.2 which (amongst other changes) introduced mDNS discovery mechanism and Version 1.3 which changed the version of the IEEE1588 standard used from IEEE1588-2002 to IEEE1588-2008 to reflect the IEEE migration on the precision timing protocol.

Products that were conformant to the 1.1 standard have continued to work almost seamlessly with products that are conformant with later versions of the standard, the only significant exception being the issues raised by IEEE1588 migration. The latest revision of the LXI Standard has now been published and maintains that backward compatibility.

So what has changed?

The new version of the standard has largely editorial changes, but they do have a dramatic effect on how the standard is organized and the migration of products that are conformant to it. It also paves the way for further additions to the standard to be made to reflect the migration of the Ethernet and IVI standardization efforts. The following explains what they mean to the Test Engineering community, both to instrument developers and to the user community.

Structural Changes

The most obvious change in the specification is that the original specification has now been broken into technical, compliance and procedural documents to make the management of the standard much simpler.Previous versions of the standard included much of this detail in a single document; Version 1.4 has broken these aspects apart so that one document can change without impacting a much larger specification. This is particularly true of the key technical documents, changes in procedural matters or compliance regime for example can be made independently of the technical part of the standard.

Technical Documents

The new version has three key technical documents that set how compliant products implement the standard. The LXI Device Specification2011 contains just those aspects of the standard that impact the LXI Device itself (the major part of the standard), the LXI Wired Trigger Bus Cable and Terminator Specification describes how the cables, terminators are implemented and how they are tested for systems that use the Wired Trigger Bus and then there is an LXI Identification document which provides the schema for identifying the device capabilities. The latter documents have always existed since Version1.2 of the standard, but have been updated and in the case of the trigger bus restructured. The LXI Device Specification 2011 has had many topics which are not related to the implementation of the LXI Device removed and placed elsewhere and is consequently a much smaller and readable document.

Introducing Extended Functions

Previous versions of the standard have included a Class model for LXI Devices, Class C covering basic LAN issues, Class B supporting Class C functionality as well as features such as IEEE1588 and Class A supporting Class B features plus the Wired Trigger Bus (WTB).

The class structure has proved difficult to manage since adding new functions to the standard requires judgments to be made as where they fit in the class structure. Most vendors have also found that despite the fact there are some compelling examples of test systems solving problems with the use of IEEE1588 that would be difficult or even impossible by other methods the majority of applications have been well served by simply complying with the Class C requirements.

Vendors were reluctant to add just parts of the Class A/B features since they did not change the status of the product within the class structure. With Version 1.4 this class structure has been removed - instead a "Core" LXI Specification is defined and a set of five optional Extended Functions have been defined, including both IEEE1588 and the WTB normally associated with the class model. The standard has also been opened up to permit other Extended Functions to be added as separate standalone documents with minimal impact on the core LXI Device Specification.

This change is allowing the specification to have new optional functions added, and two of those functions have been introduced.

The first introduction is the introduction of a function which defines how LXI Devices adopt the HiSLIP standard introduced by the IVI foundation, providing faster message based control and an alternative discovery mechanism to VXI-11. The latter will become a significant issue for many Ethernet based instruments since VXI-11 cannot be supported on IPv6 networks.

Not surprisingly, the next Extended Function was the LXI implementation of IPv6. As the world runs out of IPv4 addresses, IPv6 will become more common and LXI has to be prepared for this change. Initially it is unlikely it will have much impact on how test systems are put together on local networks, but with governments starting to require IPV6 support for all Ethernet enabled products it is crucial to get a standard framework in place that is well tested before it becomes an essential requirement, one that could be mandated by procurement policies in key markets.

The next Extended Function added was the adoption of HiSLIP, created by the IVI Foundation and designed to provide faster control on message based devices.

Eventually some of these new Extended Functions might become requirements for inclusion in to future revisions of the Core LXI Device Standard, by the time they reach that point they will be well tested by both vendors and users with well proven implementations and this will make migration of the standard much simpler and problem free experience. It might be for example that IPV6 will one day be a requirement of the standard. Other requirements, like VXI-11 discovery, will become optional requirements.

New products conformant to 1.4 can no longer be listed as Class A, B, or C. Your instrument vendor will explain any difference in ordering codes and what Extended Functions are required.

Conformance Testing

LXI has taken a route of requiring that all products are conformance tested prior to introduction to the market - a requirement that has not been mandated on any other instrumentation standard. This ensures users get a consistent experience of using LXI products and avoids any coexistence issues between different vendors products being in a system. This implementation consistency has served the users and the LXI Consortium well, it gives users confidence and provides a way of tracking introductions through the Consortium's web site. The Consortium has invested considerable effort in this conformance regime to ensure that tools are provided to the vendors to maximise the chance of first time passes at testing time.

Version 1.4 maintains this conformance regime, LXI Devices are required to be tested by third parties unless the vendor is able to state that it is a derivative product on an existing platform referred to as the Technical Justification route. With the introduction of Version 1.4 vendors are now required to demonstrate they have tested the derivative product using the LXI Consortium's test suite which is freely available to members of the Consortium. This reinforces the message that LXI is a well-tested standard with high confidence that all LXI marked products work well with each other and behaving in a controlled way on an Ethernet network. IT managers will have nothing to fear from putting LXI instruments on a network should the application require it.

In 2012 there was also another change on Technical Justification, vendors will no longer be able to use Technical Justification to introduce new versions of products that conform to Version 1.2 or 1.1 of the standard, only applications to 1.3 and 1.4 will be allowed. Other than the disappearance of the class models the differences between Version 1.3 and Version 1.4 for an LXI Device are relatively minor, so from a user stand point the two versions are very closely aligned. This change in conformance regime is intended to increase the use of mDNS discovery and IEEE1588-2008.

For WTB cables and terminators the conformance regime remains the same as it was in Version 1.3, vendors are required to declare their products are constructed in accordance with the specification.

It is possible that at some time in the future the conformance regime for LXI Devices might change to allow vendor testing,however so far experience indicates this may be a while before such a change can be made. Unlike Version 1.3 of the standard such a conformance regime change can be implemented without impacting the technical documents.

Summary

Users will not see many changes in the way their products work from the introduction of the 1.4 LXI Device Standard, but it does ensure the LXI Standard is well placed to migrate with the evolution of Ethernet standards and add new features in response to market pressure. It will result in a stable core specification with new optional features being added, implemented by vendors and tested by users before consideration of integration into future revisions of the LXI Device Standard. Version 1.4 has set a platform for the easy management of the standard and its migration, ensuring a long and successful future in test and measurement.

So what is the bottom line to users? LXI is a stable and efficient test system that can evolve in the future without revolutionary changes. That ensures users know they can invest in LXI platforms without fear of major breaking changes – ensuring a long and stable platform life. It is based on Ethernet standard designed specifically to connect discrete boxes together over long and short distances, not on standards primarily directed towards internal connectivity. That ensures the products and standard will have an incredibly long life not subject to the volatile standards associated with controller interconnection or operating system variables.

More information on the LXI Standard can be found on the LXI web site at:
http://www.lxistandard.org/

and copies of the specifications can be obtained from:
http://www.lxistandard.org/Specifications/Default.aspx

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