Using the LXI standard in switching test systems - Embedded.com

Using the LXI standard in switching test systems

Signal management, which is using switching systems to routeinstrumentationand stimulus to appropriate test point on a unit under test, has a verycrucial role to play in most electronics test.

The sharing of resources, connection of calibration references, loadmanagement,and many other functions are all controlled by signal management.Pickering Interfaces has been involved in the development of completeswitchingsystems since its first products were introduced in 1988.

Using the knowledge based developed by its sister company, PickeringRelays, Pickering Interfaces was able to develop a range of switchingsystems basedaround a proprietary standard ” its System 10 and 20 products ” whichprovided GPIB or RS232 control of the relays.

Even at that time there was a recognized demand for switchingplatforms that were based on open modular standards and Pickeringintroduced switching products based on the VXI and the PXI standards.

The PXI standard, despite doubts expressed by some, has proven to bean excellent basis for switching products. Although the PXI form factoris small, the development costs are low once the initial entry barriershave been crossed and speed of development is high, enabling a largevariety of standard and custom switching solutions to be made availablein response to user demand asadoption of the PXI standard increased.

Pickering Interfaces has introduced a number of other products,includingchassis, power supplies, DMM's, amplifiers and attenuators. The PXIstandard ismature and has been widely adopted in the test industry.

It offers very fast communication interfaces for the applicationsrequiring the exchange of the large blocks of data that may be neededfor analysis by the system controller, but it also imposes mechanicaland power restrictions on the modules.

Not all users want their instrumentation or switching products to beextensions of their PC's PCI bus or for the data analysis to beperformed in the PC rather than the instrument. Although it has beenhugely successful, PXI has not displaced the majority of rack or benchinstruments.

Ubiquitous Ethernet
GPIB instruments have more recently had their control interfacessupplemented bya LAN connection that can be used as an alternative control mechanism.Ethernetconnections are ubiquitous on system controllers, and the connectioncables are easily managed, have a latching mechanism and have virtuallyno restrictions on distance.

However, there was no agreed standard for controlling an instrumentthrough aLAN connection, so users found that manufacturers solved the problemsin different ways.

In some ways this is rather like the situation 30 years ago whenGPIB (originally HPIB) was introduced. No two manufacturers did thingsthe same way. As occurred on GPIB, a more cohesive approach is nowbeing taken.

Standardisation of LAN connections for instrumentation is taking amajor leap forward following the introduction of the LXI standard.Standardisation started in the second half of 2004 and the LXIConsortium (http://www.lxistandard.org ) was formed to develop andrefine it.

Many of the major T&M companies have joined the consortium,including Pickering Interfaces, who are now Strategic Level members.The first publicly released version of the standard was made availableat Autotestcon 2005.

The LXI standard defines LAN controlled instruments that behave in asimilarand consistent way when they are connected to a network. Eachinstrument hasits own web interface that can be used to set up the instrument andreport its status.

The LAN interface can be used to control some functions. An IVIdriver on the client PC is used to provide programmatic control of theLXI device over the LAN interface. In most cases LXI devices areconsidered to be self contained ” they have their own power source, LANconnector and case.

They are intended to be small compared to their bench instrumentversions and be controlled from an external controller, so generallythey have no front panel user interface. User connections arerecommended to be on the front of the instrument and power and LAN onthe rear. An LXI device providing this levelof performance is referred to as a Class C Device, it meets all theessential requirements of the LXI standard.

Two further classes of LXI device are defined that are intended toprovide more deterministic response to trigger events. The Class Bdevices include a timing facility based on IEEE1588 that allows usersto trigger events in the system at specified times (includingimmediately). This can be used to make the LAN timing of measurementsmore deterministic, permitting timing accuracies of a few 10's ofnanoseconds to be achieved.

A Class A device includes both the IEEE 1588 features and a wiredtrigger interface that can be used to physically connect instrumentstogether with an 8 bus trigger interconnection. This provides a moredeterministic way of responding to trigger events since only internalhardware and cable interconnectdelays affect response times.

Figure1: Comparison of the features of PXI and LXI standards

Comparison of LXI and PXI
There has been speculation that PXI and LXI are competing standards, aview thatPickering does not share. In many ways they are sufficiently differentthat there will be clear case where one is better for a particularsolution than another.

The table inFigure 1, above , clearly illustrates this differences, and thesedifferences are valid for both switching and instrumentationapplications.

For LXI instruments they are largely platform agnostic, whereas PXIis very dependent on the PC architecture (and in practice, Windows).LXI Devices do not have many mechanical or electrical constraints, butPXI have to conform to the PXI standard in order to benefit from themulti-vendor chassis platform. They can also have quite different speeddrivers as well.

Although PXI has the faster connection speeds it relies onprocessing data in the controlling PC ” so it inherently needs a highspeed interconnect for some functions. An LXI system might be expectedto process data within the LXI devices and simply has to report theresults.

For switching, speed differences are of little consequence becausein practice the speed of change of switching systems is constrained bymechanical components.

The mechanical and electrical constraints of PXI can influence whatcan be cost effectively supported in PXI, but the cost overheads of aLXI Device can limit the minimum functionality that can be costeffectively supported.

As ever, one standard does not fit all.

Revision 1.0 of the LXI standard is now available and the LXIConsortium willwork on newer revisions to cover additional areas and improve on theexisting content.

All new standards face a period of time when companies seek tolaunch productsconforming to the standard as quickly as possible in order to encourageusers to embrace the new technology, but it takes time before a newstandard has the breadth of product availability of legacy standards.

Lack of availability of a wide range of products can act asdeterrent to usersbeing able to move from, for example, GPIB to LXI as soon as they wouldlike. In the case of Pickering Interfaces the company has a very largerange of PXIswitching products that have been developed in the last 7 years and asmaller range of older designs that are housed in a GPIB chassis(System 10 and System 20).

Pickering Interfaces recognised that there are a significant numberof users for who PXI was the natural answer to their switching needs,but there are many other users who would prefer to use a differentcontrol method that provides abarrier between the system controller and the switching hardware.

Years of effort
Replicating the breadth of switching products available in PXI acrossLXI thoughwould create many years of effort in both hardware and software. Forthat reason a decision was taken to leverage the existing PXI switchingproducts into LXI, essentially producing a level playing field for thetwo formats immediately that a suitable LXI product was introduced, andthat led directly to the Pickering Interfaces 60-100 LXI Chassis (Figure 2, below ).

Figure2: The 60-100 LXI modular switching chassis

The use of the LXI architecture for switching makes sense for manytest applications. In the area of legacy test systems, where amigration path to newer systems are planned, LXI can be selected toreplace older GPIB Based switching subsystems.

From a software standpoint, there is no easy answer to migratingolder testprograms. However, modern software does make the change relativelysimple.

If the test system is primarily PC Based, LXI based switchingsimplifies the issue of PCI Bus enumeration as the switch system is noton the PCI bus. So powering down the switching system will not forcerebooting of the controller when maintenance/ upgrades are required.

When compared to a pure PXI switching application, the costs of acontroller for the LXI switch systems are far less as the applicationis slower and less bus intensive. So the overall costs of a switchsystem based on LXI are likely lower than an equivalent PXIimplementation.

The 60-100 chassis system was initially developed around acommercially available PXI backplane that could accept up to 7 PXIperipheral modules and a low cost single board PC that provides a LANinterface and a PCI bus. A PCI bridge interfaces the PC to the PXIbackplane.

An AC supply is used by the power supply to support both the PC andthe PXIbackplane. The chassis does not need to comply with the PXI standard,so issues like cooling can be solved in a different way to PXI chassis(there is for example no need to cool power hungry modules in aswitching system).

The hardware provides a means of physically controlling the PXImodules installed in the backplane, for external control and monitoringLXI requires the use of a standardized programming interface (IVI) andweb interfacing. The control route taken to supporting the PXI modulesis shown in Figure 3, below.

Figure3: The 60-199 interface model

Module control
The client PC has an installed driver and discovery software thatcommunicatesthrough the client Ethernet interface to the 60-100 chassis, shown in Figure 3, above .

The 60-100 provides an Ethernet interface, discovery server and webserver that interface to the control software that in turn controls thePXI modules. The drivers provided include IVI compliant custom andclass drivers that communicatethrough the Ethernet connection, using a proprietary command set, tothe 60-100 control sub-systems which then manage the connection andhandle the physical interface to the PXI cards. Also provided is aprogramming interface based on the Pickering kernel driver, suitablyadapted to the 60-100.

Physically the 60-100 implements these control and managementfunctions ona single board embedded PC using flash memory cards to avoid theproblems ofsupporting hard drives, ensuring the program information remains evenif unexpected power interruptions occur.

The embedded PC has a PCI interface that is connected to the PXIbackplane through a bridge, making the PXI modules appear as anextension to the 60-100 embedded PC's PCI bus.

Only a small proportion of the internal flash memory is used for thesystem functions, leaving room for the memory to be loaded with otherdata, including the drivers, manuals and data sheets.

In principle the 60-100 can accept any 3U switching module, but inpractice suitable software drivers need to be available that run themodule in accordance with Pickering's software control model.

The approach used in the 60-100 will have a long service life aheadof it, it is not simply a temporary measure to “get LXI product outthere”. As new switching modules are introduced in PXI they immediatelybecome available in LXI, it will be an incentive for Pickering todevelop modules that can be used in either environment.

This gives an immediate benefit to Pickering Interfaces in spreadingengineeringinvestment across both platforms, making it possible to continue todevelop low volume (almost custom) products and higher volume productsto the same physical and electrical standards.

User choice
Users will benefit because they do not have to make the choice of whatswitching platform to use simply on the grounds of product availability” Pickering Interfaces becomes 'platform agnostic'. Other versions ofthe 60-100 will be introduced with more available slots so that morediverse applicationsor more complex switching modules can be included.

Figure4: Functional representation of control interface for 60-100

The 60-100 LXI Chassis is supplied with software support for all itsPXI switching modules, in the future this will be expanded to includeto support key instrumentation items such as a high performance DMM LXIonly products will be developed, particularly in areas where LXI has atechnical advantage over PXI because of its relatively high mechanicaland electrical freedom compared to PXI.

However, PXI has advantages where there is a need for a mixture ofswitching functions that can be densely packed since in a chassis. Forthose applicationswhere Pickering is the selected supplier the user has a choice ofstaying witha PXI environment or choosing to operate the modules through an LXIenvironment.

It seems the future is bright for both PXI and LXI standardssupporting switching functions in test systems.

David Owen is the PXI BusinessDevelopment Manager for Pickering Interfaces.

To download a PDF version of this article, go to”.Usingthe LXI standard in switching test systems,” at.

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