Feeling Comfortablewith VXIbus

If you’ve been around electronicsvery long, you know that “rackand stack” instruments have beena mainstay in the electronicsindustry for years. Much of theirsuccess is due to a widely usedinterface, IEEE-488. Developedby Agilent Technologies in themid-1970’s, this interface allows

you easily to connect yourinstruments using a remotecomputer. In the late 1980’s,

Agilent Technologiesoffered a standardinstrument language

that was quickly adoptedby the Test and Measurement

industry as SCPI -StandardCommands for ProgrammableInstrumentation - to eliminatethe multitude of proprietaryinstrument programming languagesavailable from instrument vendors.During this time, Agilent andother instrument manufacturersproduced a growing numberof proprietary GPIB modularinstrument products. Theseinstruments could be integratedinto test systems to provideswitching, measurementsand signal source capabilities.However few of these modularproducts were compatible. TheVXIbus standard addressed thisproblem of incompatibility.Because it changes the way we

think about electronic test, there’sstill some confusion about howto apply the VXIbus standard,how complex it is, and how it fitsin with existing rack and stackinstruments. With the successof the VXIbus standard, otherstandards -like VXIplug&playand SCPI - are emerging toimprove the usefulness of VXIbustechnology in electronic test.

In this booklet, we’ll provide youwith a basic understanding ofVXIbus, SCPI, and VXIplug&play-and explain some of the advantagesof these standards. We will nottell you that they are the answerto every problem, but will showyou how to integrate these modularproducts into your current testsystem. We’ll also help youunderstand the tradeoffs inselecting various VXI devices.Please understand this is nota manual for any specific VXIinstrument, but rather anintroduction to overall VXItechnology.

We’re pleased to be a leader inVXIbus, SCPI, and VXIplug&playtechnologies. We think you’ll seethe advantages of these standardsin your test system environment.

With that in mind, let’s take a closer look.

Feeling Comfortable with VXIbus

CONTENTS

VXIbus: The Test and Measurement Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3The History of VXIbus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3Goals of the VXIbus Consortium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3GPIB &VMEbus, The Foundation for VXI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

What is GPIB? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4What is VMEbus? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

VXIbus: The Best of Both Worlds, and More! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Taking the Best from GPIB & VME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5More Features of VXIbus Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

SCPI: The Standardized Programming Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

VXIplug&play: The New Standard for Test and Measurement . . . . . . . . . . . . . . . . . . . . . .8What is VXIplug&play? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8What does VXIplug&play Offer? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

What Do VXIbus, SCPI, and VXIplug&play Mean to You? . . . . . . . . . . . . . . . . . . . . . . . . . .9Open Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Higher Test System Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10True Upgrade Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11Easy Integration with Rack & Stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Smaller Test Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Access to Switching Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Long-Term Software Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Multi-Vendor Interoperability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

The VXIbus Standard: More Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Instrumentation Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Module Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Connectors and Buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Power, Cooling, and Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

VXIbus Devices and Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16What are Devices? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Register-Based Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17Message-Based Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

More on Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19GPIB & LAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19External Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19Embedded vs. External Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

SCPI: More Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

VXIplug&play: More Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22How does VXIplug&play Work? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22Instrument Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23VISA I/O Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Putting It All Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

What has Agilent Technologies Done to Improve VXI Technology? . . . . . . . . . . . . . . . .26

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

The History of VXIbus During the late 1970’s and 1980’s,numerous electronic instrumentcompanies were producing theirown proprietary cardcagesystems. The manufacturersrecognized the advantages ofcardcage and IAC (Instrument-On-A-Card) systems, but theywere using vastly differentapproaches. At the same time,the US Air Force created aprogram to design a single IACstandard that would result insubstantially smaller electronicequipment.

In April 1987, five companies -Agilent Technologies, Tektronix,Colorado Data Systems, Racal-Dana, and Wavetek -starteddiscussions aimed at creating anIAC standard that would benefitboth the commercial and militarytest communities. The companiesformed the “VXIbus Consortium”and met for three months ofintense technical discussions.An initial draft, the VXIbusSystem Specification, wasreleased on July 14, 1987.

Like most new things, thespecification has undergoneseveral changes. Revision 1.3 wasreleased in 1989. The specificationwas submitted to the IEEE andwas adopted as the IEEE-1155Standard in 1992. In addition, theU.S. Air Force has incorporatedthe VXIbus specification into itsMATE (Modular Automated TestEquipment) program. The VXIbushas continued to evolve, withRevision 2.0 released in 1998.

Goals of the VXIbus Consortium The members of the VXIbusConsortium had a lot ofexperience with IAC systems.They understood their benefits -increased test throughput, smallerinstruments, reduced cost, etc.They wanted to create a technicallysound standard that would bringIAC systems into the “nextgeneration.” That would free updesign engineers to do what theydo best- bring new technologiesto market. The result would beproducts of greater innovation,quality and diversity.

3

VXIbus: The Test and Measurement Standard

Instrument-On-A-Card Systems

GPIB and VMEbus: The Foundation for VXIbus The Consortium membersrecognized Agilent Technologies’GPIB (IEEE-488) and VMEbus asthe two most popular standardsfor instrumentation. They decidedto take the best from thesestandards and add more featuresto create the best possible IACstandard.

What is GPIB?

In the early 1970’s, AgilentTechnologies invented an 8-bitparallel interface- GPIB. It allowedrack and stack instruments tocommunicate with each otherand with a host computer. In1975, GPIB was adopted by theIEEE as standard IEEE-488.Today, it is the leading interfaceused in automated test systemsbuilt on individual instruments.It is simple, flexible, and usedby nearly all instrumentmanufacturers.

GPIB is a widespread standard-it allows you to connect instrumentsfrom several manufacturers to ahost computer (controller), andthus build an automated, integratedtest system. Normally, you don’thave to worry about how informationis passed between the devices.Your only concern is the contentof the information, whether it beASCII or binary instructions toan instrument, or ASCII or binaryresults from an instrument.

A good way to think of GPIBinstruments is as electronic devicesthat operate by themselves. Theyhave communication intelligence,

and may also perform sophisticateddata capture and analysis. Anexample is a digital multimeter.Using GPIB, you can send themultimeter a sequence of ASCIIstrings that instruct it to take aburst of 1000 readings. You mayalso send it commands telling itto calculate statistical functionslike minimum, maximum, andstandard deviation on the readings.Then you can bring only thoseresulting values back to thecomputer.

One limitation of GPIB has beena maximum data transfer rate ofabout 1 Mbyte per second. This isusually not a problem, since mostapplications are limited by thespeed of the measurement circuits,or by the switch closure andsettling time required to routesignals. However, it can become aproblem with high-speed digitizing,digital inputs/outputs, or iflarge amounts of data must betransferred from an instrumentto the computer for specializedprocessing. Furthermore, theGPIB protocol limits the transferspeed to that of the slowestdevice on the bus.

What is VMEbus?

While GPIB is the most popularelectronic instrument interface,VMEbus is widely used in micro-computer systems. The VMEbusspecification was released inAugust 1982, and approved byIEEE and ANSI in 1987.

You can think of VMEbus as aninterface with two components-mechanical and logical. Themechanical portion specifies thephysical dimensions of plug-inboards, backplanes, subracks, etc.(The form factor of the plug-inboards is commonly known asthe Eurocard format.) The logicalportion of the interface describeshow functional modules (in thiscase, plug-in cards) communicatewith each other. A major objectiveof VMEbus is to allow communicationbetween two devices withoutdisturbing the internal activitiesof other devices in the system.VMEbus systems can havemultiple microprocessorson the same backplane.

4

Figure 1. A GPIB System: Ease of Use, Ease of Integration

One strength of VMEbus is that itallows high-speed communicationbetween devices (which weuse interchangeably here with“modules”). The specificationwas originally intended formicrocomputer systems. Asinstrument speeds increasedand printed circuit board sizesdecreased, interest grew inbringing electronic instrumentsinto the system. However, thisbrought out two shortcomingsof VMEbus: the electricalenvironment, designed for digitalcommunication, is too “noisy”for precise analog measurements,and the programming needed forhigh-speed communication has tobe done with low-level registerreads and writes.

VXIbus: The Best of Both Worlds,and More! Members of the VXIbus Consortiumrealized that for the VXIbusstandard to be successful, it mustanswer two major challenges ininstrumentation: communicationspeed and integration. The GPIBand VMEbus specifications heldthe answers to both these problems.A third challenge solved by theConsortium was to devise a well-defined environment in whichdifferent vendors’ products canoperate together properly.

The result is the VXIbus (VMEbusExtensions for Instrumentation )

Taking the Best from GPIB & VMEbus.

The VMEbus specification,originally designed for micro-computers, has a great potentialfor high-speed device-to-devicecommunication. This can increasethe throughput of your test systemconsiderably. And GPIB is wellknown for its ease of integration,which helps you to build your testsystem faster. So the two mainchallenges- speed and integration-were answered. Although theGPIB and VMEbus standards havedifferent bus communication styles,VXIbus defines two differentdevices to take advantage ofthese styles.

Remember that GPIB instrumentsare easy to use. You simply connectthe cable and program the instru-ments in whatever language theyrequire. In VXIbus systems, thecounterpart to GPIB instrumentsare “Message-Based Devices.”They are easy to integrate into asystem and communicate at ahigh level using ASCII characters.Like GPIB instruments, Message-Based Devices can containsignificant intelligence anddata processing capabilities.Like instruments on a GPIB bus,

however, Message-Based Register-Based Devices can be limitedwhen it comes to high-speed datatransfer.

The outstanding feature ofVMEbus devices is that they canmove data between themselvesvery fast. The VXIbus specificationdefines “Register-Based Devices”as the analog to VMEbus devices.These devices communicate ata lower, more basic level thanMessage-Based Devices andso can attain greater transferspeeds. Programming a Register-Based Device involves writing toand reading from individualregisters on the device.

5

VXIbus: The Best of Both Worlds, and More!

VMEGPIB

Register-BasedDevices (Binary)

Message-BasedDevices (ASCII)

Easy Fast

bus

Figure 2. A VMEbus System: Potential for High Speed And Multiple Processors

Figure 3. VXIbus: The Best of Two Worlds

More Features of VXIbus Systems.

The VXIbus Consortium fullydefined the operating environmentfor VXIbus modules. All VXIbusmainframes must state how muchpower and cooling they provide.And all VXIbus modules muststate how much power and coolingthey require. Also, there are strictlimits on how much conductedand radiated interference isallowed between modules.These parameters allow youto configure a workable systemeasily.

Two special functions mustbe performed in every VXIbussystem. The first, Slot 0, takescare of backplane management.Slot 0 is a unique physical locationin every VXIbus mainframe. Signalsfrom this slot must include thingslike clock sources, arbitrationfor data movement across thebackplane, etc. The module thatgoes into this slot must performthese hardware functions inaddition to its normal functions.If you’re familiar with VMEbussystems, you probably recognizethat this is very similar to VME’s“Slot 1 Device.” The Slot 0 devicerelieves you of the burden ofmanaging data flow across thebackplane.

The second special function ina VXIbus system is the ResourceManager. The best way to thinkof the Resource Manager is as acomputer program. This programconfigures the modules for properoperation whenever the system ispowered on or reset. This meansthat you can build your test systemsoftware from a known startingpoint. The Resource Manager isnot involved with the VXIbussystem once normal operationbegins.

6

VMEGPIB

bus

• Instrumentation Environment

• Slot 0 Functions

• Resource Manager

Easy Fast

Figure 4. VXIbus Additional Features

With the rapid growth of computer-controlled instruments, AgilentTechnologies recognized the needfor a common instrumentationlanguage. Therefore, in the late1980’s, Agilent Technologiesinvented TMSL (Test andMeasurement Systems Language)and offered to make it an openstandard. TMSL itself was basedon industry standards whereverpossible, including IEEE-488.2and IEEE-754. In April 1990,this standard was acceptedby the industry, and renamedSCPI (Standard Commands forProgrammable Instrumentation).You now have to learn only thissingle instrument programminglanguage, regardless of whosedigital multimeter (or othersimilar instrument) you purchase.With this standardized programminglanguage for VXIbus instrumenta-tion in place, you can reduce yourtest system programming time!

SCPI is now managed by aconsortium of nine instrumentmanufacturers. Today, thereare over a thousand instrumentproducts using SCPI. For moreabout how SCPI works, see theSCPI: More Details section ofthis booklet.

7

SCPI: the Standardized Programming Language

VXIbus was a significanteffort to standardize modularinstrumentation. It providedan open environment where anyvendor’s modules would plug intoany VXI mainframe. Users couldexpect the module to fit the slotsize and be adequately poweredand cooled. However, VXIbusdidn’t address the need to integratea system that was truly vendorindependent and easily usable.Wouldn’t it be nice if there werea common look and feel, or astandard soft front panel forgiven instrument types fromvarious vendors? Wouldn’t it begreat if you could develop yourapplication program on a PC andexecute it on a UNIX platform?Or, what about instrumentdrivers? An industry standardset of drivers would eliminatecustom driver design issues.Agilent Technologies is an activemember in the VXIplug&playSystem Alliance to addressthese challenges.

What is VXlplug&play?VXIplug&play is a term indicatingconformance to a new set ofsystem-level standards, producedby the VXIplug&play SystemsAlliance. Agilent Technologiesjoined the VXIplug&play Alliancein 1994 in support of the Alliance’scharter: “to improve the effective-ness of VXI-based solutionsby increasing ease-of-use andimproving the interoperabilityof multi-vendor VXI systems.” Thegoal of the Alliance is to achieveinteroperability of mainframes,computers, instruments, andsoftware through open, multi-vendor standards and practices.The Alliance consists of vendorsactively involved with end-usersto produce instrumentationsystems that meet this goal.Additionally, the Alliance isopen to all vendors and usersas a forum for working togetherto make VXI technology easierto use.

Thanks to the work of the Alliance,VXIplug&play componentsintegrate easily. The new standardsapply to instrument drivers,soft front panels, installationpackages, documentation,technical support, applicationdevelopment environments, aswell as many other areas forinstrument system integration.As with the VXIbus standard,revisions to VXIplug&play willcontinue to reduce your dependenceon any single vendor and simplifyyour job of system design andimplementation.

What does VXlplug&play offer? VXIplug&play improves productivity,portability, and interoperabilityfor both vendors and end-users.

• ProductivityUse soft front panels to operateand evaluate instrumentoperation within minutes.

• PortabilityCommunicate with instrumentsvia any controller/computerinterface supported by theVISA I/O library.

• InteroperabilityDevelop application programsportable across computerplatforms and I/0 interfaces.Add new programs withouthaving to rewrite existing ones.

With vendors and end-usersworking together, the real needsof system integration continueto be analyzed, defined, andimplemented. For more on theVXIplug&play standard, see theVXIplug&play: More Detailssection of this booklet.

8

VXIplug&play: The NEW Standard for Test and Measurement

The VXIbus, SCPI, andVXIplug&play standards canimprove your test system’s speedand flexibility, lower your product“life cycle” cost, protect yourinvestment, and provide a choiceof vendors’ products that willwork together in your test system.

Open StandardsVXIbus, SCPI, and VXIplug&playare truly open standards. So far,over 200 different manufacturershave received identification codesfrom the VXIbus Consortium, andhundreds of different instrumentsare available. This multi-vendorenvironment ensures that yourinvestment in VXIbus productswill be protected long into thefuture. If one manufacturer’sinstrument becomes obsolete, areplacement should be availablefrom another. Also, there will bemany “niche” manufacturerswilling to provide specialtymodules- just as in the computerindustry.

Because of the open standards,instrument manufacturers canprovide VXI products with thebenefits of standardized architecture,instrument programming language,and I/0 communication. Thebackplane pinouts and communi-cation techniques are alreadydefined for you. Power andcooling capabilities are completelyspecified. Also, electrical inter-ference limits have been set, soyou know your module will have a“quiet” environment. The VXIbusspecification defines all of these,and takes care of the hardwarespecifications for any VXI system.The VXIplug&play System

Alliance completes the standard-ization process with specificationsfor the operating system or“framework,” instrument drivers,and I/0 software. VXIbus andVXIplug&play, working together,give you all the tools and guidelinesneeded to successfully design acustom VXI-compatible modulefor a unique function, build a newVXI test system, or upgrade yourexisting system.

9

What Do VXIbus, SCPI, and VXIplug&play Mean to you?

Higher Test System Throughput Increased test system throughputgives you a great competitive edgeby lowering your testing andmanufacturing costs. The VXIbusbackplane has a theoretical datatransfer limit of 40 Mbytes persecond. Normally, the backplanewill not be a bottleneck for yourdata transfer. The not-so-obviousadvantage of the VXIbus is itspotential for distributed intelligence,which leads to increased systemthroughput. Because of its VMEbusbackground, VXIbus can dealwith multiple microprocessorson the backplane existing withina shared memory architecture.Arbitrating data transfers onthe backplane allows for a higherdata bandwidth than any singledevice in the system can achieve.Multiple levels of priority allowcritical processes to interrupt anduse resources only when they’rerequired.

How can this help you? Let’s sayyou have an embedded computerthat’s transferring a large block ofreadings from a digitizer into thecomputer’s memory over the databus. Simultaneously, anotherinstrument like a counter mightbe using the data bus to send avalue to the controller every fewmilliseconds. At the sametime, another intelligentdevice might be monitoringseveral channels of voltageand internally performinglimit checks. If a conditiongoes out of limit, this devicecould request the data bus at ahigher priority than the otherdevices, and send onlythe failure data to thecomputer for immediateaction. Together, thesemultiple devices, withtheir different priorities,can more efficiently fillthe data bus. This makesfor higher overall systemthroughput.

10

1 - Transfer Data Blocks: priority low

2 - Update from Counter: priority medium

3 - Service Alarm Condition: priority high

Total Backplane Utilization

500 ms

100 ms

25 ms

Figure 5. Different Priorities, Maximum Throughput

1 - Transfer Data Blocks: priority low

2 - Update from Counter: priority medium

3 - Service Alarm Condition: priority high

Total Backplane Utilization

500 ms

100 ms

25 ms

True Upgrade PathAnother great feature of theVXIbus is a true migration orupgrade path. This allows you touse your current hardware andsoftware in future test systems.Suppose your current applicationis fairly simple: scan 15 channelsof voltage, measure the frequencyof two signals, and provide low-level power for a current loop.This application could be easilyand inexpensively done by a systembased on small VXIbus modules.Your testing needs may expandin the future so that you’ll needmore high-performance instrumentssuch as a high-speed digitizer orsignal generator. These instrumentsmay only be available as larger andmore complex VXIbus modules. Butall your smaller VXIbus moduleswill easily fit right into the new,larger system! Also, VMEbus devicesthat you’re currently using, orplan to use, can be integratedinto a VXlbus system. Yourimagination is the only limit onthe size and complexity of yoursystem.

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Figure 6. VXIbus: A True Upgrade Path

FuturePresent

Easy Integration with Rack and Stack VXIbus and VXIplug&playsystems can coexist perfectly withGPIB test systems. You can usethe resources from both withoutbeing limited to one type of system.In fact, many of your test systemsprobably will be a mix of VXIbusand non-VXI instruments. Forexample, a test system may needVXIbus to solve a throughputbottleneck, and also a very highaccuracy measurement from anetwork analyzer that is availableonly as a standalone instrument.

Another common scenario mightbe a VXIbus mainframe full ofinstruments controlled by acomputer. The computer might beembedded in the VXIbus mainframeor external to it. At the sametime, this computer could easilycontrol several large programmablepower supplies via GPIB, andperhaps monitor transducersor data links via RS-232.

Both GPIB and non-VXI instrumentscan be programmed exactly likeVXIplug&play instruments, byadapting VXIplug&play instrumentdriver technology and using SCPIcommands with either the VISAI/0 Library or Agilent SICL(Standard Instrument ControlLibrary).

Smaller Test Systems Probably the most obviousadvantage of VXIbus systems isthe significant downsizing of testsystems. Much of the downsizingresults from using common powersupplies and eliminating frontpanels. This benefit has been veryimportant to military users due totheir extremely large test systems,and continues to grow in thecommercial world as devicesincrease in electronic contentand complexity.

Access to Switching ModulesMany people are introduced toVXI because of their need forsignal routing (switching). Almostevery test application requiressome sort of signal routing, andVXI offers many more choicesthan traditional rack and stack.As more instruments becomeavailable, the importance of VXIin testing continues to grow.

12

Figure 7. Mix and Match with VXIbus and Rack & Stack

1 . 2345 mV

Long-Term Software Protection VXIbus is an excellent solutionto the problem of hardwarecompatibility- i.e., protectingyour hardware investment. Butsoftware compatibility, softwaredevelopment productivity, andprotecting your investment insoftware are equally importantissues. In a lot of applications,the cost of test system softwareis actually greater than the costof the hardware!

Development of the VXIbus spec-ifications increased awareness ofsoftware incompatibility issues.SCPI (Standard Commands forProgrammable Instrumentation)can help you protect your softwareinvestment with one standardizedprogramming language for VXIbusinstrumentation. For moreinformation, see the SCPI:More Details section of thisbooklet.

Multi-Vendor InteroperabilityVXIplug&play emerged not onlyto address software issues, butalso to define standards at acomplete system level. Successfullyintegrating a multi-vendor VXIsystem requires hardware workingtogether at the electrical andmechanical level. The softwareused to control the hardwaremust also work together. In large,complex test systems, determiningthe “framework” or platform andthen finding components thatwork together may be an integrator’sgreatest challenge. VXIplug&playbecomes an excellent solution forintegrating your VXI system intoa successful multi-vendorenvironment.

With the VXIplug&play concept,you designate a standard system“framework” or platform based onthe system software you intend touse (i.e., Microsoft® Windows®3.1/95/98/Me/NT®/2000, or

HP-UX*). By using the frameworkdesignation when you choose therest of your system components,your VXIplug&play compliantcomponents will be easy to useand easy to integrate into yourtest system. No longer should thepreference for a particular softwareor hardware component needto lock you out of a system orplatform. Another benefit ofVXIplug&play is its portabilitybetween the Windows and HP-UXenvironments. This means thatyour application written on HP-UXcan run under Windows witha simple recompile of the code.VXIplug&play gives you evenmore flexibility in meeting yourtest system needs in the multi-vendor environment.

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* HP-UX is Hewlett-Packard’s implementation of UNIX.

Microsoft ®, Windows®, Windows NT®, are registeredtrademarks of Microsoft Corporation.

In this chapter, we’ll give youmore detailed information aboutVXIbus systems. This should answersome common questions you mayhave about VXIbus, and help you

“speak VXI” if you wish topursue further information.

The appendix of thisbooklet has a list ofadditional reading

materials.

Instrumentation Environment The VXIbus is an open standardthat many manufacturers arenow using to develop a wide

variety of products. Thismeans the specificationmust provide a well-definedenvironment to guaranteeproper system operation.

To define an environment fora variety of high-performanceinstruments, the specificationaddresses several importantissues in physical and electricalcompatibility.

Module Sizes

One requirement for physicalcompatibility is the size of plug-inmodules. The VMEbus standarddefined two small module sizes.They’re known as A-Size and B-Size modules in VXIbus. Thesemodules are fairly compact; it’sdifficult to fit multi-functionalanalog instruments onto them.So the VXIbus Consortiumextended the VMEbus specificationfor module sizes- it defined twolarger modules, C- and D-Size.These four module sizes allow youto make a number of price andperformance tradeoffs so youcan optimize your test system.

14

The VXIbus Standard: More Details

Figure 8. Different sized modules for manyapplications

B

A

C

D

3.9 x 6.3 in(10 x 16 cm)

9.2 x 13.4 in(23 x 34 cm)9.2 x 6.3 in

(23 x 16 cm)

14,4 x 13.4 in(36 x 34 cm)

The most common card sizes areB- and C-Size. For any system,PC-board space costs money.The B-Size format gives you anexcellent tradeoff in cost andcomplexity, and is very useful forsimpler, lower-cost instruments.Instruments that make a good fitfor B-Size modules include relaymultiplexers, some voltmetersand counters, and small numbersof digital-to-analog converters.

C-Size is a good size for morecomplex instruments requiringextra space for sophisticatedcircuitry or computation hardware.Since A- and B-Sizes came fromthe VMEbus, they retain themodule-to-module spacing of0.8 inches (2 cm) specified byVMEbus. Because C-Size is definedby VXIbus, it uses a larger spacingof 1.2 inches (3 cm). This gives C-Size modules better shieldingfrom electrical interference, andtherefore better measurementsensitivity and accuracy. Someexamples of instruments thatfit the C-Size format are high-performance multimeters, functiongenerators, high-speed digitizers,and high point-count switches.

A-Size cards are too small forprecision instruments, givencurrent technologies. Still, theyare a good size for communicationinterfaces to the non-VXIbusworld. The D-Size format is usefulin specialized applications, but itdoes result in significantly highercost and increased rack space. Asa result, almost all manufacturersare using multiple C-Size slots forcomplex instruments instead ofputting them onto D-Sizemodules.

Connectors and Buses

Closely related to module sizesare the backplane connectors,shown in Figure 9. All modulesmust have at least one 96-pinconnector, known as P1. All thepins on P1 are completely definedby the VMEbus specification.These definitions are maintainedin VXIbus. P1 contains all thenecessary lines for 16-bit datatransfer, handshaking, busarbitration, and interruptsupport.

It’s possible to add the optionalP2 connector on B-Size and largermodules. This will expand thedata transfer bus and provide alocal bus for high-speed module-to-module communication. On D-size cards, you can add anotheroptional connector, P3, whichadds resources for specializedinstrumentation. When combinedwith multiple module sizes, thisrange of connectors allows youto optimize your test system withvarious price and performancetradeoffs.

15

Agilent 34401A DMM and its VXIbus C-Size equivalent, the Agilent E1412A

Figure 9. VXIbus modules and connectors

B

A

C

D

• 16 bit Data Transfer Bus• Arbitration Bus• Prioritized Interrupt Bus

P 1

• Increase width of Data Transfer Bus• 12-line Local Bus• TTL & ECL Trigger Buses• 10 MHz Clock Bus• Power Distribution Bus

P 2 (optional)

• Increase width of Local Bus• Increase width of ECL Trigger Bus• 100 MHz Clock Bus• ECL Star Bus• Expand Power Distribution Bus

P 3 (optional)

P 1

P 2

P 3

Power, Cooling, and Interference

The second major issue ofcompatibility addressed bythe VXIbus specification is themainframe environment. VXIbusproduct vendors need to knowexactly what type of environmenttheir module will be used in, sothey can design, test, and specifyit properly. This means you canhave great confidence that VXIbussystems- including those comprisedof different vendors’ modules-will function properly.

All VXIbus mainframes mustspecify how much steady-stateand dynamic current they provideat different voltage levels. Also,mainframes must specify howmuch cooling, on a per-slot basis,they provide. VXIbus plug-inmodules must provide correspond-ing information about their ownrequirements.

Plug-in modules must also specifyboth high-frequency emissionand susceptibility. The VXIbusspecification sets strict limitsfor the amount of interference adevice may emit. Conversely, thedevice’s operation must not beaffected by neighboring modulesoperating within interferencelimits. These limits and testsapply to both radiated andconducted signals.

VXIbus Devices andCommunicationWhat Are Devices?

In this booklet, you’ll noticethe term “device” used often.The simplest way to thinkof a device is as a plug-inmodule, or card. In VXIbussystems, every device musthave a unique addressfrom 0 to 255, called itsLogical Address. Thisdistinguishes it from otherdevices in the system.

Every VXIbus device isgranted 64 absoluteaddresses on the backplane.You can think of this as 64bytes of RAM on the device thatcan be addressed and accessedby other devices in the system.This little block of memory servestwo main purposes- it containsinformation about the deviceand its communication capabilities,and it’s also the location whereall required communicationwith the device takes place.The device’s Logical Addressdetermines the address of this“information and communicationmemory” within a VXIbus system.This 64-byte chunk of dedicatedmemory contains the device’s“Configuration Registers” and“Communication Registers.”

The VXIbus specification definesfour types of devices- Register-Based, Message-Based, Memory,and Extended. Memory devicesare just specialized Register-Based Devices that are optimizedto hold and move large amountsof data. Extended Devices arecurrently reserved, and providea growth path for new types ofdevices in the future.

16

Figure 10. VXIbus device registers.Memory address is determined by VXIbus logicaladdress.

Status and Control

Device Type

Identification Register

DeviceDependentRegisters

16 bits

32 w

ords

(64

byte

s)

Memory address isdetermined by VXIbusLogical Address

Register-Based Devices

In the first section of this booklet,we said that VXIbus Register-BasedDevices communicate much likeVMEbus Devices. This means thatthey’re programmed at a low levelusing binary information. Theobvious advantage of this is speed-Register-Based Devices communicateliterally at the level of directhardware manipulation. Thishigh-speed communication canlead to much greater test systemthroughput.

All types of devices in a VXIbussystem must have some sort ofcommunication interface to theVXIbus backplane.

One advantage of Register-BasedDevices is that this interface isvery simple, and thus small andlow in cost.

The Register-Based Device is idealfor simple cards such as switches,multiplexers, basic DACs, etc.There’s little reason to put alarge, expensive communicationinterface on these devices. Theregister-based interface is also a

good choice for devices that mayneed to move large amounts ofinformation across the VXIbusbackplane at very high speeds-for example, high-speed digitizersand high-speed digital I/O cards.

You might think Register-BasedDevices are limited to being verysimple devices. However, thisis not the case. For example, aregister-based digital multimetermay communicate with otherdevices using low-level binarycommands, but it could also havean internal microprocessor forsophisticated measurementcontrol and self-diagnostics.

A lot of people like Register-Basedmodules because of their lowcost, but don’t want to have toprogram them with low-levelbinary commands. VXIbusanswers this concern with aconcept called Commandersand Servants (not a video game).A device that contains theintelligence to operate a Register-Based Device can be configuredas a Commander to that Register-Based Device. You send high-level

ASCII commands to theCommander, it interprets them,and then sends the necessarybinary information to theRegister-Based Servant. In thismode, you program Register-Based Devices exactly as if theyare Message-Based Devices.Figure 12 shows the optionsavailable for programmingRegister-Based Devices.

17

Figure 11. Register-Based communication Figure 12. Paths for register-based communication

Easy (ASCII)

Fast (binary)

Register-BasedDevice

Register-BasedDevice

Register-BasedDevice

Register-BasedDevice

You

"Commander"for

Register-Based

Devices

Register-Based Device

InstrumentControl

Registers

BinaryInformation

VXI Backplane

Message-Based Devices

Message-Based Devices, incontrast to Register-BasedDevices, communicate at avery high level using ASCIIcharacters. A good analogy forthese devices is standalone GPIBinstruments. Message-Based Devicesare very easy to integrate intoVXIbus systems. This is especiallyimportant in systems comprisedof modules supplied by differentvendors.

Message-Based Devicescommunicate with each otherwith a well-defined set of rulesknown as “Word Serial Protocol.”This asynchronous protocoldefines the handshakingnecessary to move commandsand data between devices.

One advantage of Word SerialProtocol is that it hides compatibilityconcerns from you, the user. That’swhy Message-Based Devices areeasy to integrate into a system.To program them, you simplysend and retrieve high-level ASCIIcharacters. The characters yousend must be in the device’sspecific language, but you don’thave to worry about module-specific registers, binary readingand writing, etc.

Two tradeoffs are required tohave this ease of use. First, thecommunication interface requiredto implement the Word SerialProtocol is complex. This meansit takes up more “real estate”on the circuit board than thecommunication interface of aRegister-Based Device. Hence, aMessage-Based Device will alwayscost more than an equivalentRegister-Based Device. Also,because of the space committedto communication rather thanmeasurement circuitry, Message-Based instruments will generallybe on C-Size or larger modules.

Second, the communication speedbetween devices is lower. It’sroughly equal to that of GPIB.However, most sophisticateddevices will have on-board dataprocessing, so they may onlyneed to pass a final result fromthe device to the controller. Anexample of this is an oscilloscopethat digitizes a waveform andthen automatically measures risetime. Only the rise time is passedfrom the instrument, not all thedigitized readings.

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Figure 13. Message-Based communication

Message-Based Device

ASCIIcharacters

VXI Backplane

LocalIntelligence

(microprocessor)

More on Communication The ability to communicatebetween devices, the VXIbusbackplane, and computers is anintegral part of any VXIbus testsystem. We’ve already defined thetwo primary types of devices -Register and Message - and howthey are set up for communication.How can we improve data transfer,or make measurements for testsrequiring transfers of large amountsof data? How can we continue togrow test systems and keep costlow? With new developments inexternal interfaces and bothexternal and embedded computers,the rules are changing and moreoptions are being created foranswering these questions.

GPIB & LAN

As noted earlier, one limitation ofGPIB is its maximum data transferrate: about 1 Mbyte per second. IfGPIB speeds meet your applicationneeds, you can add the flexibilityand long-distance capabilitiesof LAN to your test systemarchitecture. For instance, a LAN/GP-IB Gateway, with measurementserver software, allows testsystems, whose components haveGPIB interfaces, to be controlledby a computer via a thin or twistedpair LAN. It is particularly usefulwhen the device under test is ina protective or environmentalchamber some distance awayfrom the computer; engineerscan follow the test run’s progressfrom a workstation at their desks.You can also control multiple

GPIB test stations with onecomputer by connecting eachtest station to the LAN througha GPIB/LAN Gateway. Anotheradvantage is the ability to createmeasurement servers on thenetwork. This allows you to usea measurement server to collecttest data and analyze it frommultiple locations.

External Interfaces

There are three basic choices forinterfacing between the VXIbusbackplane and your computer.The most common is the popularGPIB (IEEE-488) interface. AGPIB Command Module residesin slot 0 of the VXIbus system,providing the required ResourceManager and Slot 0 functions,and acting as a GPIB-to-VXIbusinterface. This interfacing choiceeasily allows you to combine yourVXIbus system and other GPIBinstruments on one computerinterface.

As the computer industry evolvesand creates new industry-standardinterfaces, VXIbus vendors aretaking advantage of these interfaces.FireWire (IEEE-1394) is a low costinterface that offers the ability tomove data at much faster ratesthan GPIB. Compared to GPIB’stheoretical limit of 1 Mbyte/sec,FireWire has an upper limit of50 Mbytes/sec. For applicationsrequiring the transfer of largeblocks of data, such as uploadingsequences of digitized waveforms,FireWire is a very attractivesolution. In a manner similar to

GPIB, a FireWire CommandModule resides in slot 0 of theVXIbus system, providing therequired Resource Manager andSlot 0 functions, and acting as aFireWire-to-VXIbus interface.Since FireWire is a recognizedstandard within the computerindustry, it is built into increasingnumbers of PCs as a standardfeature. FireWire interfaces arealso available to plug into PCs asPCI cards. You can also easilyuse both FireWire and GPIBinterface cards in your PC tocommunicate with all your testsystem resources, no matterwhat kind of interface they use.

Another proprietary interface,called MXIbus, has been developedfor VXIbus systems. With thisscheme, a MXIbus module plugsinto slot 0 of the VXIbus mainframeand connects to the MXIbusinterface card in the externalcomputer. MXIbus provides thespeed and throughput of directmemory-mapped access to theVXI backplane. Due to itsproprietary nature and its designthat directly extends the VXIbusbackplane, MXIbus is higher incost than the industry-standardalternatives of GPIB or FireWire.

With these advances in externalinterfaces, VXIbus vendorscontinue to push the limits forfaster and more configurablecommunication tools.

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Embedded vs. External Computers

In addition to a rich choice ofinterfaces between an externalcomputer and the VXIbusbackplane, you can also put yourcomputer directly on the VXIbusbackplane and inside the VXIbusmainframe. Whether you choosean embedded or external computerwill depend on your test systemneeds. To select the best computer,consider several factors: operatingsystem, throughput, ease of use,physical size, configurationflexibility, and cost.

For example, if your applicationrequires the greatest possiblethroughput, a VXI embedded PA-RISC computer using HP-UX -the 9000 Series 700 -may best fityour needs. Using this computerin conjunction with VXI devicesthat use high-speed register-levelcommunication gives youmaximum throughput.

When space is at a premium inyour test system set-up, embeddedcomputers are extremely valuable.Embedded computers integratehigh functionality into smallmodules for use in C-Size VXImainframes. Embedding thecomputer in the VXI chassisallows direct computer accessof other VXI devices, systemmemory, and triggers as thoughthey were part of the computerhardware. You get the highestspeed performance and computeraccess to VXI devices while stillconserving space. An embeddedcomputer also allows you encloseand secure your computer in thesame enclosure as your VXIbussystem.

With external computers, youhave a huge choice of PCs andUnix workstations. You canchoose the combination of costand performance that best meetsthe needs of your application.An external computer allows

you to reap the benefits of thefast-paced, changing technologyof the computer industry. Youcan take advantage of continuousimprovements in price andperformance, and can upgradeyour computing power andperformance as your test systemrequires. An external computerrequires the use of an interface toconnect to the VXIbus backplane.If you need easy access to yourcomputer as you develop a testapplication and cost is a primaryconcern, an external computer -PC-based or UNIX-based - maybetter fit your needs. With anexternal computer, you can cyclethe VXIbus mainframe powerwithout having to reboot anembedded computer’s operatingsystem. If you have custom VXImodules as part of your testsystem, this can be veryimportant during applicationdevelopment.

20

SY

ST

EM

PR

ICE

S Y S T E M I / O P E R F O R M A N C E

GPIBFireWire

MXIbus

EmbeddedComputer

SCPI: More DetailsThe cost of software for a testsystem is often greater than thecost of the hardware. In the late1980’s, Agilent Technologiesinvented and offered to make TMSL(Test and Measurement SystemsLanguage) an open standard.In April 1990, this standardwas accepted, and renamedSCPI (Standard Commands forProgrammable Instrumentation).SCPI complements the VXIbusand VXIplug&play specifications,helping you protect your investmentby using an industry-acceptedstandard.

SCPI differs from earlier instrumentlanguages in that the commandsdescribe the signal you’re tryingto measure, not the instrumentyou’re using to measure it. Thismeans programs written withSCPI are more readable andintuitive. You spend less timelearning about your instruments,and more time solving yourapplication problems. This feature,known as “horizontal compatibility,”means that the same SCPIcommands apply to manydifferent types of instruments.

For example, the“TRIGGER:IMMEDIATE”command can be used with amultimeter, oscilloscope, or anyother instrument with triggercapability.

SCPI is also designed to beextensible, allowing it to growas instrument capabilities grow.Suppose that in the future youbuy a multimeter with morefeatures than your currentunit. Its basic functions will beprogrammed exactly like the oldunit. This “vertical compatibility”results in lower support costs,obsolescence protection, andan upgrade path. Headers,mnemonics, and parameterformats are standardized as well.

All Agilent Technologies VXIbusproducts are programmed withSCPI commands. You learn justone programming language that,in turn, increases your programmingefficiency and reduces the timeyou spend on development. It alsoensures that your investment ininstrument control software isprotected.

21

OUTPUT@Dmm;“*RST” ! Reset all theOUTPUT@Scope;“*RST” ! test systemOUTPUT@Counter;“*RST” ! instruments.

OUTPUT@Scope;“MEASURE:VOLTAGE:RISETIME?” ! Use oscilloscope to ENTER@Scope;Risetime ! measure wave form risetime.

OUTPUT@Dmm;“MEASURE:VOLTAGE:DC?” ! Use DMM to accuratelyENTER@Dmm;Dc_level ! measure final signal level

OUTPUT@Counter;“MEASURE:FREQUENCY?” ! Use counter to measureENTER@Counter;Frequency ! freq of another signal

Figure 14. SCPI’s horizontal compatibility

Making all the pieces fit togetheris a big challenge for any systemintegrator or engineer. If youhave ever done this, you will

be amazed at thebenefits that

VXIplug&playbrings to theTest andMeasurementindustry.

VXIplug&playcompliantsystems providethe mechanismsfor you to buildsystems thatmeet your specificapplication needs,

while removing concerns aboutsoftware interoperability. TheVXIplug&play Systems Alliancehas achieved this by defining andimplementing requirements forthe entire test system- includingsoftware, I/O communication,drivers, installation packages,computers and interfaces.

How does VXlplug&play Work?To understand how VXIplug&play-compliant components worktogether, you must understandeach of the different pieces.Of course, it begins with youand your application. Then, youchoose your system framework.Figure 15 is a simple display ofthese pieces.

Frameworks

The VXIplug&play SystemsAlliance has defined several“frameworks” based on establishedindustry-standard operatingsystems. Specifically, a frameworkrelates to a particular operatingsystem (for example, Windows3.1/95/98/Me/NT/2000, HP-UX,GWIN, and others), and specifiesthe requirements for instruments,controllers, interfaces, mainframes,and software packages thatcomply with that system.

To make the example in Figure 15work, the specification assigns aframework designation to eachdifferent piece of the system.What does all this mean to you,the test engineer or integrator?Once you choose the frameworkbased on the system softwarethat meets the needs of yourunique test application, then theframework designation is all youneed to know when choosing therest of your system components.An Alliance specification defineseach VXIplug&play framework,with detailed requirements forall system pieces – from top tobottom.

22

VXIplug&play: More Details

For example, say you’ve chosenWindows 95 as your systemframework. You will need to selecta PC, interface, VXI mainframe,and VXI instruments that areWIN 95 Framework-compatible.Manufacturers of these systemcomponents have already identifiedwhich are VXIplug&play. One ofthe instruments you may need isan Agilent E1412A Digital Multimeter.Because of VXIplug&play,you will automatically receive aninstallation disk or CD with theappropriate library files (i.e., .dll),MS WIN help files, knowledgebase file, and an executable softfront panel file. (For more detailon these features, keep reading. )The VXIplug&play Specificationrequires the manufacturer tosupply all of this support materialwith this (and every) instrumentdriver.

Instrument Drivers

One of the most exciting develop-ments from the Alliance is standardinstrument drivers. The instru-ment drivers take care of the low-level details of I/0 communication.As a test system developer, youno longer need to deal with low-level I/0 protocols! As defined bythe Alliance, VXIplug&playinstrument drivers include thefollowing features:

C function library files

C function library files contain adynamic link library (.dll or .sl),ANSI C source code (.c, .h), andfunction panel file (.fp). It usesthe VISA I/0 Library for all I/0functions. With these tools, youhave high-level, easy to use Cfunctions for your application.

Interactive soft front panel executable program

A soft front panel is a graphicaluser interface for an instrument.However, soft front panels do notgenerate code and are not for usein a program. You use the softfront panel when the instrumentis first integrated into the system.You can verify your instrumentcommunications, or use it as alearning tool to teach instrumentcontrol and capability concepts.Many test system users like thisfeature because it is quick andeasy to determine if the instrumentis ready for use.

This tool is specifically usefulfor system integration and forincoming inspection tasks. It isalso an integral part in makingyour system easier to use.

23

Agilent VEE, NI's LabView and LabWindows\CVIFunctional Panel

VXIplug&play-compliantInstrument Drivers

Agilent's VISA or NI VISA

• Delivered with your instrument• Downloaded to you controller• Communicates low-level I/O details via VISA

OtherApplications

e.g., Visual Basic/C

Interface Hardware(GPIB, FireWire, MXIbus)

YOURAPPLICATION

Contains VXIplug&play function calls

Figure 16. A VXIplug&play soft front panel

Agilent VEE, NI's LabView and LabWindows\CVIFunctional Panel

VXIplug&play-compliantInstrument Drivers

Agilent's VISA or NI VISA

• Delivered with your instrument• Downloaded to you controller• Communicates low-level I/O details via VISA

OtherApplications

e.g., Visual Basic/C

Interface Hardware(GPIB, FireWire, MXIbus)

YOURAPPLICATION

Contains VXIplug&play function calls

Agilent I/O Libraries, including VISA I/OLibrary, is provided with your interface orembedded computer, and links yourapplication to your VXI instrument.

Figure 15. A VXIplug&play-compliant system

Knowledge base file

A knowledge base file describesall specifications for an instrumentas an ASCII file. These specificationsinclude mechanical, electrical,and environmental information.This is useful when used as asystem integration tool for multi-vendor systems.

Help file

A help file provides helpinformation for the C functionlibrary, programming examples,instrument overview, and thesoft front panel. You can copy orpaste the programming examplesdirectly into your applicationprogram. This can greatly reduceyour development time.

VXIplug&play requires deliveryof the instrument driver withyour instrument. All you need todo is load it onto your computer.

VISA I/0 Library

Without a common I/0 library,interoperability of systemcomponents is not possible.Therefore, a standard I/0 librarybecame a primary goal of theVXIplug&play Alliance. I/0software takes care of thecommunication over a physicalconnection, and is the foundationon which other standards arebuilt. To move away from existingproprietary systems, languages,and I/0, the library needed tobe independent of instrument,interface, operating system,language, or networkingmechanisms.

The VXIplug&play Alliance callsthe new standard I/0 library“VISA”- Virtual InstrumentSoftware Architecture. VISAprovides a single foundation formulti-vendor instrumentationsoftware. The development ofVISA provides access to newcapabilities as technology changes,as well as maintaining a migrationpath for existing systems. VISAoffers all of these as a single,easy-to-use set of I/0 controlfunctions very similar to existingI/0 libraries, such as Agilent’sSICL.

VISA provides a set of corefunctions- Location and LifeCycle Control (open, close),Events (enable, disable), andMessage-Based Control (write,read, print), just to name a few.Each of these functions applies toall instruments. This means youcan use VISA’s open ( or viOpen)function call for any device inyour test system.

VISA’s development andstandardization offer you evenmore benefits. With fewer functionsto learn, you reduce the time spentlearning a new tool. Since it issimilar to existing I/O libraries,you probably already have theknowledge to begin developmentof your application. Another greatbenefit of VISA is its portabilitybetween frameworks. Yourapplications written on HP-UXcan run under Windows withjust a simple recompile of thecode. You are no longer platform-dependent! VISA gives yousystem-level confidence andremoves your dependency on asingle vendor’s I/O strategy.

24

GPIB Instruments

GPIB

FireWire

GPIB

VXI Embedded Computer

External Computer/Interface

C O M P U T E R / I N T E R F A C E I N S T R U M E N T S I N T E R C O N N E C T & W I R I N G

A VXIbus system can be thoughtof as a collection of tasks to beperformed. Most of these taskswill be specific to your application,while some of them must be donein every VXIbus system.

Yesterday, a discussion about the“typical” VXIbus system woulddiscuss Resource Manager, Slot 0Functions, GPIB, and CommandModules. Perhaps you couldconfigure one or more CPUsinto the system to store and processinformation. Even though ResourceManager and Slot 0 Functions areunique to VXIbus systems, todaywe look at test systems differently.Never before has the systemintegrator been able to benefitfrom the large selection of opentest system components! Nowavailable from Agilent Technologiesand a growing list of instrumentmanufacturers is standard,off-

the-shelf test system hardwareand software: PCs or UNIXcomputers (standalone orVXI format); IEEE-488, IEEE-1394, and MXIbusinterfaces; and VXIbusinstrumentation. VXI is acompletely open environmentfor both hardware and instrumentcontrol software. You specifythe VXIplug&play frameworkbased on the operating system,and then select the rest of yoursystem components based onthat framework. If you alreadyhave a test system set-up, you cancontinue with your GPIB solutionand begin moving to higher-performance VXI solutionsas your needs grow.

25

Putting It All Together

By now, you can see that VXIbusand VXIplug&play standardsprovide a well-defined hardwareand software environment, inwhich modules from many differentmanufacturers can work together.How is Agilent Technologiesinvolved in this process?

In the past, Agilent participatedin the standardization processby providing technology anddevelopments of our own toeveryone in the industry- GPIB(lEEE-488) and SCPI. Thus,Agilent was one of the firstinstrument manufacturers torealize the need for industrystandards. Today, Agilent’scommitment to these and tonew standards continues togrow with the needs of theTest and Measurement industry.

Agilent VEE, Agilent’s VisualEngineering Environment, isVXIplug&play-compliant forAgilent instruments, as well asfor non-Agilent products. How?Agilent VEE can access and loadany instrument driver written tothe VXIplug&play standard. Theinstrument driver then providesa procedural interface to theinstrument for programmaticcontrol. Agilent VEE uses agraphical user interface to makeit easy for you to develop thecode necessary for controllingyour instruments.

Agilent also provides soft frontpanels and robust help files withall instrument drivers to help youunderstand and operate yourinstruments quickly. Since mostcomplex test systems are notcompletely populated with VXI,Agilent is a leader in providing acommon driver strategy for bothVXI and non-VXI (GPIB)instruments.

Even though VXIbus andVXIplug&play define boththe hardware and softwareenvironments, these standardsstill do not address everythingyou’ll need for a total systemsolution. Agilent offers aninstrument family that has aconsistent “look and feel,” providesmany cost and performancealternatives, is easy to integrate,and allows you to get your testsystem operating quickly.

26

What has Agilent Technologies Done to Improve Upon VXI Technology?

During its first decade, VXIbushas spawned an unexpectedlevel of industry cooperation inachieving a truly open modularinstrumentation architecture. It’seasy to understand how industrystandards deliver benefits. Oneonly has to observe the success ofthe PC, as thousands of computerusers switched to this open platformso broadly supported by thecomputer industry. The VXIplatform now brings the samebenefits to instrument users:

• Today, VXI is a stable, highlystandardized platform withhundreds of products available,making your investment secure.As you combine hardware andsoftware from a variety ofvendors, you’re assured thatyour system will work.

• VXI provides scalable, configurablesolutions that adapt to yourunique measurement situationallowing you to trade-off costand performance. And VXI iseasily integrated into existingor new rack and stack systemsfor complete solutions that canreduce your cost of test.

• VXI has spawned other openstandards, that help you savedevelopment time, and protectyour system investments -forinstance, SCPI and VXIplug&play.

• VXI has influenced thedevelopment of test softwareprogramming languages, suchas Agilent VEE, that complementand support the VXI environmentand allow an unprecedentedlevel of test software develop-ment productivity, saving timeand money.

With VXI technology rapidlyemerging as the industrystandard in all facets ofinstrumentation, you can expectmanufacturers to continue toprovide even greater productivitygains, as well as to reduce testsystem costs. Agilent Technologieswill continue to be the leadingmanufacturer in VXI technologyand products, and will continueto drive industry standards thattruly benefit instrumentationusers.

27

Summary

By internet, phone, or fax, get assistancewith all your test & measurement needsOnline assistance:www.agilent.com/find/assist

Phone or FaxUnited States:(tel) 1 800 452 4844Canada:(tel) 1 877 894 4414(fax) (905) 282 6495Europe:(tel) (31 20) 547 2323(fax) (31 20) 547 2390Japan:(tel) (81) 426 56 7832(fax) (81) 426 56 7840Latin America:(tel) (305) 269 7500(fax) (305) 269 7599Australia:(tel) 1 800 629 485 (fax) (61 3) 9210 5947New Zealand:(tel) 0 800 738 378 (fax) 64 4 495 8950Asia Pacific:(tel) (852) 3197 7777(fax) (852) 2506 9284

Product specifications and descriptions in thisdocument subject to change without notice.Copyright © 2001 Agilent TechnologiesPrinted in the USA April 16, 20015988-2372EN

Agilent Technologies’ Test and MeasurementSupport, Services, and AssistanceAgilent Technologies aims to maximize thevalue you receive, while minimizing your riskand problems. We strive to ensure that youget the test and measurement capabilities youpaid for and obtain the support you need. Ourextensive support resources and services canhelp you choose the right Agilent products foryour applications and apply them successfully.Every instrument and system we sell has aglobal warranty. Support is available for atleast five years beyond the production life ofthe product. Two concepts underlie Agilent’soverall support policy: “Our Promise” and“Your Advantage.”Our PromiseOur Promise means your Agilent test andmeasurement equipment will meet itsadvertised performance and functionality.When you are choosing new equipment, wewill help you with product information, includingrealistic performance specifications andpractical recommendations from experiencedtest engineers. When you use Agilentequipment, we can verify that it works properly,help with product operation, and provide basicmeasurement assistance for the use ofspecified capabilities, at no extra cost uponrequest. Many self-help tools are available.Your AdvantageYour Advantage means that Agilent offersa wide range of additional expert test andmeasurement services, which you canpurchase according to your unique technicaland business needs. Solve problems efficientlyand gain a competitive edge by contractingwith us for calibration, extra-cost upgrades,out-of-warranty repairs, and on-site educationand training, as well as design, systemintegration, project management, and otherprofessional engineering services. ExperiencedAgilent engineers and technicians worldwidecan help you maximize your productivity,optimize the return on investment of yourAgilent instruments and systems, and obtaindependable measurement accuracy for the lifeof those products.