VXI Technology Tutorial

- summary or tutorial giving information about the basics of VXI technology and the VXI bus, VXIbus, used as a standard for test equipment in the production, repair, and maintenance industries.

VXI or VXIbus is a powerful test instrumentation technology that is based on the VME bus. Indeed the letters VXI stand for VME eXtension for Instrumentation. Its development was born out of the need to be able to produce smaller yet effective solutions for test systems. At the time of its development GPIB controlled equipment was the main way in which automatic test equipment systems were built up to provide functional test solutions. While these were effective they were relatively slow and occupied considerable amounts of space because each item of test equipment was contained in its own case.

VXI technology overcame these problems by developing a system based on the VME bus. The test solution would be contained in a single 19 inch rack, with individual test instruments being contained on a card occupying one of sometimes more slots. In this way a complete test solution containing several instruments could be contained in a much smaller space, making the solution far more attractive for many field and factory applications.

VXI technology development

The VXI standard was developed by the VXI Consortium. This was formed in 1987 with the aim of defining a multi-manufacturer standard that would enable test solutions to be more compact by having instruments contained on cards. The VXI Consortium originally consisted of GenRad, Hewlett Packard, National Instruments, Racal Instruments, and Tektronix, but as the importance of VXI increased, so did the number of members of the Consortium.

A typical module used for incorporation within a VXI chassis

The VXI Consortium initially developed a system that gave hardware interoperability and this was adopted by the IEEE in 1993 under their standard IEEE 1155.

While VXI in its early form was able to fulfil many of its roles, it did not provide software interoperability. As a result, in September 1993, another consortium called the VXIplugandplay Alliance was founded. This focussed its activities towards enabling VXI instrumentation to become plug and play. Accordingly the VXI Systems Alliance provided the framework around which a much higher level of system standardisation was developed. This was primarily achieved through addressing the software elements.

With full system standardisation, VXI is able to offer vendor independent solutions. This has considerably enhanced the easy of use and the number of applications that use VXI. As a result there are more than 250 vendors and 1500 products that have been developed for VXI technology.

VXI technology advantages

Each test technology has its own advantages. The need for VXI technology arose out of the need for a more compact and integration format for test solutions, but in addition to this there are a number of other advantages that VXI brings to any solution:

  • Compact test solution by the fact that all test instruments can be contained on a card within a multislot rack.
  • VXI is an open standard and allows the use of instruments from a variety of manufacturers.
  • High speed operation resulting from the use of a single high speed rack and backplane.
  • The ability for accurate timing and synchronisation as a result of the high speed communication possible between instruments and the triggering facilities available.
  • Ease of configuration resulting from the plug-and-play software.

VXI system configuration

The basic building block of a VXI system is the mainframe or chassis. This contains 13 slots into which various modules (instruments) can be added. The mainframe also contains all the power supply requirements for the rack and the instruments it contains.

VXI employs the standard VME bus structure, although further functions are allocated to the uncommitted backplane pins. This allows for functions that are required by test instrumentation to be implemented quickly and easily without the need for external connections. Functions that fall into this category include chassis wide clocks, timing and additional control.

The VXI standard also imposes a variety of requirements on the module. These are both electrical and mechanical.

There are many electrical requirements. These range obviously from the way in which the module communicates across the bus to elements such as the fact that each module is required to perform a self test on power up. A bit in the status register in the module contains information indicating whether the module has passed or not. Another register includes a number of identifications including the Manufacturer ID, and the Module Type ID assigned by the manufacturer to indicate the type of module. There is also a Module Serial Number and Module Hardware / Firmware revision number.


VXI technology has become a very successful test equipment standard. There is a wide variety of test equipment cards that are available, making the standard capable of handling most test requirements. Accordingly it is widely used, especially in situations where space must be reduced on that required by a GPIB rack and stack system or where fast control and communication is required.

By Ian Poole

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