VXI and its VXIbus is a powerful test instrumentation technology based around a rack system based on the 19 inch rack based VME bus: test instruments were constructed on single cards or modules that could slot into the VXIbus.
The VXI test system is used for many automated test systems contained within a rack, and it finds many uses for testing for production, field repair and maintenance, often being found in avionics and military applications.
The VXI test system was possibly the first rack based ATE standard to gain major acceptance in the test industry, although its relatively high cost meant that it only gained acceptance in a real like military and defence where these costs could be tolerated.
At the time, the VXI test system was seen as a successor to GPIB, saving space and cost as well as making the test systems more robust and much smaller.
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.
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.
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