- a tutorial covering VXI instrumentation technology, used for test and
data acquisition applications, defined under IEEE standard IEEE1155.
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.
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 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 mechanical configuration
The specification also includes the mechanical dimensions. Three card
heights are allowed 3U, 6U, or 9U. On "U" is 43.6 mm. Lengths may either be
160 mm or 340 mm. This makes the A sized card 3U x 160 mm, B sized card 6U x
160 mm, C sized card 6U x 340 mm and finally the largest is the D sized card
at 9U x 340 mm. A single card width is then 30.5 mm. Of these sizes the C
sized card is probably the most widely used as it is sufficiently large for
most instruments.
The mainframes come in various sizes. The full size
version a standard 19 inch rack that can house up to 13 VXI C or D sized
cards. Smaller mainframes are also available that can accommodate C sized
modules. Module positions or slots are designated 0 to 12, slot 0 being on
the left.
The controller card, required in each solution, fits in
slot 0. As a result the backplane for this slot is slightly modified to
support all the requirements for the controller card. The controller is
responsible for performing all the resource management at start up and for
controlling the system in operation. Accordingly each rack must have a
controller card, in the slot itself, or a link to a controller capable of
controlling the system.
VXI connectors
VXI cards can have up to three module connectors. The actual number depends
upon the size of the card. They are given the designations P1, P2, and P3. A
sized cards only possess the P1 connector, whereas B and C sized cards have
P1 and P2, and the D sized cards posses all the connectors.
The functions contained on the connectors include the
following:
- P1 VME computer bus - 16 bit data transfer, 16 Megabyte
addressing, multimaster arbitration, priority interrupt.
- P2 VME 32 bit data, 4 Gigabyte addressing, 10 MHz clock,
TTL and ECL bus, 12 pin local bus, analogue sum bus, module
identification bus, power distribution bus.
- P3 100 MHz clock, star trigger bus, ECL trigger bus, 24 pin
local bus, power distribution bus.
Overview
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.
