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Feeling Comfortable
with VXIbus
If you've been around electronics think about electronic test, there's
very long, you know that "rack still some confusion about how
and stack" instruments have been to apply the VXIbus standard,
a mainstay in the electronics how complex it is, and how it fits
industry for years. Much of their in with existing rack and stack
success is due to a widely used instruments. With the success
interface, IEEE-488. Developed of the VXIbus standard, other
by Agilent Technologies in the standards -like VXIplug&play
mid-1970's, this interface allows and SCPI - are emerging to
you easily to connect your improve the usefulness of VXIbus
instruments using a remote technology in electronic test.
computer. In the late 1980's,
In this booklet, we'll provide you
Agilent Technologies
with a basic understanding of
offered a standard
VXIbus, SCPI, and VXIplug&play-
instrument language
and explain some of the advantages
that was quickly adopted
of these standards. We will not
by the Test and Measurement
tell you that they are the answer
industry as SCPI -Standard
to every problem, but will show
Commands for Programmable
you how to integrate these modular
Instrumentation - to eliminate
products into your current test
the multitude of proprietary
system. We'll also help you
instrument programming languages
understand the tradeoffs in
available from instrument vendors.
selecting various VXI devices.
During this time, Agilent and
Please understand this is not
other instrument manufacturers
a manual for any specific VXI
produced a growing number
instrument, but rather an
of proprietary GPIB modular
introduction to overall VXI
instrument products. These
technology.
instruments could be integrated
into test systems to provide We're pleased to be a leader in
switching, measurements VXIbus, SCPI, and VXIplug&play
and signal source capabilities. technologies. We think you'll see
However few of these modular the advantages of these standards
products were compatible. The in your test system environment.
VXIbus standard addressed this
problem of incompatibility. With that in mind,
Because it changes the way we let's take a closer look.
Feeling Comfortable with VXIbus
CONTENTS
VXIbus: The Test and Measurement Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
The History of VXIbus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Goals of the VXIbus Consortium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
GPIB &VMEbus, The Foundation for VXI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
What is GPIB? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
What is VMEbus? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
VXIbus: The Best of Both Worlds, and More! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Taking the Best from GPIB & VME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
More Features of VXIbus Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
SCPI: The Standardized Programming Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
VXIplug&play: The New Standard for Test and Measurement . . . . . . . . . . . . . . . . . . . . . .8
What is VXIplug&play? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
What does VXIplug&play Offer? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
What Do VXIbus, SCPI, and VXIplug&play Mean to You? . . . . . . . . . . . . . . . . . . . . . . . . . .9
Open Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Higher Test System Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
True Upgrade Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Easy Integration with Rack & Stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Smaller Test Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Access to Switching Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Long-Term Software Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Multi-Vendor Interoperability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
The VXIbus Standard: More Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Instrumentation Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Module Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Connectors and Buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Power, Cooling, and Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
VXIbus Devices and Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
What are Devices? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Register-Based Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Message-Based Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
More on Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
GPIB & LAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
External Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Embedded vs. External Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
SCPI: More Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
VXIplug&play: More Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
How does VXIplug&play Work? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Instrument Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
VISA I/O Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
Putting It All Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
What has Agilent Technologies Done to Improve VXI Technology? . . . . . . . . . . . . . . . .26
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
VXIbus: The Test and Measurement Standard
The History of VXIbus
During the late 1970's and 1980's,
numerous electronic instrument
companies were producing their
own proprietary cardcage
systems. The manufacturers
recognized the advantages of
cardcage and IAC (Instrument-
On-A-Card) systems, but they
were using vastly different
approaches. At the same time,
the US Air Force created a
Instrument-On-A-Card Systems
program to design a single IAC
standard that would result in
substantially smaller electronic
equipment.
In April 1987, five companies -
Agilent Technologies, Tektronix,
Colorado Data Systems, Racal-
Dana, and Wavetek -started
discussions aimed at creating an Goals of the VXIbus Consortium
IAC standard that would benefit The members of the VXIbus
both the commercial and military Consortium had a lot of
test communities. The companies experience with IAC systems.
formed the "VXIbus Consortium" They understood their benefits -
and met for three months of increased test throughput, smaller
intense technical discussions. instruments, reduced cost, etc.
An initial draft, the VXIbus They wanted to create a technically
System Specification, was sound standard that would bring
released on July 14, 1987. IAC systems into the "next
Like most new things, the generation." That would free up
specification has undergone design engineers to do what they
several changes. Revision 1.3 was do best- bring new technologies
released in 1989. The specification to market. The result would be
was submitted to the IEEE and products of greater innovation,
was adopted as the IEEE-1155 quality and diversity.
Standard in 1992. In addition, the
U.S. Air Force has incorporated
the VXIbus specification into its
MATE (Modular Automated Test
Equipment) program. The VXIbus
has continued to evolve, with
Revision 2.0 released in 1998.
3
GPIB and VMEbus:
The Foundation for VXIbus
The Consortium members
recognized Agilent Technologies'
GPIB (IEEE-488) and VMEbus as
the two most popular standards
for instrumentation. They decided
to take the best from these
standards and add more features
to create the best possible IAC
standard. Figure 1. A GPIB System: Ease of Use, Ease of Integration
What is GPIB? and may also perform sophisticated What is VMEbus?
data capture and analysis. An
In the early 1970's, Agilent While GPIB is the most popular
example is a digital multimeter.
Technologies invented an 8-bit electronic instrument interface,
Using GPIB, you can send the
parallel interface- GPIB. It allowed VMEbus is widely used in micro-
multimeter a sequence of ASCII
rack and stack instruments to computer systems. The VMEbus
strings that instruct it to take a
communicate with each other specification was released in
burst of 1000 readings. You may
and with a host computer. In August 1982, and approved by
also send it commands telling it
1975, GPIB was adopted by the IEEE and ANSI in 1987.
to calculate statistical functions
IEEE as standard IEEE-488.
like minimum, maximum, and You can think of VMEbus as an
Today, it is the leading interface
standard deviation on the readings. interface with two components-
used in automated test systems
Then you can bring only those mechanical and logical. The
built on individual instruments.
resulting values back to the mechanical portion specifies the
It is simple, flexible, and used
computer. physical dimensions of plug-in
by nearly all instrument
boards, backplanes, subracks, etc.
manufacturers. One limitation of GPIB has been
(The form factor of the plug-in
a maximum data transfer rate of
GPIB is a widespread standard- boards is commonly known as
about 1 Mbyte per second. This is
it allows you to connect instruments the Eurocard format.) The logical
usually not a problem, since most
from several manufacturers to a portion of the interface describes
applications are limited by the
host computer (controller), and how functional modules (in this
speed of the measurement circuits,
thus build an automated, integrated case, plug-in cards) communicate
or by the switch closure and
test system. Normally, you don't with each other. A major objective
settling time required to route
have to worry about how information of VMEbus is to allow communication
signals. However, it can become a
is passed between the devices. between two devices without
problem with high-speed digitizing,
Your only concern is the content disturbing the internal activities
digital inputs/outputs, or if
of the information, whether it be of other devices in the system.
large amounts of data must be
ASCII or binary instructions to VMEbus systems can have
transferred from an instrument
an instrument, or ASCII or binary multiple microprocessors
to the computer for specialized
results from an instrument. on the same backplane.
processing. Furthermore, the
A good way to think of GPIB GPIB protocol limits the transfer
instruments is as electronic devices speed to that of the slowest
that operate by themselves. They device on the bus.
have communication intelligence,
4
VXIbus: The Best of Both Worlds, and More!
One strength of VMEbus is that it
allows high-speed communication
between devices (which we
use interchangeably here with
"modules"). The specification
was originally intended for
microcomputer systems. As
instrument speeds increased
and printed circuit board sizes
decreased, interest grew in
bringing electronic instruments
into the system. However, this
brought out two shortcomings Figure 2. A VMEbus System: Potential for High Speed And Multiple Processors
of VMEbus: the electrical
environment, designed for digital Taking the Best from GPIB & VMEbus. however, Message-Based Register-
communication, is too "noisy" Based Devices can be limited
for precise analog measurements, The VMEbus specification,
when it comes to high-speed data
and the programming needed for originally designed for micro-
transfer.
high-speed communication has to computers, has a great potential
be done with low-level register for high-speed device-to-device The outstanding feature of
reads and writes. communication. This can increase VMEbus devices is that they can
the throughput of your test system move data between themselves
VXIbus: considerably. And GPIB is well very fast. The VXIbus specification
The Best of Both Worlds, known for its ease of integration, defines "Register-Based Devices"
and More! which helps you to build your test as the analog to VMEbus devices.
system faster. So the two main These devices communicate at
Members of the VXIbus Consortium a lower, more basic level than
challenges- speed and integration-
realized that for the VXIbus Message-Based Devices and
were answered. Although the
standard to be successful, it must so can attain greater transfer
GPIB and VMEbus standards have
answer two major challenges in speeds. Programming a Register-
different bus communication styles,
instrumentation: communication Based Device involves writing to
VXIbus defines two different
speed and integration. The GPIB and reading from individual
devices to take advantage of
and VMEbus specifications held registers on the device.
these styles.
the answers to both these problems.
A third challenge solved by the Remember that GPIB instruments
Consortium was to devise a well- are easy to use. You simply connect Easy Fast
defined environment in which the cable and program the instru-
different vendors' products can ments in whatever language they GPIB VME
operate together properly. require. In VXIbus systems, the
counterpart to GPIB instruments
The result is the VXIbus (VMEbus
are "Message-Based Devices." Message-Based Register-Based
Extensions for Instrumentation ) Devices (ASCII) Devices (Binary)
They are easy to integrate into a
system and communicate at a
high level using ASCII characters. bus
Like GPIB instruments, Message-
Based Devices can contain
significant intelligence and
Figure 3. VXIbus: The Best of Two Worlds
data processing capabilities.
Like instruments on a GPIB bus,
5
More Features of VXIbus Systems.
The VXIbus Consortium fully