Feeling Comfortable
with VXIbus
If you’ve been around electronics
very long, you know that “rack
and stack” instruments have been
a mainstay in the electronics
industry for years. Much of their
success is due to a widely used
interface, IEEE-488. Developed
by Agilent Technologies in the
mid-1970’s, this interface allows
you easily to connect your
instruments using a remote
computer. In the late 1980’s,
Agilent Technologies
offered a standard
instrument language
that was quickly adopted
by the Test and Measurement
industry as SCPI -Standard
Commands for Programmable
Instrumentation - to eliminate
the multitude of proprietary
instrument programming languages
available from instrument vendors.
During this time, Agilent and
other instrument manufacturers
produced a growing number
of proprietary GPIB modular
instrument products. These
instruments could be integrated
into test systems to provide
switching, measurements
and signal source capabilities.
However few of these modular
products were compatible. The
VXIbus standard addressed this
problem of incompatibility.
Because it changes the way we
think about electronic test, there’s
still some confusion about how
to apply the VXIbus standard,
how complex it is, and how it fits
in with existing rack and stack
instruments. With the success
of the VXIbus standard, other
standards -like VXIplug&play
and SCPI - are emerging to
improve the usefulness of VXIbus
technology in electronic test.
In this booklet, we’ll provide you
with a basic understanding of
VXIbus, SCPI, and VXIplug&playand explain some of the advantages
of these standards. We will not
tell you that they are the answer
to every problem, but will show
you how to integrate these modular
products into your current test
system. We’ll also help you
understand the tradeoffs in
selecting various VXI devices.
Please understand this is not
a manual for any specific VXI
instrument, but rather an
introduction to overall VXI
technology.
We’re pleased to be a leader in
VXIbus, SCPI, and VXIplug&play
technologies. We think you’ll see
the advantages of these standards
in 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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
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 (InstrumentOn-A-Card) systems, but they
were using vastly different
approaches. At the same time,
the US Air Force created a
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, RacalDana, and Wavetek -started
discussions aimed at creating an
IAC standard that would benefit
both the commercial and military
test communities. The companies
formed the “VXIbus Consortium”
and met for three months of
intense technical discussions.
An initial draft, the VXIbus
System Specification, was
released on July 14, 1987.
Like most new things, the
specification has undergone
several changes. Revision 1.3 was
released in 1989. The specification
was submitted to the IEEE and
was adopted as the IEEE-1155
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.
Goals of the VXIbus Consortium
The members of the VXIbus
Consortium had a lot of
experience with IAC systems.
They understood their benefits increased test throughput, smaller
instruments, reduced cost, etc.
They wanted to create a technically
sound standard that would bring
IAC systems into the “next
generation.” That would free up
design engineers to do what they
do best- bring new technologies
to market. The result would be
products 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 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.
What is GPIB?
In the early 1970’s, Agilent
Technologies invented an 8-bit
parallel interface- GPIB. It allowed
rack and stack instruments to
communicate with each other
and with a host computer. In
1975, GPIB was adopted by the
IEEE as standard IEEE-488.
Today, it is the leading interface
used in automated test systems
built on individual instruments.
It is simple, f lexible, and used
by nearly all instrument
manufacturers.
GPIB is a widespread standardit allows you to connect instruments
from several manufacturers to a
host computer (controller), and
thus build an automated, integrated
test system. Normally, you don’t
have to worry about how information
is passed between the devices.
Your only concern is the content
of the information, whether it be
ASCII or binary instructions to
an instrument, or ASCII or binary
results from an instrument.
A good way to think of GPIB
instruments is as electronic devices
that operate by themselves. They
have communication intelligence,
and may also perform sophisticated
data capture and analysis. An
example is a digital multimeter.
Using GPIB, you can send the
multimeter a sequence of ASCII
strings that instruct it to take a
burst of 1000 readings. You may
also send it commands telling it
to calculate statistical functions
like minimum, maximum, and
standard deviation on the readings.
Then you can bring only those
resulting values back to the
computer.
One limitation of GPIB has been
a maximum data transfer rate of
about 1 Mbyte per second. This is
usually not a problem, since most
applications are limited by the
speed of the measurement circuits,
or by the switch closure and
settling time required to route
signals. However, it can become a
problem with high-speed digitizing,
digital inputs/outputs, or if
large amounts of data must be
transferred from an instrument
to the computer for specialized
processing. Furthermore, the
GPIB protocol limits the transfer
speed to that of the slowest
device on the bus.
What is VMEbus?
While GPIB is the most popular
electronic instrument interface,
VMEbus is widely used in microcomputer systems. The VMEbus
specification was released in
August 1982, and approved by
IEEE and ANSI in 1987.
You can think of VMEbus as an
interface with two componentsmechanical and logical. The
mechanical portion specifies the
physical dimensions of plug-in
boards, backplanes, subracks, etc.
(The form factor of the plug-in
boards is commonly known as
the Eurocard format.) The logical
portion of the interface describes
how functional modules (in this
case, plug-in cards) communicate
with each other. A major objective
of VMEbus is to allow communication
between two devices without
disturbing the internal activities
of other devices in the system.
VMEbus systems can have
multiple microprocessors
on the same backplane.
4
Figure 1. A GPIB System: Ease of Use, Ease of Integration
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
of VMEbus: the electrical
environment, designed for digital
communication, is too “noisy”
for precise analog measurements,
and the programming needed for
high-speed communication has to
be done with low-level register
reads and writes.
VXIbus:
The Best of Both Worlds,
and More!
Members of the VXIbus Consortium
realized that for the VXIbus
standard to be successful, it must
answer two major challenges in
instrumentation: communication
speed and integration. The GPIB
and VMEbus specifications held
the answers to both these problems.
A third challenge solved by the
Consortium was to devise a welldefined environment in which
different vendors’ products can
operate together properly.
The result is the VXIbus (VMEbus
Extensions for Instrumentation )
Taking the Best from GPIB & VMEbus.
The VMEbus specification,
originally designed for microcomputers, has a great potential
for high-speed device-to-device
communication. This can increase
the throughput of your test system
considerably. And GPIB is well
known for its ease of integration,
which helps you to build your test
system faster. So the two main
challenges- speed and integrationwere answered. Although the
GPIB and VMEbus standards have
different bus communication styles,
VXIbus defines two different
devices to take advantage of
these styles.
Remember that GPIB instruments
are easy to use. You simply connect
the cable and program the instruments in whatever language they
require. In VXIbus systems, the
counterpart to GPIB instruments
are “Message-Based Devices.”
They are easy to integrate into a
system and communicate at a
high level using ASCII characters.
Like GPIB instruments, MessageBased Devices can contain
significant intelligence and
data processing capabilities.
Like instruments on a GPIB bus,
however, Message-Based RegisterBased Devices can be limited
when it comes to high-speed data
transfer.
The outstanding feature of
VMEbus devices is that they can
move data between themselves
very fast. The VXIbus specification
defines “Register-Based Devices”
as the analog to VMEbus devices.
These devices communicate at
a lower, more basic level than
Message-Based Devices and
so can attain greater transfer
speeds. Programming a RegisterBased Device involves writing to
and reading from individual
registers on the device.
5
VXIbus: The Best of Both Worlds, and More!
VME
GPIB
Register-Based
Devices (Binary)
Message-Based
Devices (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 fully
defined the operating environment
for VXIbus modules. All VXIbus
mainframes must state how much
power and cooling they provide.
And all VXIbus modules must
state how much power and cooling
they require. Also, there are strict
limits on how much conducted
and radiated interference is
allowed between modules.
These parameters allow you
to configure a workable system
easily.
Two special functions must
be performed in every VXIbus
system. The first, Slot 0, takes
care of backplane management.
Slot 0 is a unique physical location
in every VXIbus mainframe. Signals
from this slot must include things
like clock sources, arbitration
for data movement across the
backplane, etc. The module that
goes into this slot must perform
these hardware functions in
addition to its normal functions.
If you’re familiar with VMEbus
systems, you probably recognize
that this is very similar to VME’s
“Slot 1 Device.” The Slot 0 device
relieves you of the burden of
managing data flow across the
backplane.
The second special function in
a VXIbus system is the Resource
Manager. The best way to think
of the Resource Manager is as a
computer program. This program
configures the modules for proper
operation whenever the system is
powered on or reset. This means
that you can build your test system
software from a known starting
point. The Resource Manager is
not involved with the VXIbus
system once normal operation
begins.
6
VMEGPIB
bus
• Instrumentation
Environment
• Slot 0 Functions
• Resource
Manager
Easy
Fast
Figure 4. VXIbus Additional Features
With the rapid growth of computercontrolled instruments, Agilent
Technologies recognized the need
for a common instrumentation
language. Therefore, in the late
1980’s, Agilent Technologies
invented TMSL (Test and
Measurement Systems Language)
and offered to make it an open
standard. TMSL itself was based
on industry standards wherever
possible, including IEEE-488.2
and IEEE-754. In April 1990,
this standard was accepted
by the industry, and renamed
SCPI (Standard Commands for
Programmable Instrumentation).
You now have to learn only this
single instrument programming
language, regardless of whose
digital multimeter (or other
similar instrument) you purchase.
With this standardized programming
language for VXIbus instrumentation in place, you can reduce your
test system programming time!
SCPI is now managed by a
consortium of nine instrument
manufacturers. Today, there
are over a thousand instrument
products using SCPI. For more
about how SCPI works, see the
SCPI: More Details section of
this booklet.
7
SCPI: the Standardized Programming Language
VXIbus was a significant
effort to standardize modular
instrumentation. It provided
an open environment where any
vendor’s modules would plug into
any VXI mainframe. Users could
expect the module to fit the slot
size and be adequately powered
and cooled. However, VXIbus
didn’t address the need to integrate
a system that was truly vendor
independent and easily usable.
Wouldn’t it be nice if there were
a common look and feel, or a
standard soft front panel for
given instrument types from
various vendors? Wouldn’t it be
great if you could develop your
application program on a PC and
execute it on a UNIX platform?
Or, what about instrument
drivers? An industry standard
set of drivers would eliminate
custom driver design issues.
Agilent Technologies is an active
member in the VXIplug&play
System Alliance to address
these challenges.
What is VXlplug&play?
VXIplug&play is a term indicating
conformance to a new set of
system-level standards, produced
by the VXIplug&play Systems
Alliance. Agilent Technologies
joined the VXIplug&play Alliance
in 1994 in support of the Alliance’s
charter: “to improve the effectiveness of VXI-based solutions
by increasing ease-of-use and
improving the interoperability
of multi-vendor VXI systems.” The
goal of the Alliance is to achieve
interoperability of mainframes,
computers, instruments, and
software through open, multivendor standards and practices.
The Alliance consists of vendors
actively involved with end-users
to produce instrumentation
systems that meet this goal.
Additionally, the Alliance is
open to all vendors and users
as a forum for working together
to make VXI technology easier
to use.
Thanks to the work of the Alliance,
VXIplug&play components
integrate easily. The new standards
apply to instrument drivers,
soft front panels, installation
packages, documentation,
technical support, application
development environments, as
well as many other areas for
instrument system integration.
As with the VXIbus standard,
revisions to VXIplug&play will
continue to reduce your dependence
on any single vendor and simplify
your job of system design and
implementation.
What does VXlplug&play offer?
VXIplug&play improves productivity,
portability, and interoperability
for both vendors and end-users.
• Productivity
Use soft front panels to operate
and evaluate instrument
operation within minutes.
• Portability
Communicate with instruments
via any controller/computer
interface supported by the
VISA I/O library.
• Interoperability
Develop application programs
portable across computer
platforms and I/0 interfaces.
Add new programs without
having to rewrite existing ones.
With vendors and end-users
working together, the real needs
of system integration continue
to be analyzed, defined, and
implemented. For more on the
VXIplug&play standard, see the
VXIplug&play: More Details
section of this booklet.
8
VXIplug&play: The NEW Standard for Test and Measurement
The VXIbus, SCPI, and
VXIplug&play standards can
improve your test system’s speed
and flexibility, lower your product
“life cycle” cost, protect your
investment, and provide a choice
of vendors’ products that will
work together in your test system.
Open Standards
VXIbus, SCPI, and VXIplug&play
are truly open standards. So far,
over 200 different manufacturers
have received identification codes
from the VXIbus Consortium, and
hundreds of different instruments
are available. This multi-vendor
environment ensures that your
investment in VXIbus products
will be protected long into the
future. If one manufacturer’s
instrument becomes obsolete, a
replacement should be available
from another. Also, there will be
many “niche” manufacturers
willing to provide specialty
modules- just as in the computer
industry.
Because of the open standards,
instrument manufacturers can
provide VXI products with the
benefits of standardized architecture,
instrument programming language,
and I/0 communication. The
backplane pinouts and communication techniques are already
defined for you. Power and
cooling capabilities are completely
specified. Also, electrical interference limits have been set, so
you know your module will have a
“quiet” environment. The VXIbus
specification defines all of these,
and takes care of the hardware
specifications for any VXI system.
The VXIplug&play System
Alliance completes the standardization process with specifications
for the operating system or
“framework,” instrument drivers,
and I/0 software. VXIbus and
VXIplug&play, working together,
give you all the tools and guidelines
needed to successfully design a
custom VXI-compatible module
for a unique function, build a new
VXI test system, or upgrade your
existing system.
9
What Do VXIbus, SCPI, and VXIplug&play Mean to you?
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