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Introduction
to VXI
Abstract
VXI
is an industry standard (IEEE-STD-1155) defining the
specifications for compatible computer-controlled,
modular instrumentation. It describes the hardware
and software requirements, including power, cooling,
communication protocols, triggers, clocks, and memory
moves. A related standard, the VXIplug&play
specifications, takes these requirements further and
defines the software I/O layer between the hardware
and application software as well as requiring user
tools such as executable soft front panels, instrument
drivers in source code and standardized descriptions
of the instrument in a Knowledge Base.
VXIbus SPECIFICATION
VXIbus
stands for VMEbus Extensions for Instrumentation.
This standard defines a specification for instruments
on a card. It includes the requirements for building
the modules as well as the mainframes in which the
modules work. This covers the electrical parameters,
communication protocols and all mechanical characteristics.
Figure
1. VXIbus Modules and a Mainframe
The
original members of the VXIbus Consortium decided
to build upon existing technology for the new specification.
They chose the widely used VME computer bus (IEEE-STD-1014)
as the basis for the new specification. It provided
a well understood and documented high speed backplane
technology. This capability was a critical factor
in the development of VXI, permitting it to be the
next generation bus for high performance instrumentation.
The VXIbus Consortium extended the VMEbus specification
in a number of ways to optimize it for test and measurement
instrumentation. These extensions include two larger
module sizes and corresponding mainframes, cooling
specifications, EMC specifications and additional
clocks, interrupts and triggers.
VXI Brings Advantages to
Lab Automation
VXI
brings a number of advantages over rack and stack
test equipment to automating a lab. These include
standardization of multi vendor solutions, small size
and computer control. These advantages result in ease
of integration and use as well as repeatability of
measurements.
Common Backplane Resources
VXI
enhances systems use and integration because of the
availability of multiple resources on the backplane.
These include power, clocks, triggers and interrupts
that are used in the same way by all vendors. These
capabilities are ideal when automating test functions
as they are easily accessed without external cabling
and are integrated into the control software provided
by VXI. For example, the backplane contains eight
TTLTRG lines accessible by all modules. This is a
very convenient way to trigger a scan sequence between
a switch card and a DMM. All of the trigger lines
and the commands to implement the sequence are located
on the module or in the chassis, simplifying what
could be a messy setup in a rack-and-stack system.
Figure
2. VXI Backplane Structure
Another
example of a common backplane signal which is frequently
used is the 10MHz clock. Instruments such as a counter,
signal generator and spectrum analyzer rely on a common
clock. The VXI backplane provides a 10MHz common clock,
called CLK10, which is sourced from the Slot 0. This
clock is an ECL differential signal with specified
impedance and is available to all slots. While the
VXI specification requires that the accuracy need
only be 100 parts per million, Slot 0 devices can
accept an external reference source, for distribution
onto the backplane.
This feature represents a very large savings in cable
length, a potential problem when distributing signals
to multiple devices. It also solves a number of other
problems such as buffering and impedance. Refer to
Table 1 for examples of some of these problems and
the solutions provided by the VXI backplane CLK10.
The VXIbus uses a standard method for system development
and simplifying the system design task. New users
who figure out how to use the backplane functions
on one module in one setup will be able to use the
knowledge when setting up the next measurement with
different modules.
| Problem |
Rack and Stack Description |
VXI CLK10 Solution |
| Sensitivity |
Signal levels at Reference Inputs/Outputs are
not standard. |
Signal level is differential ECL, 50% duty cycle,
+/- 5% at 50% amplitude point. |
| Impedance |
Input/output impedances of devices are not standard. |
Backplane stripline = 50ê |
| Loading |
Loading not specified or inadequate to drive
multiple instruments from one source. |
Modules must not exceed 2 ECL loads. |
| Buffering |
Insufficient buffering for all instruments using
a common clock. |
All slots individually buffered on backplane.
|
| Skew |
Time between clock signal is undefined and difficult
to control, changes if system is reconfigured. |
<8 nanoseconds between any module on the backplane.
|
| Noise |
Each system or reconfiguration requires new
design and verification. |
Specification requires high level of inter-module
isolation. |
| Connectors |
Physical constraints of coaxial cabling impact
design as well as noise, loading and buffering. |
Signals are available on P2 connector of each
slot.
|
Table
1. Problems CLK10 Solves
Small Size Enhances System
Setup and Use
The
modular, instrument-on-a-card approach of VXI reduces
system size. Users typically have significant space
constraints for test equipment. VXI mitigates this
since there is typically a 50% to 80% reduction in
required space when migrating from rack-and-stack
to VXI.
Following is an example of a 2:1 reduction of an automated
test system with common instruments used in general
purpose as well as RF test and measurement functions.
The original two-bay system was reduced to one bay
by placing two VXI chassis into a single rack.
Figure 2a. shows the original two-bay configuration.
The cabling on the rear illustrates only the connections
for the common 10 MHZ signals from the distribution
unit. The interrupts, triggers and other signals which
are routed or daisy-chained among instruments are
not shown.
Figure 2a. 2-Bay Rack-and-Stack
System
Figure
2b. shows the reduced one-bay configuration. Again,
only the wiring for the 10 MHZ clock is included.
The routing is originally sourced from a rubidium
oscillator built into a 2-slot VXIbus module. It is
then cabled to the Slot 0 controller in the top chassis
and to the rack-and-stack spectrum analyzer. To ensure
that modules in VXI Chassis #2 are also locked to
the same standard, the signal is routed from the mainframe
extender MXI in Chassis #1 to the Slot 0 MXI in Chassis
#2.
Figure 2b. Equivalent 1-Bay
VXI System
Common Computer Choices Mean
Flexible System Development
When
the founding members of the VXI Consortium developed
the standard, they wanted to ensure that there would
be an easy migration path between current software
and hardware and future technology. As the founders
were primarily from the ATE community, they selected
bus standards which would interface with VXI but were
also compatible with existing platforms. The most
common implementations of the specification include
three ways to control and communicate with VXI products:
IEEE-STD-488 (GPIB), embedded computers and MXIbus.
Figure 3 illustrates the three types of controllers.
Figure
3. Three Ways to Control VXI Instruments
IEEE-STD-488
(GPIB) Controllers
Using
GPIB to control VXI devices is identical to using
GPIB to control rack-and-stack instruments. Each chassis
must have a Slot 0 Resource Manager. The Slot 0 sources
a number of specified signals including the CLK10.
The Resource Manager controls the power up sequence
as well as allocating the hierarchy of communications
between any masters and slaves. There are a number
of GPIB-to-VXI Slot 0 modules available which handle
the translation from GPIB protocol to VXI Message-based
and Register-based protocol. They also permit users
to toggle interrupts and triggers and to pass them
to the host GPIB controller.
This
method of control is very simple. Users who are familiar
with GPIB can add VXI instruments to a system in a
seamless manner. The command structure is identical
to GPIB ASCII style commands, so there is little or
no learning curve. The drawback is that as a bus it
is very slow. However, in applications where speed
is not critical, as in making measurements with long
integration times, this is not a problem. It is also
a simple way to start using VXI without purchasing
new controller platforms.
Embedded
Controllers
The
easiest way to use VXI in an automated setting is
with an embedded controller. The choices for embedded
controllers are similar to the types of computers
used in the control of rack-and-stack test equipment.
These specially packaged controllers reside in the
Slot 0 position of a chassis. Most require two slots,
although single slot versions are available. They
are fully functional computers with all the standard
features of hard and floppy drives, VGA port, COM
and Serial ports and usually a GPIB port. They may
also provide capability for expansion cards to allow
communication or control over a LAN via Ethernet or
other communication media.
There are several advantages to embedded controllers.
The primary advantage is their speed. Embedded controllers
permit simplified use of the Shared Memory function
of VXI, effectively a DMA capability. Because the
control is so tightly coupled to the VXI backplane,
it is very easy to move large amounts of data between
controller and module for processing. This enables
the user to retrieve data or results in a meaningful
format.
Another advantage to embedded controllers is their
small size. By configuring a chassis with an embedded
computer, the user effectively has a self-contained
system in a mainframe. While this setup does not qualify
as a truly portable system, it is certainly possible
to roll this type of a system up to a device and perform
complete testing and verification with immediate data
logging and certification.
MXIbus Control
MXIbus
stands for Multisystem Extension Interface. This method
provides almost the same communication speed as an
embedded-control solution. User software is identical
to that used in embedded controllers, allowing the
tight coupling between the controller and VXI backplane.
MXI solutions are available for DOS/Windows and Unix
computers, (PCI, ISA, NuBus and SBus) as well as several
HPUX versions.
VXIplug&play
The
VXIplug&play Systems Alliance developed a mission
statement which called for the vendors to put themselves
in the shoes of their customers and work together
to develop user tools and common low-level I/O software.
The group has grown to more than 60 members and has
created and published a number of specifications outlining
how to design and build these tools. Convergence has
also been reached on the low level I/O software between
VXI hardware and application software. This section
will describe some of the user tools being provided
as part of VXIplug&play compatible instruments
to make VXI equipment very easy to use.
VXIplug&play Frameworks
To
ensure compatibility of user tools on various software
platforms, the Alliance developed the concept of Frameworks.
A Framework defines a set of requirements that ensures
component compatibility. These components include
computer hardware and software, I/O communications
software, instrument Soft Front Panels, and instrument
drivers. The two Framework specifications which are
complete at this time are the WIN and the GWIN, based
on Microsoft Windows 3.1 applications. These are primarily
based on LabWindows/CVI and LabVIEW application software.
VISA--Virtual Instrument Software
Architecture
All
VXI equipment runs on a low level I/O communications
layer of software. Prior to VXIplug&play, each
controller manufacturer provided their own unique
I/O layer. The Alliance standardized this by developing
VISA. To the average user, this layer is transparent.
It allows application programs from various suppliers,
such as National Instruments, Hewlett-Packard, Radisys,
ICS, GenRad and Teradyne, to run on hardware from
other vendors.
Installation Disks
To
be fully VXIplug&play compliant, all products
must be shipped with an Installation Disk. This disk,
or set of disks, must contain an executable soft front
panel, an instrument driver written in an acceptable
language or program for the Framework and a Knowledge
Base. These components are at the heart of making
VXIplug&play compatible instruments easy to
use.
Soft
Front Panel
The
soft front panel allows a user with any compatible
computer containing VISA to load the front panel and
start operating the instrument. The Alliance has a
number of documents which describe what should be
included in a front panel and some basic guidelines
on how it should be laid out.
These
requirements include such capabilities as an initialization
panel, a limitation on the number of functions which
can be crammed into one panel and some logical, intuitive
groupings.
This
promotes a common look and feel between instruments,
independent of manufacturer or supplier of the software.
Refer to the example of a VXI chassis monitoring system's
front panel for a sense of these attributes in Figure
4.
Figure
4. Chassis Monitoring System Front Panel
The
Instrument Driver
The soft front panel is typically derived
from an instrument driver written in a program language
which lends itself to some graphical interfaces. Common
examples are LabWindows/CVI and LabVIEW from National
Instruments and Hewlett-Packard's Visual Engineering
Environment (HP VEE). The manufacturer develops the
driver according to the specifications of the Alliance
and then uses an Application Builder program to turn
it into an executable program for the user.
As the basic requirements are outlined in the VXIplug&play
specifications, there is a common look and feel to
all VXIplug&play drivers. They also must feature
an Initialize panel as well as other specified panels
for common functions such as triggering and interrupts.
Refer to Figure 5 above for an example of these panels
for an arbitrary waveform generator.
Figure
5. Instrument Driver Panel for VXIplug&play
The
Knowledge Base
To
ensure that all users are able to easily determine
the minimum required capabilities to run the executable
front panel, instrument driver and the instrument
itself, the Alliance requires that each manufacturer
include an ASCII file on the Installation Disk. This
Knowledge Base File contains all of the pertinent
information for operating the instrument as well as
its VXIplug&play components. This includes
specifications for computer RAM and hard disk space
for their driver and VXI cooling and power requirements
for the hardware. It is easy to read and the format
is prescribed in a VXIplug&play document. Again,
there is the commonality of look and feel across modules
and manufacturers once users start using VXIplug&play
compatible products. Refer to Figure 6 for a sample
of a part of a Knowledge Base File.
################################################################################
# This file defines the components delivered with the
Racal Instruments 3151
# Arbitrary Waveform Generator
################################################################################
[File Name]="RI3151.KB";
[File Revision]=1.1;
[VPP-5 Specification Revision] = 3.0;
################################################################################
# Define the 3151 software information
################################################################################
[New Record]
[Device Category]=5;
[Device Class]= 5;
[Disk]=-1000,-5;
[Framework]="WIN";
[Manufacturer]="Racal Instruments, Inc.";
[Product Name]="RI3151";
[Product Description]="Instrument Driver and SFP for the
Racal Instruments 3151 Arbitrary Waveform Generator";
[VISA I/O]=3.0;
################################################################################
# Define the 3151 hardware information
################################################################################
Figure
6. Part of a Knowledge Base File for a VXI Arbitrary
Waveform Generator
Summary
The
VXI Consortium developed a robust standard for modular
instruments-on-a-card. Inter-operability among products
from different vendors has been tested and proven
reliable. Over the years, a large number of products
with a wide degree of functionality have been produced.
Their small size, high-performance, integrated signals
on the backplane, and common computer control have
made them ideal instruments.
The VXIplug&play Systems Alliance has led the
way in developing a number of user tools which ease
start-up and make VXI products as easy, if not easier,
to use than traditional rack-and-stack equipment.
These include defining common software and hardware
platforms, developing common I/O communications software
and requiring that all VXIplug&play compliant
instruments ship with an Installation Disk which contains
an easy to use soft front panel, an instrument driver
and an easy-to-read ASCII file in a common format
describing the instrument.
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