PXI
VXI, SCXI, PXIDHAIRYA GOEL (61)UTKARSH SAURABH (20)NISHANT RATH
(11)VXI(VME eXtensions for Instrumentation)About VXIVXI- VME
eXtensions for InstrumentationVME- Versa Module Europa bus. It is
acomputer busstandard.IEEE adopted the VXI specification in
1993.Used for test, measurement, and data acquisition both in
research and automation sectors.It is being integrated by users
into data acquisition boards.User can control a VXI system by using
a remote-general purpose PC using MXI or GPIB.One of the most
exciting and fast growing platform in instrumentation systems.
FeaturesOpen, multi-vendor standards maximize
flexibility.Smaller size reduces floor space, increases mobility
and gives close proximity.Better timing and synchronization
improves measurement capability.Modular, rugged design improves
reliability.Standardized VXI plug and play software eases system
configuration, programming, and integration.Need For VXINeed for
reduction in size of rack and stack instruments.Tighter timing and
synchronization between multiple instruments.Higher transfer rates
than 1Mb/sec.The VXI specification adds standards necessary to
combine VME bus with GPIB to create a new platform.
Real Life VXI
Mechanical ConfigurationConsists of a mainframe chassis-it has
physical mountings and backplane plug-ins.Uses industry standard
VME bus as base architecture.The connecters are connected exactly
as defined by the VME specifications.The VXI bus specification
includes packaging requirements, electromagnetic compatibility,
power distribution, cooling and airflow for VXI bus mainframes and
plug-in modules.
Hardware RegistersThe upper 16kb of the 64 kb are reserved for
VXI bus devices.Each VXI device has a 8-bit logical address.A
single VXI system can have upto 256 VXI devices.Register
ArchitectureRegister
ApplicationsMeasurements modules ( Digitizers, Digital
Multimeters (DMM), Counters, power meters)Stimulus modules (
Arbitrary Waveform Generators (AWG), D/A converters)Scalable, high
density switches for up to 3000 channelsData Acquisition
(DAQ)Automated TestingAdvantagesCompact test solution since all
test instruments can be contained on a card within a multi-slot
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 synchronization 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.Signal Conditioning eXtension for
Instrumentation (SCXI)Introduction-SCXI stands for Signal
Conditioning eXtension for Instrumentation.It is a signal
conditioning module used to condition the physical signals obtained
from sensors and transducers.Signal conditioning is the general
term for the process of making a signal more useful by improving
its precision, accuracy, or signal to noise ratio.
To get a general idea about the need and working of a SCXI
module, let us take the example of a thermocouple (sensor).
In order to get quality temperature readings, we need to amplify
the thermocouple signal. We should also filter out high
frequencies, because while noise is high-frequency in most
environments, temperature changes almost always happen very
slowly.
SCXI can also handle switching, and has some multiplexers in the
background so that a separate ADC can capture the conditioned
signals. So, to put these two together, a real-world signal from a
transducer goes into SCXI, which might then send the conditioned
signal into a DAQ board in a PXI system. SCXI is a signal
conditioning and switching platform for measurement and automation
systems. Whether you need to measure sensors or raw signals,
generate voltages or currents, monitor digital lines, or route
signals with switching, SCXI data acquisition (DAQ) hardware
provides a single, integrated platform for all of your signal
conditioning and switching needs.
Signal Conditioning-Working of SCXITo understand SCXI, one
should have an understanding of the process of signal
conditioning.Signal conditioning is the process of measuring and
manipulating signals to improve accuracy, isolation, filtering, and
so on. Many stand-alone instruments and DAQ devices have built-in
signal conditioning.Signal conditioning also can be applied
externally, by designing a circuit to condition the signal or by
using devices specifically made for signal conditioning. Signal
conditioning accessories amplify low-level signals and then isolate
and filter them for more accurate measurements. In addition, some
transducers use voltage or current excitation to generate a voltage
output.
SCXI devices and other devices are designed for the purpose of
signal conditioning . Signal conditioning accessories can be used
in a variety of important applications. Common types of signal
conditioning are amplification, isolation, multiplexing, filtering,
and transducer excitation.
Amplification-Amplification is the most common type of signal
conditioning. Amplifying electrical signals improves accuracy in
the resulting digitized signal and reduces the effects of noise.
Signals should be amplified as close to the signal source as
possible.If you amplify the signal at the DAQ device while
digitizing and measuring the signal, noise might have entered the
lead wire, which decreases SNR. However, if you amplify the signal
close to the signal source with an SCXI module, noise has a less
destructive effect on the signal, and the digitized representation
is a better reflection of the original low-level signal.
Isolation-Another common signal conditioning application is
isolating the transducer signals from the computer for safety
purposes as the system being monitored may contain high-voltage
transients that could damage the computer without signal
conditioning. An additional reason for isolation is ensuring that
the readings from the plug-in DAQ device are unaffected by
differences in ground potentials or common-mode voltages. When the
DAQ device input and the signal being acquired are each referenced
to ground, problems occur if there is a potential difference in the
two grounds. Isolation prevents that from happening.
Multiplexing-A common technique for measuring several signals
with a single measuring device is multiplexing.Signal conditioning
hardware for analog signals often provides multiplexing for use
with slowly changing signals like temperature.SCXI modules for
analog signals employ multiplexing so that as many as 3,072 signals
can be measured with one DAQ device. With the AMUX-64T analog
multiplexer, you can measure up to 256 signals with a single
device. This feature is in addition to any built-in multiplexing on
the DAQ device.
Filtering-The purpose of a filter is to remove unwanted signals
from the signal that you are trying to measure. If the signals were
not removed, they would erroneously appear as signals within the
input bandwidth of the device. Devices designed specifically for
AC-class signal measurement the NI 455x, NI 445x, and NI 447x
dynamic signal acquisition (DSA) devices, the NI 61xx
simultaneous-sampling multifunction I/O devices, and the SCXI-1141
module have built-in antialiasing filters.
Transducer Excitation-Signal conditioning systems can generate
excitation, which some transducers require for operation.Strain
gauges and RTDs require external voltage and currents,
respectively, to excite their circuitry into measuring physical
phenomena. This type of excitation is similar to a radio that needs
power to receive and decode audio signals. Signal conditioning
modules for these transducers usually provide these signals.The
SCXI-1121 and SCXI-1122 have onboard excitation sources,
configurable as current or voltage that you can use for strain
gauges, thermistors, or RTDs.
Signal from outside world goes into transducer , then signal is
sent to SCXI, from which it is sent to a PXI DAQ board. The
electrical signals generated by the transducers must be optimized
for the input range of the DAQ board. Signal conditioning
accessories can amplify low-level signals, and then isolate and
filter them for more accurate measurements. In addition, some
transducers require voltage or current excitation to generate a
voltage output.
SCXI System Components
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Architecture of an SCXI System
The SCXI modules consist of 32 multiplexed input channels.Each
input channel has its own low pass filter.The Digital Control
Circuitry consists of the Address Handler and registers that are
necessary for identifying the module, starting calibration
information, setting the gain, and selecting the appropriate
channel.The Analog Circuitry per channel consists of a low pass
filter and amplifier.The channels are multiplexed to a single
output buffer.
The National Instruments SCXI Modules and Accessories
PXI stands for PCI eXtensions for Instrumentation PXIWhat is Pxi
?PXI is a rugged PC-based platform for measurement and automation
systems. PXI combines PCI electrical-bus features with the modular,
Eurocard packaging of Compact PCI and then adds specialized
synchronization buses and key software features. PXI is both a
high-performance and low-cost deployment platform for applications
such as manufacturing test, military and aerospace, machine
monitoring, automotive, and industrial test.Developed in 1997 and
launched in 1998, PXI is an open industry standard governed by the
PXI Systems Alliance (PXISA), a group of more than 70 companies
chartered to promote the PXI standard, ensure interoperability, and
maintain the PXI specification.
PXI ARCHITECTURE
PXI ARCHITECTURE DESCRIPTION 1) LOCAL BUSThe PXI local bus is a
daisy-chained bus that connects each peripheral slot with its
adjacent peripheral slots to the left and right. Thus, the right
local bus of a given peripheral slot connects to the left local bus
of the adjacent slot, and so on.Each local bus is 13 lines wide and
can be used to pass analog signals between modules or to provide a
high-speed sideband digital communication path that does not affect
the PXI bandwidth.
PXI ARCHITECTURE DESCRIPTION (CONTD.)2) SYSTEM REFRENCE CLOCKThe
PXI 10MHz system clock (PXI CLK10) is distributed to all peripheral
modules in a system. This common reference clock can be used for
synchronization of multiple modules in a measurement or control
system3) TRIGGER BUSThe PXI bused trigger lines are eight in number
and are highly flexible that can be used in a variety of ways . For
example, triggers can be used to synchronize the operation of
several different PXI peripheral modules.
PXI ARCHITECTURE DESCRIPTION (CONTD.)4) Star triggerThe PXI star
trigger bus offers ultra-high performance synchronization features
to users of PXI systems. The star trigger bus implements a
dedicated trigger line between the first peripheral slot (adjacent
to the system slot) and the other peripheral slots. The star
trigger can be used to communicate information back to the star
trigger controller, as in the case of reporting a slots status, as
well as responding to information provided by the star trigger
controller.
PXI ARCHITECTURE DESCRIPTION (CONTD.)5) System Expansion with
PCIPCI Bridge TechnologyA PXI system can be built with more than
one bus segment by using standard PCIPCI bridge technology. The
bridge device takes up one PCI load on each of the bus segments
that it links together. Thus, a 33MHz system with two bus segments
offers 13 expansion slots for PXI peripheral modules where two bus
segments +8 slots per segment +1 system controller slot +2 slots
for PCI-PCI Bridge = 13 available expansion slotsDIFFERENT OVERVIEW
OF PXI ARCHITECTUREThere are various overviews for the
architecture:-Hardware ArchitectureMechanical
ArchitectureElectrical ArchitectureSoftware Architecture1) Hardware
architectureThe PXISAhardware specificationstipulates all feature
requirements pertaining to the mechanical, electrical, and software
architectures. The PXI Express specification is an implementation
of the CompactPCI and CompactPCI Express specifications.
2) Mechanical architecture High-performance connectors
Environmental testing Electromagnetic testing
3) Electrical architecture Industry-standard PC buses System
reference clocks Star trigger buses PXI trigger bus
4) Software architectureMicrosoft Windows software frameworks
Software components that define HW configuration and
capabilitiesVirtual Instrument Software Architecture (VISA)
implementation.
Pxi system overview
Types of PXI controllers
Embedded PXI System Controllers
Remote PXI System ControllersEmbedded PXI system controllers1)
Windows-Based Embedded Controllers High-performance Integrated
peripherals Entire system in one chassis
2) Real-Time Embedded Controllers reliability with LabVIEW
Real-Time Select high-performance or low-cost/low-power
Remote pxi system controllers1) PC Control of PXI Use latest
high-performance PCs High-speed, software transparent links Up to
110 MB/s sustained throughput Build multi-chassis PXI systems
Copper and fiber-optic cabling options
Remote pxi system controllers (contd.)2) Laptop Control of PXI
Use latest high-performance laptop computers High-speed, software
transparent links Up to 110 MB/s sustained throughput PXI
controllers for portable applications Use with DC-powered chassis
for mobile systems
Pxi chassisIt offers 4, 6, 8, 14, and 18-slotPortable, benchtop,
and rack-mount.AC and DC optionsPXI/SCXI combination chassis with
integrated signal conditioning.
PCI express cards
NI PCIe-GPIBInstrument Control(x1)PCI Express Graphics
Card(x16)
NI PCIe-1429Image Acquisition(x4)Examples of Different PCI
Express Link Widths: x1, x4, and x16PXI slots
Figure: The specifications define the connector that is used for
the communication to the PXI chassis.Applications of
PXiSemiconductor testMilitary Electronics testAudio and Video
testConsumer electronic testHardware in loop test
The difference between PXI and SCXIThe major difference between
PXI and SCXI is that SCXI simply manipulates and conditions the
signals whereas PXI sends this signal to the computer software for
processing.THANKS
