PXI vs CompactPCI, PCI and VXI
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Why should system designers consider PXI?
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Systems designers have many options when developing test systems. Developers can design their own propriety hardware and software, incorporate stand alone GPIB instruments, select modular systems such as VXI or use PC-based instruments. Each of these possibilities has benefits and limitations to consider. For example, developing propriety hardware and software gives the designer a lot of flexibility but systems tend to take a long time to develop and often lack interchangeability. Standalone GPIB and RS232 instruments incorporated in test systems typically have good price/performance but data transfer are often slow and the availability of drivers and software is often questionable. High-end modular systems, VXI for example, often fill the need for high performance, fast data transfer and channel density but also include a high price. PC-based instruments are good for some general-purpose applications and are typically lower cost and lower performance. PC-based instruments tend to be susceptible to noise, lack robustness and are limited by physical space in a desktop PC.
In recent years CompactPCI and CompactPCI with extensions or PXI were introduced as platforms that strike a good price/performance balance within a compact, rugged, modular architecture. The extensions offered in PXI are especially valuable for the test and measurement applications and include mechanical, electrical and software requirements in addition to those defined by
CompactPCI.
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How is PXI different from CompactPCI, PCI and VXI?
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The PCI Bus (Peripheral Component Interconnect Bus) was introduced by Intel in 1993 as a local, low level, processor independent, multi mastering, 32 bit synchronous bus running at 33 MHz. The PCI bus can be found on almost any new PC and has become the standard in the PC industry. One of the main reasons for the popularity of PCI is the fast data throughput it offers between add-on devices with up to 132 MB/s (32-bit) and 264 MB/s (64 bit) peak transfer rates. Additional features of PCI include system expansion via PCI-PCI bridges and plug and play capability.
It soon became apparent that there was a growing need in test and measurement applications for a more rugged form factor for
PCI. In 1995 an organization was formed PICMG to discuss and release a specification for
CompactPCI. This CompactPCI combines the benefits of PCI with Eurocard mechanical specifications, IEC connector technologies and more slots per bus segment compared to PC-based PCI systems.
In 1997 National Instruments took the lead in extending the CompactPCI specification and introduced PXI
(PCI eXtensions for Instrumentation) as an open standard. An important feature of PXI is the interoperability with CompactPCI or the fact that a PXI module can be used in a CompactPCI mainframe and vice versa. PXI leverages the CompactPCI specification and with electrical, mechanical and software enhancements and requirements well matched to the needs of industrial applications and instrumentation.
The electrical extensions of PXI include a 10 MHz reference clock on the PXI
backplane, trigger bus and star trigger control for precise trigger synchronization between modules and a local bus for communication between adjacent modules. The mechanical extensions of PXI include requirements for documenting environmental and EMC tests, specify active cooling and define the location of the controller within the PXI chassis. The software extensions of PXI include a software support framework
as well as WIN32 device drivers.
VXI was introduced and defined around the highly popular VMEbus architecture as a high-end test platform. PXI was introduced as a mainstream platform defined around the PC standard PCI bus. PXI is physically smaller (typically 3 U), less expensive and lower performance compared to
VXI.
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What is the technology used in LeCroy’s PXD Digitizers?
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Recently, LeCroy announced the introduction of eight digitizers in PXI format with bandwidths ranging from 150 MHz to 1 GHz. There are also choices for optional memory to increase recording depth to 8 Million samples. These products feature advanced "digitizer on a chip" technology which combines a high performance analog-to-digital-converter (ADC) and long acquisition memory in a single hybrid. This allows LeCroy to produce PXI digitizers with highest performance in the PXI market, high density of channels.
At the heart of the LeCroy PXI product line is the "digitizer on a chip" or HAM631 Multi Chip Module (MCM). This chip combines two monolithic integrated circuits into one package that includes both an 8 bit ADC and up to 4 Mbytes of memory. This MCM was designed with inputs to control gain, offset and sampling delay to allow easy interleaving of ADC channels. This chip is packaged in a thermally enhanced BGA and is 19 mm x 29 mm. The small geometry of the ADCs and memory allows LeCroy to deploy them into compact form factors and specifically in PXI digitizers.
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DATA TRANSFER AND THROUGHPUT
What are typical transfer rates in a PXI system and what is the fastest way to store data?
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Transfer rates to the hard drive are heavily dependent on the application,
the software involved, etc. so it's hard to give a single number. However, we
have done benchmarks in various applications that result in 4 to 7 Mbytes/s
streaming continuously to the onboard hard drive. Using a Wide Ultra-SCSI
interface (e.g. PXI-8210) to an external SCSI drive will deliver the highest
performance (theoretical limit for wide ultra SCSI is 40 Mbytes/s).
There is a theoretical limit of 254 linked chassis in a PXI system, however, there are some limitations for real systems. Having one PC or one PXI embedded computer control hundreds of cards presents a serious bottleneck in that the overall bus bandwidth to the controller/PC is still only a maximum of 132 Mbytes/s. In other words, the more modules you add in a linked system - you don't get any more bandwidth - you just divide the existing bandwidth by more modules. We've tested configurations that go up to about 6 bridging links (6 chassis). But in general, we don't recommend linking more than 3 chassis simply because of the load on a single PC/PXI controller (this is especially true for high performance cards like digitizers). For cards that don't require a lot of processor bandwidth (i.e. switching cards), more chassis and modules may be acceptable for some applications.
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How are multiple device service requests handled in a PXI system?
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The PXI standard requires an arbitration type bus. What ever module puts up a flag first is serviced,then the next flag on line downloads all its data, etc. In regards to the multi-tasking, this will be software dependent. The PXI platform is currently supported by MS Windows which supports LabView, LabWindowsCVI, MS Visual Basic, MS Visual C/C++, and Turbo C/C++ from Borland. Any multi-tasking will be governed by these software programs.
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What type of throughput can I expect from a PXI system?
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Throughput is governed by the PCI bus. The spec is 33 MHz x 4 bytes wide = 132 Mb/sec transfer rate. However, burst rates are measured at 100 Mb/sec. The LeCroy PXD digitizers will use a stacked readout technique in certain configurations. We should use safe estimates for throughput calculations until these can be benchmarked. Throughput can be calculated as follows:
10 Mbytes/second x 4 bytes = 40 Mbytes/second maximum readout if the bytes are stacked.
10 Mbyes/sec for our calculations with 1 byte transferred per cycle at the
backplane.
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SYSTEM CONFIGURATION
Are there PXI Chassis rack mount kits and what are the dimensions?
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The rack mount kits for National Instruments and PXIT chassis adapt to a standard 19" instrument rack and are 4U tall.
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POWER REQUIREMENTS
What is the typical power consumption for the PXD Series?
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PXD514/214/114 |
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Supply |
Current (A) |
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12 V |
0.28 |
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5 V |
7.3 |
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3.3 V |
5.8 |
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-12 V |
-0.35 |
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Total Power |
70 W |
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PXD512/212 |
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Supply |
Current (A) |
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12 V |
0.11 |
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5 V |
4.2 |
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3.3 V |
3.7 |
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-12 V |
-0.22 |
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Total Power |
41 W |
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Applications and advantages
What are some typical applications for PXI digitizers?
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TOF - Time Of Flight
Automotive Ignition Testing
Engine Analysis
Non-Destructive Testing
Vibration Analysis
Testing Motor Windings
Ultrasonic Testing
Laser Doppler Anemometry
Laser Characterization
LIDAR
Radar Signal Analysis
Modem Testing
Telephony
Explosion Testing
Disk Drive Testing
Ultrasonic Materials Testing
Acoustic Emission Testing
Ultrasound Imaging
High-End Video Testing
Spectrum Analysis
Nuclear Instrumentation
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