Evaluating PXI Chassis for Test, Control, and Design Applications

A fundamental component of PXI or CompactPCI measurement and automation systems is the chassis. Choosing the appropriate PXI chassis is crucial for the overall accuracy, safety, and reliability of the system. Factors to be considered when selecting a PXI chassis for test and measurement systems include number of required slots (with room for future expansion), power availability, cooling capability, chassis design and testing procedures, acoustic noise emissions, software configuration tools, safety certification procedures, and purchase price.

Because there is not one single PXI chassis that fits each of the needs of a given system, it is helpful to ask yourself the following questions when choosing a PXI chassis: How much power is required for the system? In what kind of temperature environment(s) will I be operating? Are low acoustic noise emissions important? Does my PXI chassis have the necessary safety and compliance certification? Will my system be rack-mounted, portable, or operated on a desktop? How important is manufacturer system level testing and technical support?

National Instruments designs and offers PXI chassis that meet and exceed the demands of today’s complex test and measurement applications. The following paragraphs provide answers to the tough questions you should ask when deciding what chassis is best for a given set of system requirements.

1) How much power is required for your PXI system?

The development of new, higher performance PXI modules such as 6½ digit DMMs, dynamic signal acquisition (DSA) modules, 100 MHz signal generators and digitizers, and RF devices, requires more power from the PXI chassis, especially at high ambient temperatures (up to 55 °C).

The latest PXI specification (revision 2.1) outlines the following current requirements on each of the four voltage rails of the PXI chassis power supply. The specification does not dictate a specific temperature range, which will become an important point as shown in subsequent paragraphs.

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When specifying a PXI system, it is helpful to perform a power budget to determine how much power is required by your system versus the amount of power your chassis can supply. Let’s assume you want to build a system with three 14-bit, 100 MS/s digitizers, two 1.3 GHz switch modules, and two 16-bit, 100 MS/s AWGs (Arbitrary Waveform Generators) – operating up to 50 ºC. The following is an example of a power budget calculation for this system.

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Although a simple power budget is valuable in determining power consumption, it does not completely determine the overall required power of the system. When determining how much power is needed for a given PXI system, you must also consider the operating temperature of your system.

2) At what ambient temperatures will your system be operating?

The amount of power available for a PXI system is directly related to the ambient temperature. Generally, as ambient temperature increases, available power decreases. The ambient temperature of a system is defined as the temperature at the chassis fan inlet. A simple room temperature measurement is a good estimate of ambient temperature, but may not be the most accurate representation of the actual ambient temperature. For example, heat dissipation from surrounding equipment, such as the equipment in a 19 in. rack, can artificially increase the ambient temperature, which may ultimately lead to measurement inaccuracy, system shutdown, or premature system failure.

Many PXI chassis meet the required PXI specifications for available power at lower ambient operating temperatures (20 to 35 °C), but may become unstable or completely inoperable at higher temperatures (>40 °C) because of power derating. Many National Instruments chassis have power margins to maintain operation up to 55 C with specified derating of power supply. NI performs extensive design testing to ensure chassis operation above the specified temperature range before shut down or significant reduction in functionality occurs.

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As seen in the table above, the National Instruments PXI-1042 8-slot chassis supplies more power than is recommended by the PXI specification for temperatures up to 50ºC.
The National Instruments PXI-1042 8-slot, PXI-1044 14-slot, and PXI-1045 18-slot chassis have a power margin to enable operation up to 55C (with specified derating of power supply), and extensive design testing is performed to ensure the chassis can operate above its specified temperature range before shut down or significant reduction in functionality occurs.

3) Does your PXI chassis have certified safety markings?

The safety of employees is critical in today’s laboratories, manufacturing facilities, and industrial settings. Safety awareness among users of equipment for test and measurement is increasing because of new high-voltage measurement categories and extra precautions required for safe operation. For example, some test and measurement products produce voltages up to and greater than 1000V. These high voltages can be accessed during measurement procedures via sensors and probes.

The European Conformity (CE) marking is the equipment manufacturer’s self-declaration signifying that the product meets applicable directive and standards. Today’s market surveys suggest that many consumers are beginning to scrutinize the CE marking, especially in the context of product safety and due-diligence in product compliance and testing. For this reason, many countries are requiring a third-party certification mark, such as UL and CSA (North America), and VDE, TUV, and Demko (Europe). The PXI Specification (all Revisions), clearly outlines the requirement of both CE markings and third-party certified safety markings on all PXI chassis. The following is per the latest PXI specification (Revision 2.1, section 3.9.2):

“Testing shall be performed, either by a competent organization or qualified manufacturer, marked accordingly, and documented showing compliance to the following electrical safety standard.”
· IEC 1010-1:1990 + A1:1992

All National Instruments PXI chassis are marked with both self-declared CE markings and certified third-party safety markings, including UL, Demko, and TUV. In addition to safety and compliance testing, National Instruments performs extensive electromagnetic interference (EMI) and electromagnetic compatibility (EMC) testing to ensure proper functionality of PXI test equipment in environments with electromagnetic disturbances. Details for these safety certifications are clearly outlined in a “declaration of conformity” for each respective PXI chassis. These declarations of conformity can be found at www.ni.com/support/reference => Product Certification.

4) Are you planning to implement triggering in your system?

PXI systems, by definition, provide integrated triggering and synchronization features for measurement systems. For simple to intermediate synchronization, software tools such as DAQmx can be used to program PXI timing and triggering in a series of straightforward steps using the DAQ Assistant. For more complex triggering schemes, use NI Synch in conjunction with the PXI-6653 Timing and Synchronization module (see links below for more information).

The following figure shows the user interface for configuring triggers for PXI Data Acquisition devices.

Figure 1. NI DAQmx and the DAQ Assistant in LabVIEW 7

In larger applications requiring trigger signals routed across PCI bridges, specialized cabling and configuration software may be required. National Instruments MAX (Measurement and Automation Explorer) provides a simple software utility so you can quickly and easily configure triggers to chassis slots of each bus segment.

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Figure 2. NI MAX 3.0 and the trigger configuration utility

5) Will it be necessary to route clock signals between multiple chassis/systems?

The PXI 10 MHz reference clock is routed to each peripheral slot in the PXI chassis. The 10 MHz clock can be used to synchronize multiple modules from a common TTL, low-skew reference source (<1 ns skew between slots). Some applications may require synchronization of instrument devices in multiple chassis. For accurate synchronization, it is best to use a single clock source for each module in the system.

The National Instruments PXI-1042 8-slot chassis and PXI-1045 18-slot chassis include two BNC connectors on the rear of the chassis, one for exporting 10 MHz clock signals, and one for importing 10 MHz clock signals from a primary/master PXI chassis, or another 10 MHz reference source. If a 10 MHz clock signal is imported from another source via the BNC connector, the imported signal overrides the onboard chassis 10 MHz reference clock and is routed to each peripheral slot with less than 1 ns of skew. In addition, a timing and synchronization module in slot 2 of the PXI chassis (such as the PXI-665x timing and synchronization module) can be utilized to route 10 MHz clock signals and triggers between devices in a single chassis or multiple chassis.

6) What are the acoustic requirements for your PXI system?

Acoustical noise is one of the most common occupational health hazards. Noise emissions from test and measurement equipment is a major concern in offices, laboratories, and classrooms. In addition, unwanted acoustical noise can severely alter the accuracy of high resolution measurements, such as audio measurements and NVH (noise, vibration and harshness) measurements.

National Instruments offers PXI chassis with low acoustic noise emissions for office and laboratory environments and acoustically sensitive applications. Measured in accordance to International Organization of Standardization (ISO) 7779, the PXI-1031 4-slot, PXI-1042Q 8-slot, and PXI-1052 PXI/SCXI chassis have the following sound pressure levels (measured at 25 °C, operator position).

Table 4. Sound Pressure Levels of common desktop equipment




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7) Does your PXI chassis require mounting in a 19 in. rack?

Many applications require rack-mounted equipment, including instruments, monitors, and PXI chassis. The mechanical stability of a PXI chassis in a rack-mounted system is one factor that determines the longevity of a product’s life cycle. Many PXI vendors offer front rack-mounting kits for standard 19 in. rack systems.

In addition to standard front rack mount kits, National Instruments offers chassis with rear rack mount kits for increased stability, especially for large systems such as the PXI-1042 8-slot chassis, PXI-1052 combination PXI/SCXI, PXI-1044 14-slot, and PXI-1045 18-slot chassis.

8) Is software configuration required for your PXI system?

All PXI chassis include software initialization files for the configuration of PXI systems. To assist the end user, the manufacturers of PXI chassis and system modules must document the capabilities of their products in ASCII based .ini files. The information in these files is combined with information about the system controller to create a single initialization file called the pxisys.ini file (PXI system initialization file).

Configuration of National Instruments PXI systems is handled directly through MAX, included with the software pre-installed on your controller. For example, the NI PXI-8186 uses MAX to automatically generate the pxisys.ini file from the chassis.ini file. Other PXI chassis and controller manufacturers may require arduous system configuration steps that involve manually transferring and copying of .ini files as well as setting dip switches on the chassis.

9) Will you require your PXI system to be ready for use directly out of the box?

National Instruments offers several options to help you get up and running when configuring your system with a system assurance program . The Basic system assurance program ensures that you have the latest drivers correctly installed and that all your modules are functioning correctly. The Standard system assurance program goes a step further and provides you with flexible software configuration options and custom system documentation. THe Standard system assurance programs delivers your PXI system in one box with the software configuration that you selected in the PXI Advisor tool, including previous System Driver Sets and courtesy installation of National Instruments development environments. The Standard program also includes a printed pass/fail test result from the system-level functional test, so that you can be sure that all the modules are working in your chassis. System assurance programs ensure viable operation of the PXI system right out of the box and minimize the amount of time spent on installation and configuration of hardware or software. This service from National Instruments also includes recovery media to restore the hard disk to its original state.

10) Is system technical support valuable for your application?

In today’s world of tight budgets and rigid timeliness, superior technical support may be the difference in the overall success of an application. A competent technical support organization facilitates proper configuration and installation of key system components, minimizes downtime through troubleshooting techniques, and provides key information for optimized use of your PXI equipment.

In offices around the globe, National Instruments has staff local to the country so you have access to field engineers who speak your language and are available to consult based on your needs. National Instruments also has a worldwide support organization staffed with Applications Engineers trained to quickly provide superior technical assistance via phone, email, or web (www.ni.com/support). In addition, you can use online resources to access thousands of Knowledge Base entries and more than 14,000 worldwide measurement and automation professionals within NI Developer Exchange Discussion Forums (www.ni.com/zone).

Conclusion

Choosing the appropriate PXI chassis for your test and measurement application is dependant on several factors. These factors include power requirements of the system, operating temperature, environmental acoustic requirements, safety certifications, system configuration tools, device timing and triggering requirements, system level functional testing, and technical support.

National Instruments designs and offers PXI chassis that meet and exceed the demands of today’s complex test and measurement applications. For more information regarding the National Instruments PXI chassis offering, as well as other PXI products offered by National Instruments, visit the National Instruments PXI website at www.ni.com/pxi.

PXI Chassis Scorecard

[+] Enlarge Image *Sound Pressure Level (LP) at Operator Position and low fan speed. Tested in Accordance with ISO-7779.

Related Links:

NI PXI Website
PXI-1042 Series Model Page
PXI-6653 Timing and Synchronization Module Model Page

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