PXI System Architecture

Overview

The rugged PC-based PXI platform is both a high-performance and low-cost deployment platform for measurement and automation systems. PXI serves applications such as manufacturing test, military and aerospace, machine monitoring, automotive, and industrial test.  When designing a PXI system, it is valuable to take advantage of the complete PXI system architecture.  PXI provides a flexible, modular hardware platform for you to design a system according to your needs, but part of the flexibility and ease of use comes from the key software features of PXI.  To maximize the flexibility and performance of the hardware and software, use a layered system architecture.  This system architecture provides additional abstraction with each layer making the system upgrades and equipment reuse easier.  The system architecture shown below starts from the device layer where the engineer selects specific IO to meet requirements and then builds on each layer up to the optional system management layer.

Figure 1. National Instruments offers a complete hardware and software solution for designing systems.

Architecture Layer #1: Measurement and Device I/O

Fundamentally, there are two types of instrumentation architectures today – traditional and virtual. Figure 2 illustrates the similarities in these two approaches. Both have measurement hardware, a chassis, a power supply, a bus, a processor, an OS, and a user interface.
The most obvious difference from a hardware standpoint is how the components are packaged. A traditional, or stand-alone, instrument puts all of the components in the same box for every discrete instrument. The measurement functionality, analysis, displays, and control of the instruments is defined by the supplier.

By contrast, modular, software-defined virtual instruments incorporate general-purpose measurement hardware that helps users go beyond the standard capabilities and define their own measurements and user interfaces in software. With a modular approach, engineers can define the measurement functionality and build systems that scale to meet future demands. In this layer, users can focus on the specific measurement requirements for their system including dynamic range, sampling rates, frequency etc. Through a modular, software-defined approach, engineers make custom measurements, perform measurements for emerging standards, or modify the system if requirements change (for example, to add instruments, channels, or new measurements). This combination of user-defined software and scalable hardware components is key to flexibility.

Architecture Layer #2: Computing and Measurement Bus

At the center of every system today is a computer in the form of a desktop PC, server workstation, laptop, or embedded computer as used with PXI. An important aspect of the computing platform used is the ability to connect (and communicate) to the wide variety of devices in a test system. There are several different buses available for devices including GPIB, USB, LAN, PCI, and PCI Express. These buses have differing strengths making some more suitable for certain applications than others. For example, GPIB has the widest adoption for instrument control and wide availability of instrumentation; USB provides wide availability, easy connectivity, and high throughput; LAN is well-suited for distributed systems; and PCI Express provides the highest performance.

The widespread use of the PC has generated the proliferation of high-performance internal buses including PCI and PCI Express, which provide the lowest latency and highest data throughput or bandwidth. The PCI bus provides up to 132 MB/s of bus bandwidth and PCI Express, an evolution of PCI, can scale up to 4 GB/s to meet growing bandwidth needs while still providing complete software compatibility with PCI. Figure 3 illustrates the latency and bandwidth performance of the most popular instrument control buses.

Architecture Layer #3: Measurement and Control Services

Measurement and control services play an important role by providing connectivity to various hardware assets in the system, system configuration, and diagnostic tools. For example, NI Measurement & Automation Explorer (MAX) automatically detects hardware assets including data acquisition and signal conditioning hardware; GPIB, USB, and LAN-controlled instruments; PXI systems; VXI devices; and modular instrumentation so developers can configure them all in one place. Integrated diagnostic tests ensure that devices function properly, and test panels provide a quick way to check the functionality of the hardware before developers begin programming. Measurement and control services should also provide integration with the application development software layer through application programming interfaces (APIs) so developers can easily program their instruments. In fact, the components of this services software – hardware drivers, application programming interfaces (APIs), and a configuration manager – must seamlessly integrate within the ADEs to maximum performance, increase development productivity, and reduce overall maintenance.

Architecture Layer #4: Application Development Software

The application development environment (ADE), such as National Instruments LabVIEW and LabWindows/CVI, plays a critical role in test system architectures. With these tools, test system developers can communicate to a variety of instrumentation, integrate measurements, display information, connect with other applications, and much more. Ideally, the ADE(s) used to develop test and measurement applications provide ease of use, compiled performance, integration of a diverse set of I/O, and programming flexibility to meet the requirements for a range of applications.

Ease of use goes beyond how quickly someone can get up and running. With easy-to-use ADEs, developers can easily integrate processing routines with multiple measurement devices, create sophisticated user interfaces, deploy and maintain applications, and modify the application as product designs evolve and system needs expand.

Architecture Layer #5: System Management Software [Optional]

Some applications, such as in automated test, a final system management software layer might be necessary. Such systems might require the implementation of several tasks and measurement functions – some specific to a portion of the system and others repeated for every part of the system. To minimize maintenance costs and ensure system longevity, it is important to implement a strategy that separates the device-level tasks from the system-level tasks so engineers can quickly reuse, maintain, and change programs (or modules) created throughout the development cycle to meet specific requirements.

This is particularly common in test applications. In any test system, there are often operations that are different for each device tested and operations that are common for each device tested such as system-level tasks.

Operations different for each device: • Instrument configuration • Measurements • Data acquisition • Results analysis • Calibration • Test modules

Operations common for each device: • Operator interfaces • User management • DUT tracking • Test flow control • Storing results • Test reports

Some companies have written their own test executives and spent valuable engineering resources to develop test management software from the ground up. This often results in reduced productivity and ties up resources over time for software maintenance. To maximize productivity, engineering teams should use commercially available system management software, such as NI TestStand, to reduce the development of operations that are common for each device. By using this software, engineers can focus their development efforts on the operations that are different for each device.

Summay

When designing measurement and automation systems it is important to incorporate strategies that increase system flexibility, deliver higher measurement and throughput performance, lower system cost, and expand longevity. Modular, software-defined systems overcome the shortfalls of past solutions based on stand-alone instrumentation or cost-prohibitive propriety systems. A modular hardware platform based on the widely adopted PXI platform allows engineers to develop scalable test systems that tightly integrate the functionality from a variety of suppliers. In addition, it also allows engineering teams to integrate current equipment investments lowering the initial cost of implementation. Along with software-defined measurements that make use the latest PC technology such as multiple core processors and PCI Express, these layered systems can significantly improve throughput performance and scale to meet the demands of different product generations and business segments.

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