Tips for building PXI systems

Create a hybrid system

The PXI system controller often includes integrated peripheral interfaces for buses such as Gigabit Ethernet, USB 2.0, GPIB, and RS-232. The PXI system can also connect to VXI, LXI, and other PXI systems. Because of this capability, you can use PXI to create hybrid systems that keep system costs down by reusing instruments you already have. Many Tektronix digital oscilloscopes, for instance, offer remote connectivity through PCI, GPIB, or Ethernet ports and the company's OpenChoice software. These instruments can serve as the data-acquisition portion of a hybrid PXI test system, eliminating the need to acquire a separate module. Such re-use thus helps keep system costs down.

A hybrid system design allows developers to reuse equipment to keep costs low. Keeping a modular system approach in mind will maximize the return on the efforts at system development. Courtesy of National Instruments.

Plan for the future

While it is tempting to focus solely on the current project when designing a test system, such an approach is not the best use of the PXI environment. If you build a system that can perform only a single set of measurements, you will have difficulty adapting the system for future applications. A flexible design will be easier to modify and expand, saving time and money in the long run.

You can start by defining the I/O you need and may already have in PXI or stand-alone instruments. When evaluating existing instruments or considering purchases, however, be sure to calculate the number of bus transactions that will be needed for control and data transfer. Then, verify that the bandwidth and latency of the bus that will link the instrument to the test system will meet the system's performance needs. GPIB may be good enough for DMM measurements, while Ethernet may be needed for data acquisition. Keep in mind that the PCI bus underlying PXI is one of the highest-performing bus options available, so the highest performance requirements may be best met by using a PXI module.

Unless the performance of a dedicated instrument is essential, consider implementing the functions of that instrument by combining PXI cards. A PXI cable tester module, for instance, embodies the functions of a continuity checker and a switch in an integrated instrument that performs its dedicated function with high efficiency. Populating the PXI system with a DMM module and a switch module allows recreation of that same functionality with somewhat less efficiency but with greater flexibility. If needs change, for instance, the DMM can make voltage measurements rather than simply check continuity, and the switch can be used to route signals other places within the instrument rather than just to the continuity checker.

Next, provide the bus interfaces (such as GPIB, VXI, or USB) and the device drivers that the equipment requires. To keep software development costs down, look for one of the many free device drivers that are available. Both instrument and module manufacturers often supply drivers for their products. In addition, National Instruments provides a large library of drivers on its Web site (www.ni.com).

Define the user interface and measurement functions you want in your system, and use a modular structure for the applications software. One way to achieve a modular structure is to make each basic measurement function a callable operation and then create a framework that combines the basic measurements into the function that you need. You may also want to consider using test-management software to manage and execute test procedures automatically and data-management software to store the measurement data.

Plan your module placement

Placing PXI modules in slots arbitrarily will result in suboptimal system performance. Slot 2, for instance, offers a tuned and matched set of trigger connections to the next 12 system slots. These matched triggers have less than 1 ps of skew between them. Therefore, to achieve top performance when creating a multichannel, synchronized data-acquisition system, you must place the triggering source in slot 2.

Keep in mind, too, which instruments will need to be located next to one another. An RF downconverter and its associated digitizer will work best when located next to one another. Bus segmentation should also be considered. To keep loading and skew from compromising bus performance, PCI and PXI systems limit direct backplane connections to four slots. Larger backplanes break the bus into segments that connect together through PCI bridges.

Functions such as direct-to-disk data recorders, which require a high-bandwidth data path among only a few modules, achieve their top performance when those modules inhabit a single system bus segment dedicated to that function. Similarly, modules needing precision timing (<6 ns) between them should be located on the same segment. Check the needs and interactions of system modules and assign them to backplane segments appropriately.

Allow for expansion and upgrades

Test system requirements inevitably change over time, and in general become more demanding. To avoid having to develop a new system as needs grow, build in some expansion options. If the system needs a data-acquisition card, for instance, choose a multifunction card that meets the present system needs with one of its functions. Such cards typically have additional I/O capabilities that may come in handy as new system requirements develop.

Another, relatively simple, way to ensure expansion capability is to use a PXI chassis that has at least two more slots than you need for your initial system. Alternatively, leave the right-most slot open. This provides you with the option of using a PXI remote controller to expand your instrument to a second chassis should your system needs grow significantly.