Drive down the size and cost of test with PXI and AXIe

Author : Keysight

12 May 2017

Figure 1. PXI star and daisy-chain chassis configurations.

Across many industries, manufacturers are transitioning from standalone benchtop instruments to modular test platforms. For some, the move towards modular is essential as they address the test challenges of multichannel technologies, cost and time-to-market.

Benefits of going modular

There are many reasons to transition from benchtop to modular – one of which is the reduced footprint that modular enables. Multiple channels are supported in a compact space without duplicating displays and front panels. The open system allows engineers the flexibility to configure modules from different vendors and to integrate a range of instruments. Scalability of modular systems is an attractive feature for multichannel applications, as it allows for synchronised, phase-coherent measurements. Besides that, high throughput is achieved with optimised drivers providing direct access to backplane speed. All this leads to significant savings in total cost of ownership.

From benchtop to modular

Transitioning from a benchtop to a modular test system can be a big step, and implementing the right strategy can save substantial engineering time and cost. A strong understanding of your options and the pros and cons upfront will help you succeed as you develop a new modular test system or update your existing test system with PXI or AXIe instruments.

Although we may focus more on the PXI test platform, some of the information also applies to AXIe. Essentially, AXIe may be considered a high-performance big brother of PXI. It looks the same as PXI to a host controller and shares the same standard PCIe I/O interface. The main difference is that AXIe is optimised for high-end applications like high-energy physics and multichannel antennas by offering larger board space and higher power per slot.

The infrastructure

PXI test system development starts with the infrastructure by selecting the chassis, controller and interface. The chassis provides a single power source and cooling for a selection of plug-in PXI instruments, switching and control modules. As there are multiple sources for PXI chassis and controllers, and thousands of PXI modules available from many different vendors, it is important that test system developers are well informed on PXI interoperability [1].

Tips:

1.  To support future needs, select a chassis with a large number of hybrid slots to provide for both PXI-1 hybrid and PXIe-style modules.

2.  If using an external controller, choose a PCIe adaptor card optimised for driving long PCIe lines and one which provides clock isolation with low clock/data jitter. Select one that has been pre-tested to verify BIOS and whether signal characteristics are suitable for full enumeration of PXIe chassis.

3.  If using a PXIe chassis with embedded controller, ensure consistent form factor: PXIe chassis require PXIe controllers. PXI-1 chassis require PXI-1 controllers.

4.  Select PXI modules which are PXI hybrid slot compatible as older PXI-1 modules may not be.

5.  To minimise system downtime, select a test platform that has longer duration standard and optional warranty and calibration plans, low failure rates and flexible delivery choices.

6.  To avoid false pass/fail of your products, select test equipment that provides periodic calibration options with measurement uncertainties towards warranted specifications.

Figure 2. PCI driver stacks for multi-vendor interoperability.

7.  To future-proof a test system, select a PXI or AXIe platform that can evolve as test requirements evolve, particularly if options can be quickly added with licence key upgrades.

For larger test systems, multiple chassis can be connected in various configurations. Multiple chassis can be connected directly to the PC in a star configuration or from chassis to chassis in a daisy-chain configuration. Some examples of star and daisy-chain configurations are shown in Figure 1.

Software and programming

As with any automated test systems, software plays an important role. For each module, you will have several software options. As instrument modules are selected, it is important that system developers understand the modules’ communication path – how the module is controlled through software, and trade-offs in programming time and test throughput.

PCI is the primary communications path for PXI instruments.

Just as the PCI electrical specifications provide a framework for electrical connectivity, the PCI device driver provides a common framework for software access. To provide scalability and support for the many different instruments, the driver stack is usually partitioned into multiple layers. Figure 2 shows how the kernel, VISA and IVI driver stacks work together and form a tightly coupled group, enabling co-existence of PCI driver stacks from multiple vendors.  

A kernel driver provided by the instrument module is used to interface the instrument with the PCIe bus and to communicate with the virtual instrument software architecture (VISA) within the IO software layer. The interchangeable virtual instrument (IVI) drivers, for example IVI-C, -COM and -NET drivers, are typically supplied by PXI or AXIe module manufacturers to provide a programming interface with test system software.

PXI instruments are provided with a soft front panel driver: a graphical user interface that allows you to easily verify communication between the PC and instruments, create and execute instrument commands, and display results. Soft front panels are most helpful during initial startup and when troubleshooting or creating new instrument control programming code.

Test systems that prioritise measurement speed often use programs written by engineers in programming environments such as C, C++, C# and Direct IO commands to optimise test throughput. These and other programming environments use SCPI commands to achieve the fastest test execution times. Fast throughput is also possible when using modern instrument drivers in programming environments such as LabVIEW, Microsoft Visual Studio, VEE and MATLAB.

Although instrument drivers provide greater flexibility for speed, they require more development time and the measurement accuracy depends on the engineer’s code. On the other hand, application-specific software, such as 89600 VSA software, Signal Studio and SystemVue, is created for ease-of-use, faster development and measurement integrity. At the same time, it also provides specialised measurement and analysis capabilities.

Tips:

1.  Use Windows Device Manager to determine if PXI modules are present on the PCI bus and if a driver corresponds to the instrument.

2.  If using VISA alias or resource descriptors, use the corresponding supplier’s tools for setting. For example, Keysight Connection Expert for Keysight modules or National Instruments MAX for NI modules.

3.  To ensure consistent, reliable test-result validation from R&D to manufacturing, use common application software.

4.  To ensure maximum flexibility during system development, look for modular instrument vendors who provide software tools and applications that support all of the leading test development environments, including National Instruments LabVIEW, LabWindows, Microsoft Visual Studio, Keysight VEE and MathWorks MATLAB.


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