Pickering Interfaces' Blog - Insights into Switching Technology

If you think about it, sensors control much of our daily lives. Sensors ensure that the food in your refrigerator stays cold, count your steps on a smartphone, and even protects you in an automobile accident. So many devices in our personal lives as well as in business and other markets contain sensors.

All these sensors add a layer of complexity to your test strategy, as you need to simulate them when testing the circuit board that makes decisions based on a sensor’s response. Since it is usually not practical to incorporate actual sensors into a test fixture, Pickering Interfaces has created external hardware that is designed to replace these sensors in a test program. In this paper, we will talk about sensor simulation and how to select these products. With Pickering’s history of more than 15 years designing programmable resistors, we have the expertise and product depth to play an important role in testing sensor-driven products.

Learn the right questions to ask when testing sensor-driven products.

• What is a programmable resistor and why is it used for sensor simulation in test?
• What are the different types and parameters for programmable resistors?
• What if I need more accuracy in my test?
• Are there different types of configurations I can use?

As you can see, there are many things to consider when selecting programmable resistor modules in test. It is undeniable that sensors are in virtually every application for electronics, making testing important, and sensor simulation can help you get there. We recommend you take a look at our latest whitepaper -

Understanding Programmable Resistors for Sensor Simulation in Test

To learn more, take a look at our web page: Programmable Resistor Solutions for Sensor Simulation or contact our simulation application engineers.

In its day, VXI technology was a definite step forward. Built upon the modular VME computer bus, VME eXtensions for Instrumentation, or VXI, reduced the size and increased the performance of high-end test systems. Instead of bulky rack-and-stack systems, whose instruments were connected by the relatively slow GPIB interface, VXI systems featured modular instrumentation that plugged into a modified VMEbus that provided higher data transfer rates and real-time performance not possible with rack-and stack systems.

The VXIbus has served test & measurement engineers well, but it is more than 30 years old now, and it may be time to move to a more modern platform. Today, more test systems designers are choosing the PCI eXtensions for Instrumentation (PXI) platform when designing new modular test systems or replacing or updating existing test systems. Originally introduced in 1997 by National Instruments, PXI is now supported by nearly 70 companies working under the umbrella of the PXI Systems Alliance (PXISA), which publishes the PXI specification and ensures instrumentation interoperability.

PXI's main advantages: availability and lower cost

PXI's biggest advantage over VXI is product availability. In the nearly 30 years since VXI was introduced, the Consortium that manages the specification has gone from over 40 companies down to just eleven members. Even Pickering has dropped their VXI line due to key part obsolescence and reduced market requirements. A large number of the products that were available in VXI in the past are now obsolete and are often only available on the used market. That is not to say that VXI is no longer viable. In some specialized applications like Data Acquisition, there are VXI vendors producing new products. In other applications, VXI is mostly recommended for legacy applications and not for new designs.

A second reason to look closer at PXI for your test & measurement needs is that hardware costs are lower than VXI. Because PXI is based on the PCI bus, which is used in many personal computers, it can take advantage of the advances being made to serve the PC marketplace. Because many of the components used in PXI modules are also used for PCI modules, they cost less because so many more of them are made. This leads to overall reduced cost for PXI modules when compared to VXI modules. PXI systems typically cost one-half to one-third the cost of an equivalent VXI system.

Some of the other advantages that PXI enjoys over VXI include:

• Data transfer. Standard PXI can transfer data that is 8, 16, 32, or 64 bits wide. VXI can only transfer data that's 8, 16, or 32 bits wide.
• Data throughput. Because PXIe (Express version of PXI) systems can transfer data over high speed serial busses, the maximum throughput for PXI systems is higher than that for VXI systems. The maximum throughput for VXI systems is only 160 Mbytes/s, while the maximum throughput for PXIe systems is 12 Gbytes/s.
• PXI modules are smaller than equivalent VXI modules. In fact, PXI systems have the smallest footprint of any open-architecture instrumentation system. This is a big advantage for PXI test systems because as test systems become more complex, test system developers need to cram more functionality into their test systems. PXI lets them do this while at the same time keeping test system size manageable.
• Instrument availability. Aside from perhaps some very specialized instrumentation (an example would be Bustec with their specialized data acquisition systems), whatever you can buy in VXI, you can also buy in PXI. And, the trend is clear. There are fewer new VXI modules being developed than new PXI modules, and that will continue into the future as more designers specify PXI over VXI. In some applications, such as fault insertion during simulation, there are no VXI modules available. Also, because PXI is based on the CompactPCI specification, you can use CompactPCI boards in a PXI system.

Making the right choice

While the availability of PXI switching and instrumentation modules makes the choices relatively easy, there is the issue of migrating test programs from VXI to PXI, specifically in switching applications. The ability to replicate original test configurations to support legacy FRUs (Field Replaceable Units) is important to continue supporting older test programs while planning for future requirements.

To help ease this transition, we have developed PXI switching and simulation modules that closely match the operation of VXI switching modules from Racal, VTI Instruments, Agilent (Keysight), and other manufacturers. And, our policy of supporting our PXI Switching products for 15 to 20 years, or even longer, means that your next-generation test systems will have as long a life as their predecessors. An example of our VXI to PXI can be seen below.

Our PXI modules listed in the table above are in many cases a close equivalent to the VXI modules. Because of the large PCB real estate of C-Size VXI modules, channel counts may be different, and specifications may not be exactly the same. Depending on your application, a different Pickering PXI module may be closer to your requirements than what is shown here. We also have cross-references to other manufacuturer's VXI switching.

Please contact our support team for more assistance in finding the correct module for your requirements.

Migration issues

While PXI is clearly the modular platform of choice when designing new test & measurement systems or upgrading existing systems, you may encounter some issues when doing so. One of them is module real estate. Clearly, a C-size VXI module has a lot more area to mount components, meaning that some VXI modules may have more features or functionality than a PXI module that you might choose to replace it. On the whole, however, VXI module designs are not as dense as today’s PXI module, so this is usually not an issue.

A related issue that you will have to address when migrating to PXI is cabling. VXI modules have a wider front panel (1.188-in.) than PXI modules (0.8-in.), meaning that many of the connectors used in VXI systems cannot be used in a PXI system. So, plan on building or buying new cables or buying adaptors.

If your instruments dissipate a lot of power, you may have cooling issues when migrating to a PXI system. The PXI cooling spec is less stringent than the VXI specifications, and generally, PXI chassis do not have the cooling power of VXI chassis. You may use multiple slots to achieve the necessary cooling.

Software is another area of concern. There is no easy way to migrate test programs. But, given the number of good test-development software packages available for the PC platform, this may not be as big an obstacle as it might appear.

Despite these drawbacks, if you're an automatic test system developer, the question is clear. It's not if, but when you should move to PXI-based systems from VXI. The answer is pretty clear, too. The answer is now.

Let us know your thoughts on moving from VXI to PXI.

With over 25 years of signal switching and instrumentation experience, we have learned that when it comes to electronic test –"One Size Does Not Fit All" – that is why we now offer over 1000 PXI modules. 

Below are a list of frequently asked questions on why and how we offer such a large range.

Why does Pickering offer so many modules?

We entered the PXI market in 1998 with just a few modules—since then, primarily through customer demand, our PXI range has grown to over 1000 modules. This has been driven by a number of application requirements including:

• Number of input and output channels
• Voltage
• Power
• Current
• Bandwidth
• Switch time
• Switching life
• Switching technology
  • Reed Relay
  • EMR
  • Solid State
  • MEMS
  • Microwave Relay
• Crosstalk/isolation
• Budget limitations
• Low noise
• Switching configuration and density

These requirements are so varied that test engineers demand a very wide and ever increasing range of solutions. “One size fits all” just doesn’t work in switching for Test & Measurement.

Take a look at our PXI Switching web page, on the left under product navigation it shows a product count for the number of modules available by main category and for each sub-category.

What types of PXI modules are produced by Pickering?

While PXI Switching & Programmable Resistors (for sensor emulation) are our main focus, we serve your electronic test requirements in other ways. Our PXI product line includes:

• DC power supplies
• Digital I/O
• Breadboards
• Signal conditioning (amplifiers and attenuators)
• Signal generation

With so many modules, how can I find the exact one I need?

We have recently redesigned our website so you can quickly drill down to the exact module you need. We also have easy-to-use product reference maps with an overview of our entire range on one sheet.

In addition, we have highly experienced sales and applications people who understand switching and sensor emulation, and are just a phone call or email away. 

With so many modules, is delivery time compromised?

No. Typical PXI module delivery time is three weeks for almost all module types, often faster if required.

So how does Pickering offer quick delivery for such a large product range?

All module production processes take place in our two factories on flexible, demand-based manufacturing lines. We have complete control of the whole manufacturing process—we do not subcontract any process; everything is designed and manufactured in-house.

How can Pickering possibly offer long-term product support on such a large range?

As stated above, our in-house manufacturing gives us complete product control. From time to time, we update designs to improve performance and reliability while seamlessly managing any component obsolescence issues, all while preserving form and fit compatibility. Typical PXI module lifetime is in excess of 20 years, which is very important for many of our customers, especially in the Mil/Aerospace and Transportation markets.

How can you maintain and repair all these modules?

Pickering offers fast repair on a quick turnaround basis at very reasonable cost.

In addition, Pickering has a strong background in diagnostics. Modules with our BIRST™ feature can quickly diagnose down to component level. For most other modules, we have our eBIRST™ Switching System Test Tools, this toolset connects externally to the module and can diagnose faults quickly. All of our PXI modules carry a three-year warranty which is unaffected if users choose to undertake their own relay replacement.

Why not just offer software-configurable modules instead of so many different variants?

We do offer several software-configurable modules for customers who require this flexibility. However, these have not proved popular due to the added cost of configuration relays and the inherent disadvantages of lower switching density, reduced AC performance and crosstalk/isolation compromises.

Are Pickering PXI modules compatible with “NI PXI”?

This is a common question. Yes, of course, all of our PXI modules are 100% compatible with PXI products from all other vendors, including NI, Keysight and the other 60 members of the PXI Systems Alliance (PXISA).

Interested in learning more about the PXI Standard? Take a look at the article: "What is PXI" or get a copy of the 5th edition of our PXImate book. 

What about Software?

Our module drivers support all popular software languages, including LabVIEW, Visual Studio, ATEeasy, LabWindows/CVI. We also support Real Time operating systems like Real Time LINUX as well as LabVIEW RT.

Can I use Pickering PXI Modules with other common buses?

Almost all of our PXI Modules can also be used in our LXI Ethernet chassis.

With such a big PXI range, where will I find suitable cabling and connectors?

We don’t leave you struggling to figure out your connectivity. We offer a similarly large range of connectivity options and in we have an on-line Cable Design Tool where you can design your own custom cable assemblies. 

We also have strong partnerships with the two mass interconnect companies, Mac Panel and VPC, who have 100’s of off-the-shelf as well as custom solutions.

Does Pickering really understand switching?

We are experts in switching technology. We have been designing and manufacturing modular switching systems since 1988, and our sister company, Pickering Electronics, has been designing and manufacturing instrumentation grade reed relays since 1968.

Switching expertise is in our DNA!

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