Advances In PXI Switching - Enabling High-Performance ATE

Miniaturized ATE

As the range of PXI (PCI eXtensions for Instrumentation) instrumentation continues to expand, more and more complete automated test systems are being built on the PXI platform. With multiple vendors contributing to the growth, almost every traditional rack-and-stack automated test equipment (ATE) system can be miniaturized using this modular, flexible form factor. In the past, ATE switching was largely absent from PXI due to the available real estate of large form-factor VXI and standalone GPIB switch boxes. Recently, advances in PXI switching play a key role in deploying a complete PXI test solution.

Innovative approaches to designing 3U PXI switch modules are now a requirement to match the density, price, and breadth of offering available in other legacy platforms. When dealing with mechanical objects such as relays, space is at a premium in high-density switching modules. This is especially true for 3U PXI, having roughly 1/4th the surface area for relays as a 6U VXI counterpart. Therefore, PXI switch vendors must identify solutions to match the density, price, and performance without requiring 8 PXI switch modules to do the work of 2 VXI modules. National Instruments has dedicated resources to meet these challenges, resulting in a number of recent PXI switch product releases and technology improvements that far surpass the conventional approach to switch module design.

Overcoming Density Limitations

With ATE systems traditionally occupying large equipment racks, anything that can be done to minimize space is welcomed on a factory floor. Considering the physical size of a 3U PXI module, it appears to be a step in the right direction. However, small size is a huge limitation to high-density switching. Overcoming that constraint is new relay technology and improvements in manufacturing processes that allow much greater usage of the space available.

At National Instruments, we have increased the density of our matrix, multiplexer, and general-purpose switch offerings by large percentages over the last few years. In some cases, like PXI matrix density, the number of crosspoints in a single 3U PXI card has grown by a factor of five. Many of these increases can be attributed to advances in armature and non-armature relay types (like reed relays) and their shrinking footprints. Additionally, we can use surface-mount relays to improve the production process and populate relays on both sides of a printed circuit board. This maximizes not only the surface area available, but also the width/volume of a PXI slot.


Certainly these compact relay form factors and process improvements can be applied to every switching platform. But while it’s possible to create a VXI module employing these improvements, few companies have shown interest lately in expanding high-density VXI switch offerings. It’s also important to keep in mind the law of diminishing returns. In other words, you can put 10,000 crosspoints in a single slot, but would the cost to build and low volumes ever justify the small number of systems that need density of that magnitude? The advances in NI PXI switching provide the ideal density per module to maintain realistic prices for use in today’s test systems.

Integrated Timing and Triggering with Instrumentation

Switch systems can uniquely benefit from the additional timing and triggering lines of the PXI backplane. Many instruments use input and output triggers to indicate when to start a measurement and when it’s complete. Switch modules often have complementary triggers to identify when to switch and to communicate back to the instrument that the switch has finished closing.
With the 8 trigger lines built into the PXI backplane, you can route these signals back and forth between the switch and the instrument. This is even more convenient if the switch modules support scanning, as most National Instrument’s switches do. Scanning improves throughput by downloading a list of connections to the switch modules and cycling through the list without any interruption from the host processor. This process ensures optimal throughput because the entire measurement sequence is hardware-timed and has no additional software delays.

A good example of this synchronization involves the NI PXI-2530, a 128-channel, high-speed multiplexer. Using an NI PXI-4070 6 ½-digit FlexDMM in the second slot of an 18-slot chassis and filling the other 16 available slots with PXI-2530s, an application could scan more than 2000 channels in just over 2 seconds without any intervention from the host operating system. All handshaking would take place through the PXI backplane, and the speed of the PXI-4070 and the PXI-2530 would both be maximized.

Hardware-timed scanning is well recognized for multiplexer applications as it is the most effective way to cycle through many channels (sometimes even across multiple modules). However, scanning is not limited to multiplexing applications. It’s possible to program hardware-triggered state transitions in large matrices, lending a similar amount of determinism to many channels in large row/column configurations. Similarly, when general-purpose switching is being used in control loops, the ability to send triggers to and receive triggers from the relays tightens the process in time-sensitive applications.

These timing signals can also be shared between multiple chassis with the NI PXI-6653 Multichassis Synchronization Module. This allows hundreds of switch modules to be tightly synchronized across several chassis - a feat only possible at this point in PXI.

Standalone PXI Switching

PXI excels as a rugged, compact platform for measurements, but it’s also very effective without measurement modules – specifically as a non-measurement, switching platform. Considering the aforementioned switch density increases, it is advantageous to use a PXI chassis filled with switch modules in place of standalone GPIB- or VXI-based switch mainframes. A PXI chassis full of switch modules offers a switch platform based on an open, industry standard and greatly reduces the size of traditional standalone switch mainframes. In effect, a PXI chassis itself can function as a standalone switch box or datalogger. Enabling this concept are low-cost, high-speed control options.

MXI-3

MXI-3 is a PCI master/slave system implementing the PCI-PCI bridge register set. It couples two physically separate PCI, CompactPCI, or PXI buses with either a copper or fiber-optic data link capable of 1.5 Gbytes/s serial data rates (the limitations of the PCI bus set the realistic data transfer rates closer to 132 MB/s). At this speed, MXI-3 is significantly faster than the throughput of GPIB or RS-232 connections used on many switch mainframes.



Real-Time Controllers

With an embedded real-time controller, like the NI PXI-8186RT, you can communicate with a headless PXI chassis through Ethernet. This not only gives you the bandwidth common in Ethernet hardware, but it also allows the chassis to be placed in remote locations. The switch application will also have the added determinism provided by LabVIEW RT.

When used as a standalone switch box, PXI can be employed in hybrid GPIB, PXI, PCI, or VXI solutions. This allows for maximum equipment reuse and keeps development times down when new test systems are deployed.

Multiple Connectivity Options

Switch signal connectivity often comes down to a choice between simple prototyping connectivity and custom-cabling labor. GPIB systems are somewhat notorious for their low-range of connectivity options; the only option is usually screw terminals inside the chassis. This makes it very difficult to translate prototype systems to a factory floor.

PXI switch modules offer a larger range of connectivity options. This includes (but is not limited to) front-mounting terminal blocks, cables, connectors/backshells, direct-connect (SMA, SMB, MCX, etc), and mass interconnect solutions. Considering these options, you could use the same switch module with a terminal block for design validation testing and a mass interconnect solution for deployment to the factory floor.

At National Instruments, we offer front-mounting terminal blocks for many of our modules specifically for system prototyping or small-density applications. Connectivity inside these terminal blocks includes screw terminals, spring terminals, ribbon cable headers, or solder points. Using terminal blocks can greatly simplify the setup of a system, removing weeks of custom cabling labor from the overall development time.
On the other hand, terminal blocks may not be ideal for many high-density applications. Therefore, we reference mating connectors or provide connector and backshell kits so that a custom cable can be created. When the system or interconnect needs warrant it, a mass interconnect solution from companies such as Virginia Panel Corporation or MAC Panel can greatly reduce test time and maintenance. Using connector technology from these vendors helps increase the lifespan of your connection points and makes connecting a multitude of test signals achievable in one motion.

Finally, in RF applications, direct connectivity to the switch is provided on the front panel of the module in an SMA, SMB, MCX, or other high-frequency connector. These connectors are chosen to match signal bandwidths and make the switch easy to connect to high-frequency instruments.

Just the Beginning

As the physical size of ATE systems continues to be miniaturized by the PXI platform, a range of switching options in PXI is becoming more of a necessity than a luxury. The application opportunities for switching are growing, and more and more vendors are embracing the PXI platform - driving the market in new and exciting areas. While the smaller footprint of 3U PXI once served as a deterrent to PXI switching, it now serves as the vehicle on which advanced timing, control, and connectivity options are improving automated test and enabling high-performance ATE.

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