Electronic Design

Downsize ATE Systems With PXI Modular Instrumentation

Smaller footprint, lower component and maintenance costs, and faster test times make PXI the way to go.

Everyone knows that testing is an essential part of the manufacturing process, but no one wants to spend a lot of money on it. This is mostly because it doesn't add any tangible features to the product that can justify an increase in cost. Nonetheless, testing must be done. And while it can't be done for free, with PCI eXtensions for instrumentation (PXI), testing can be performed for less.

Engineers first automated test equipment with PCs as a way to spend less on testing. Now with PXI modular instrumentation technologies, it's possible to downsize automated test equipment (ATE) in unprecedented ways and reduce overall test costs. Downsizing ATE with PXI modular instrumentation doesn't only mean lower component costs and a smaller footprint. It also amounts to lower maintenance costs, faster test times, faster development times, and better test coverage. It's not the ideal approach for every aspect of testing, however. In many cases, a hybrid test approach leveraging the strengths of PXI modular instrumentation, VXI modular instrumentation, and standalone GPIB instrumentation is the best way to go.

Companies such as Hewlett-Packard (now Agilent Technologies), GenRad, and Tektronix pioneered much of the first test equipment with instruments such as oscilloscopes, digital multimeters, and arbitrary waveform generators. While these devices proved extremely useful, the benefit of controlling and monitoring these instruments from a PC was apparent. In the late 1960s, the general-purpose interface bus (GPIB), or IEEE-488 bus, was developed as a way to allow instruments to communicate with each other.

With the rise of the PC in the late seventies and early eighties, GPIB was expanded to not only communicate between instruments, but to communicate between instruments and PCs as well. This enabled the creation of ATE systems that completed complicated tests quickly and efficiently.

While GPIB had improved the performance and functionality of test systems, it was obvious that ATE systems needed to be downsized to be smaller and less expensive. At the same time, they had to improve in performance. In 1987, the VXI Consortium was formed. The consortium defined an open industry standard to combine elements of the VME bus and GPIB to create a powerful modular instrumentation platform. VXI allowed for tighter synchronization between instruments, faster data transfer, and a reduction in size. Plus, it marked a shift from standalone box instruments to modular instrumentation.

Standalone box instruments require their own processors, input devices (knobs and dials), and display devices (CRT or LCD). Modular instrumentation, on the other hand, eliminates these costly and bulky individual components. This is because all of the instruments can leverage off one processor, one input device, and one display device.

As PC technology progressed, it became possible to create PC-based instruments. PC-based instruments take advantage of the low cost and small size of PCs while achieving higher data throughput rates than VXI or GPIB instruments. But PCs are not ideal for ATE applications. They typically have only three or four peripheral slots and aren't built for a rugged rack-mount environment. Another drawback is that PCs don't have standards for passing timing and synchronization between instruments. Nevertheless, because of its performance and flexibility, PC technology remains highly desirable on manufacturing floors.

CompactPCI arose in an effort to address some of the shortcomings of PCs. CompactPCI is an open industry standard that features rugged, modular Eurocard packaging like that found in VME and VXI. It combines this packaging with a high-speed PCI bus and high-performance connectors that allow for eight slots without a PCI-to-PCI bridge.

But CompactPCI didn't address the needs of ATE systems. So in 1997, National Instruments created the open PCI eXtensions for Instrumentation (PXI) specification. The following year, the PXI Systems Alliance was established and took the next step by ad-opting ownership of the specification.

PXI adds dedicated timing and synchronization lines, environmental and EMC testing requirements, and system-level specifications that simplify integration of CompactPCI. In addition, it maintains complete interoperability with CompactPCI while adding the features necessary to build successful, high-performance ATE systems. It also allows for downsizing of ATE systems never before realized. PXI-based ATE systems can be built at a lower cost and use significantly less space than either GPIB or VXI-based systems.

PXI defines both a 3U and 6U form factor. Yet most PXI products are 3U, which is a quarter of the size of the VXI 6U form factor. The 3U size takes maximum advantage of the miniaturization of electronic components to create small, high-performance instruments. This smaller footprint offers manufacturers two options. They can either create ATE systems that test more products in the same amount of space, or use the extra space for more profitable areas of their manufacturing processes. With PXI, it's possible to implement a system into a single PXI chassis (7 by 11 by 15 in.) that would normally require an entire rack of GPIB and VXI instruments. These chassis can then be integrated into a complete ATE system (Fig. 1).

PXI is based on a modular instrumentation architecture that uses one processor, one set of input devices, one display device, and one mechanical enclosure. It's also based on the PC industry-standard PCI bus. For these reasons, products can be offered at a much lower cost than comparable GPIB or VXI products. This is possible because they use many of the same high-volume, low-cost, board-level components that are used in consumer systems.

Since PXI is based on standard Microsoft operating systems, there's a wide array of software available for it. Due to this vast software selection and the familiarity users have with Microsoft operating systems, ATE systems can be created faster and at a lower cost. PXI also defines locations for the system controller and requires that the chassis include integrated cooling and power supplies.

By setting system-level requirements, integrators don't need to be concerned with controllers being incompatible with the chassis, and valuable slots aren't wasted. Nor do integrators have to specify and secure power supplies and cooling equipment.

Driver Software Included
Another benefit of PXI products is that they must include appropriate driver software. In the past, some vendors would only supply documentation on how to write driver software or nothing at all. This forced integrators to spend large amounts of time both writing the driver software and learning the nonessential product details needed to do so. It also excluded integrators who were without the programming ability to write drivers. PXI has changed this by placing the burden of driver software development on the vendor, which is where it belongs.

The modular architecture of PXI makes maintenance and serviceability easier. In the event of a failure, PXI modules can quickly be replaced. In addition, PXI requires and recommends a variety of environmental tests to ensure the reliable performance of an ATE system. Faster system integration, more reliable performance, and simplified maintenance lower the total ownership cost of ATE systems.

With so many benefits, the best attributes of PXI are sometimes overlooked. With PXI, ATE systems can be built to provide better test coverage and improved test time. PXI runs the high-speed PCI bus and has high-performance timing and triggering features. The PCI bus can operate at 132 Mbytes/s, while the VXI is limited to 40 Mbytes/s and GPIB to 8 Mbytes/s. PXI contains all the necessary extensions to increase bus bandwidth to 256 Mbytes/s without the obsolescence of current PXI products. This higher data throughput permits higher test throughput. Therefore, more products can be tested in less time. Fewer test stands may be used to test the same amount of product. Furthermore, extensive testing that was impossible or too time-consuming in the past can now be implemented.

Although PXI holds many advantages for ATE, it doesn't meet every ATE need. PXI is only a few years old, while VXI and GPIB instruments have been produced for years. Hundreds of instruments and switch products exist for PXI, but the selection has not reached the level offered in VXI or GPIB. For example, over 30,000 GPIB instruments have been designed, many of which are for highly specific applications. The 50-plus member companies of the PXI Systems Alliance are constantly increasing the number of products available for PXI. Their web site, www.pxisa.org, has links to the products offered by each company.

The small size of PXI is not always ideal for all technologies (i.e., switching supplies). In cases where GPIB instruments or VXI seems appropriate, a hybrid system can be used. An effective hybrid system may contain PXI as the center of a test system along with a highly specific GPIB instrument and VXI switching. You might consider creating a hybrid test system if the need or desire exists to leverage off existing test equipment. With a hybrid system, users can benefit from the low cost and simple integration of PXI, the larger form factor of VXI, and the application-specific instruments of GPIB. Table 1 compares GPIB, VXI, PCs, and PXI.

Several technologies exist to create hybrid systems with PXI, PCs, VXI, and GPIB. MXI-3 is the fastest, most flexible technology for controlling PXI systems from standard PCs or from other PXI systems. MXI-3 effectively splits a standard PCI-to-PCI bridge with a high-speed 1.5-Gbit/s serial link. With MXI-3, it's possible to implement ATE systems that have PXI modules in different chassis while the system performs as if the modules are in the same chassis. MXI-3 is ideal for adding additional slots to a PXI chassis or connecting PXI chassis that are up to 200 m apart.

Control VXI From PXI Or PC
MXI-2 and IEEE-1394 are technologies for controlling a VXI system from a PXI device or a PC. MXI-2 is the highest-performing control solution in all categories of VXI data transfer, including large transfers, message-based transfers, and random register accesses. IEEE-1394 is optimized for performing large transfers and is a lower-cost control solution. GPIB interfaces exist for PXI, PCs, and VXI. Figure 2 illustrates an example configuration integrating PXI with GPIB and VXI.

With PXI, we've taken a giant step forward in the development of smaller, lower-cost ATE systems. As more and more PXI products become available, PXI-based test systems will continue to decrease the size and cost of ATE.

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