Configuring Functional ATE Systems

Many years ago, when a company's product was in the manufacturing cycle and needed to be tested, it was common for product verification to be performed manually by test technicians or operators. Test procedures detailed required test equipment necessary for running the test. It was not unusual to visit a factory test floor and find storage lockers or racks full of multimeters, oscilloscopes, custom load boxes, power supplies and test leads that were used during verification. Manual test introduces the element of human error, and increases the chances of false failures or worse yet, passing faulty product. The introduction of the general purpose instrumentation bus (GPIB) as an industry standard a few decades ago, allowed test designers to implement the automation of product verification, thus increasing confidence in test and also greatly improving product throughput.

Your ATS may be required to be transportable in the field. In this case, a smaller size is a definite advantage, as is durability. For data acquisition, there are a number of notebook-sized signal conditioning cards that communicate directly to a laptop. For test and measurement, high-density instruments on the VXI platform such as the VMIP™ can also be controlled by a laptop. Small VXIbus chassis typically come with a handle for portability and weigh around 20 lbs.

Are there environmental concerns?

Extremes in temperature and humidity can become a major disruption to the test process. Instrumentation is usually specified by the vendor to meet prescribed accuracies at nominal temperatures and humidity. If testing is to be performed in a very hot environment, it is critical to ensure some form of cooling method, otherwise, the test equipment might not meet the stated specs and false failures could result. Additionally, shock and vibration specs might be necessary for portable applications. Vendors should be able to provide this environmental data.

What is the mix and volume of products to be tested?

If a number of different types of products need to be supported by the same tester, or there is a large volume of product to be tested, then the interconnect from the UUT to the instrumentation I/O must be carefully constructed to minimize test set down time due to faulty connections. This includes broken pins on instrumentation, broken cabling, etc. You may want to consider an efficient method for mass interconnecting such as MacPanel or Virginia Panel interconnect assemblies.

Can you make use of signal switching?

I/O. Multiplexers or switch matrices may allow you to limit the number of dedicated test resources you need to purchase, which in turn reduces the overall cost of the system. The VXIbus has proven to be the defacto standard for switching applications because of the incredible densities that can be achieved as well as the ability to handle low-level or high-voltage and current requirements

Will the system need to be expandable?

Accounting for expandability for future growth is critical, particularly when floor space is limited. If the ATE rack is fully loaded, new requirements may necessitate adding another rack which might be difficult in tight spaces. It is always a good idea to allow for future growth in the initial system layout. If a VXI-based subsystem is utilized, it is recommended to leave a couple of slots open (unpopulated) for a seamless expansion path.

How soon do you need to be up and running?

And also, how much test expertise is in house? If there is a very aggressive schedule and manpower is scarce, there are many ATE system providers available who can build to print or provide a turnkey system including all test software. Documenting the system is also an unwanted burden and ATE system providers have years of experience in providing fully documented test stands utilizing VXI products.

What software platform do you plan on using for test development?

Typically this is answered by what expertise is in house. However, there are a number of languages (such as CVI, LabVIEW or VEE) that are tailored toward the test development environment. The graphical languages (LabVIEW or VEE) will require some level of training and months of working knowledge before developing the proficiency required for completing even middle-

of-the-road applications. Additionally, VXI products are Signal switching is typically the heart of every ATE provided with VXIplug&play drivers that provide and API as it is responsible for routing test assets to product

that can be implemented in other popular languages such as Visual C++ and Visual Basic.

The brain of any test stand is the CPU since it manages all aspects of test execution and data processing. The only real question here is whether to take that 'brain' and embed it in a chassis, or employ a standalone or desktop unit.Table 1. can be used to assess whether an embedded or standalone processor is a better fit for your application.

In general, for most functional test applications, cost is usually somewhat of a factor, as are maintenance and sparing, while portability is not as critical. Therefore, unless there are some unusual circumstances dictating otherwise, a traditional PC is a more logical approach.

Software is typically developed in either text-based languages such as C/Visual C++ or Visual Basic, or graphical languages such as HP VEE or National

Instruments LabVIEW. Experienced instrument vendors will develop a software driver set that allows their devices to work in any software environment. Instrument drivers take the form of a library (like a Windows .dll) that programmers can link to and call instrument specific functions. Most VXI vendors will supply VXIplug&play drivers with their product. A VXIplug&play driver includes a virtual instrument soft front panel for direct control of a device, a collection of functions in a compiled .dll for program use, and a help file that quickly brings programmers up to speed on device capabilities.

Software is typically developed in either text-based languages such as C/Visual C++ or Visual Basic, or graphical languages such as HP VEE or National Instruments LabVIEW. Experienced instrument vendors will develop a software driver set that allows their devices to work in any software environment. Instrument drivers take the form of a library (like a Windows .dll) that programmers can link to and call instrument specific functions.

Most VXI vendors will supply VXIplug&play drivers with their product. A VXIplug&play driver includes a virtual instrument soft front panel for direct control of a device, a collection of functions in a compiled dll for program use, and a help file that quickly brings programmers up to speed on device capabilities.

Two major software development efforts for ATE include a test executive and the test program sets. A TPs is developed for each type of product being tested. The TPS may contain the sequencing of steps necessary for running the test from start to finish, as well as making pass/fail determination and logging data. However, if multiple product types are to be tested on the same stand, the sequence management, pass/fail determination and data logging would need to be duplicated for each test program. This is where a test executive fits in. The test executive manages all generic operations, while each TPS will contain product specific code. A well written test executive can also be beneficial during product troubleshooting by allowing easy access to breakpoints, looping etc. A test executive should be viewed as an asset that is developed once, and all test program sets are modules that are called by the executive. IVI drivers (interchangeable virtual instruments) have emerged as the next level of drivers within the software community. This is a natural extension of the existing plug and play drivers. IVI drivers rely on instrument class specifications that are common within a particular type of instrumentation (e.g., multimeters). The biggest benefit of writing code using an IVI layer is that the code becomes vendor independent - you no longer need to worry about obsolescence. Additionally, IVI drivers offer performance benefits such as state caching. For more information on how IVI drivers might work for you, visit the IVI website at www.ivifoundation.org/.

The bus interface is the means through which the host controller communicates with the instruments in a test stand. The instruments may have a GPIB (IEEE488), Ethernet, FireWire (IEEE1394) or VXI-MXI2 interface, for example. The rate of data transfer and communication speed varies as does the cost. RS232 is the simplest form of communication, least expensive and also the slowest. GPIB has been in existence for almost 30 years and offers data transfer rates at about 1MB/s. FireWire achieved its credentials in the consumer electronics market and has seen limited usage in the industry, primarily in data acquisition applications. It is a high-speed serial link and offers data transfer rates in the neighborhood of 14 MB/s, although there is a first byte latency that is apparent when passing commands. MXI-2 offers the fastest bus speed, rated around 20 MB/s-25 MB/s. A VXIbus FireWire can communicate to the host controller through GPIB, MXI-2, and FireWire interfaces. These interfaces populate the leftmost slot of the VXI mainframe and are commonly referred to as Slot 0 interfaces. In a multiple chassis subsystem using MXI2, each chassis requires a Slot 0 interface; one chassis will connect directly to the host, and the rest will be daisy-chained together.

Selecting Instruments

Once a test requirements document has been defined (TRD), it is time to select test instrumentation necessary for powering the unit under test (UUT), applying test stimulus and measuring the response. Test instrument specifications are compared to test requirements to determine which device is the best fit. Many system developers find that circumstances like minimizing footprints or cost will dictate the selection of a particular platform. It is common for a system to contain a mix of platforms. For example, demanding power supply requirements are most often satisfied on the GPIB platform because providing this function in a chassis-based system presents many difficult challenges. If there is a large mix of instrumentation, and/or higher channel count requirements, the VXIbus offers the best cost and performance solution. Modular VXIbus designs, like the VMIP™, allow up to 36 discrete test devices in a minimal amount of space.

Handling Special Cases

Occasionally test requirements may be unique and there will not be test instrumentation available off-the-shelf that will be able to provide the necessary functionality. This is where communication between the product design team who defines the test requirements and the test development team in charge of implementing the test is crucial. It is often the case that the product designers have written difficult to perform tests into the TRD that are not absolutely necessary for acceptance testing. In these cases, it is much more efficient for the test team to determine which tests may be eliminated from the test procedure. The alternative to this is approaching vendors for custom or modified solutions, which more often than not results in non-recurring engineering expenses. VTI Instruments started business as a “custom engineering house” and still maintains a custom engineering group. In addition, prototyping modules on the VMIP™ allow users to implement custom designs on an open-architecture platform.

Once the racks are populated with the test instrumentation, a system designer is left with the task of interfacing the instrumentation I/O with the UUT. It is common practice to develop some sort of cabling method removed and reseated on the instrument front panel. If multiple UUT's are to be tested by the same tester, then a mass interconnect system, whether it be a patch panel or interface connector assembly (ICA) should be considered. An interface test adapter (ITA) which is specific to the UUT, mates to the ICA and provides I/O routing from the instruments to the UUT interface.

An ITA mates directly to the face of the ICA. The face of the ITA may take the form of a patch panel from which cables will connect to the UUT. In board level testing it is common for an enclosure to be mounted to the ITA. This enclosure will house cabling that runs from the ITA to an edge connector that mates to the UUT.

There are obvious advantages to using this type of interconnect system. Wear and tear at the instrument front panel is minimized since all of the insertions take place at the ITA/ICA interface. It is much easier to maintain and service ITA/ICA modules and pins as opposed to returning a test instrument to the vendor for repair. In production environments, these systems prove to be very valuable in reducing setup times since mating the ITA to ICA takes seconds and the need for operator intervention for routing cables is either trimmed or eliminated altogether.