A system-of-components approach to functional test system design

When developing a functional test system, there are many choices for instrumentation, interfaces, and software. A good design maximizes throughput, minimizes development time, and allows for easy future upgrades.

Maximizing throughput involves more than simply selecting fast instruments. Minimizing development time goes beyond selecting the latest software. Systems designed to accommodate more instruments, switches, and DUTs do not have to be completely redesigned, which would require more large chassis. Using system components in the design can help address these issues.

System components include system-off-the-shelf instruments, industry-standard software, and industry-standard I/O . System-off-the-shelf instruments must have the following characteristics:

Industry standard software is readily available and can be used anywhere. Standard software has a good support network. Especially Microsoft 's Visual Studio.NET firmware.

Industry standard I/O consists of interfaces that have public specifications, broad acceptance, and general availability. GPIB serves a purpose and is being rapidly replaced by Ethernet and USB . Fire Wire remains a valid option. They are all recognized as IEEE standards.

To illustrate the process of system component-based design, the following example uses the design of a low-frequency, medium-pin-count, medium-power module ( such modules are commonly used in the automotive and aerospace / defense industries ) as an example.

design

The first step in good design is to design an architecture that can meet current needs and accommodate future growth. For maximum flexibility, designers choose external PCs rather than embedded PCs . Mix modular instruments with industry-standard interfaces and rack-mounted instruments. To handle future needs, leave 20% of the slots or extra rack space for larger racks or nearby instruments.

To avoid placing slow switches in a card rack designed for high-speed instruments, place switches in isolated subsystems. To minimize wire length and rack space, place the bulk of the interconnection in the front of the switch subsystem. To minimize development time, develop applications using Visual Studio .NET with instrument extensions and standard base drivers.

Once the high-level architecture decisions are made, the engineer focuses on the detailed instrumentation requirements appropriate for the specific DUT , in this case an electronic conditioning module with 14 pins on three connectors . Based on the test performance specifications, the following instruments are required:

Based on these requirements, the test system block diagram is shown in Figure 1 , including a rack-mounted arbitrary waveform / function generator and a dedicated switch card box or switch box. And a 4 -slot VXI rack is selected, which includes a digitizer, a 16- channel DAC , and a high-speed DMM . An RS-232-C- based CAN interface is placed on the rack behind the PC . A second DMM with a panel is used during debugging .

This system has 4 GPIB instruments: power supply, switch box, oscilloscope, and DMM . A USB/GPIB converter was added. Therefore, no PC slot is required for the GPIB interface . Fire Wire was chosen to control the VXI instruments because it is a fast industry standard interface.

Connect the arbitrary waveform / function generator to the PC 's LAN using a crossover cable . Adding an interface or router allows the system to handle more LAN- based instruments. The LAN provides the opportunity to remotely view and edit configuration information using the instrument's built-in server.

Connect instruments efficiently

The next step is to determine the best way to physically connect the components in a way that will allow for further expansion. Figure 2 shows the entire switch subsystem.

With the short array method, engineers can connect any instrument to any DUT pin and easily add instruments. Except for the CAN bus, all connections to the DUT are switch transitions, which makes pin-to-pin measurement connections possible.

In such complex devices, ground loops, parasitic current paths, short circuits, open circuits, signal loss and stray capacitance are common problems. These problems can be solved by the following methods.

Any test engineer should carefully consider the extensive interconnect or fixture systems of the DUT .

For small DUTs , simple clip leads are sufficient. However, adding an interface board is appropriate for the following reasons. First , the interface board provides a physical configuration for mounting interface components ( such as terminal units, fuses, and user electronics between the system and the DUT ) . These components can be mounted to the interface structure or structure frame. Without a large number of interconnections, it is desirable to place these components elsewhere. Second , with terminal units, it is easy to modify the wiring when the DUT is changed and obtain the appropriate test connection during debugging. Third , it provides a means to quickly and robustly change connections for different DUTs using the same system .

Switch conversion structure

For a given test, the maximum number of measurements or stimulus sources that must be applied at the same time determines the best switching structure. A 4- wire bus was chosen because it allows 4- wire impedance measurements of the DUT using the DMMC .

By routing two matrix points, POT1 and POT2 ground ( Figure 2) to the same pin of the DUT , the impedance measurement is very accurate because the remote measurement location is in the DUT . Instead of using 2 wires, less accurate 4- wire ohms measurements are still possible in the relay matrix.

It is seldom necessary to have more than 2 isolated instruments or 4 single-ended instruments active at the same time, since electronic modules usually contain a built-in test routine that performs one function at a time. However, a fifth bus can be added as a common reference for single-ended devices ( such as an oscilloscope or floating devices ) . For this purpose, a 5- wire measurement bus can be selected .

When using a matrix, multiple signal sources can be connected to the same pin. It is important that these signal sources do not accidentally short together. The switching program should be carefully programmed to eliminate the possibility of a short circuit or to provide an alarm when a short circuit occurs.

Special care must be taken when using an oscilloscope. As an earth-referenced device, the oscilloscope requires a star ground connection to the chassis. This task is handled by relays ( see 1a/1b in Figure 2 ) .

In addition, the oscilloscope cannot measure the binned drive because it is a floating H- bridge circuit. It can measure Mot+ or Mot- relative to ground , but it cannot measure MOT+ to Mot- , so the system requires an isolated digitizer.

Although the test system of Figure 2 is suitable for 16 channels, only one DAC channel is shown connected to the matrix. It is generally the case that all DAC lines are controlled to a general interconnect so that they can be connected to different DUT pins in the fixture .

This method does not provide the flexibility to programmatically connect the DAC to any pin. If more simultaneous DAC signals are needed and you want to permanently assign them to the DUT pins, you can feed the simultaneous DAC signals into an expansion matrix.

Utilizing the drive

Combining many interfaces together can cause programming problems. Using Visual Studio.NET with specialized instrument libraries , control programs can easily communicate with various types of instruments. Several drivers are available that run under VS.NET .

In particular, IVI-COM developed by the IVI ( Interchangeable Virtual Instrument ) Development Group is based on Microsoft 's Component Object Model (COM) standard. It also requires controller independent software modules determined by the VXIp1ug & Play Systems Alliance , which include VXIplug&Play drivers and VISA (Virtual Instrnment Software Architectrre) I/O libraries. With these drivers, engineers can compose applications with highly independent hardware.

In addition, by combining VXIplug & Play with Microsoft 's IntelliSense function, it is easy to make the best command in the VS.NET environment to obtain the best configuration that meets the required speed, accuracy and sensitivity. Using IntelliSense , the user can write the symbolic name of the instrument ( such as MyHp34401) and a decimal point; then, the menu presents all the useful instrument functions and descriptions. If the function requires any parameters, these parameters can be presented with their data types.

In the design and development environment, engineers need to develop tests using graphics and text-based languages. In manufacturing, standards are the basis for test execution, and engineers can write pre-written programs using a mixed language of graphics and text. Specific examples are using Visual Basic and Visual C++ , VS.NET to provide a wide range of support environments for testing and GUI development.

Conclusion

Designing a functional test system requires pre-planning for system flexibility, scalability, and speed. The system already has instruments, industry-standard software, and industry-standard I/O to connect the process into a whole.