VXI Test—Applications From the Front Line

In less than 10 years, VXI technology has become a significant force in the test industry because it supports high-speed data rates to 100 MB/s and offers communication protocols that ensure high throughput. The modular instrument standards and open architecture permit easy integration of apparatus and allow you to solve applications with equipment ranging from portable field testers and remote data acquisition systems to high-performance combinational and functional testers.

EE asked some VXI manufacturers to share their hard-fought battles from the trenches of test. The applications focus on recording measurements in a medical environment, synchronizing subsystem timing, testing circuit cards that required an easily reconfigured system and converting 300 manual test procedures to an automated routine. These are just a few applications that vendors have turned from a difficult test situation into a success using a VXI system.

A Personality for Automation

A customer of MicroCraft needed to reduce downtime losses that were approaching $18,000 per hour. To lower costs, the customer decided to convert 300 intricate and cumbersome manual test procedures for PCBs to an automated VXI system that could perform test, troubleshooting and burn-in functions for each of the boards.

The difficulty was finding a method to test the variety of PCBs. Although all the boards shared a common edge connector, each board’s pin-out was unique, requiring special testing.

For example, one family of boards that needed special tests used a 96-pin connector. Originally, the company considered using a switch matrix to access the system instruments and signal sources for all 96 pins. But a switch matrix that was large enough to accommodate all interconnection requirements was cost-prohibitive. Routing the signals on a fixture-by-fixture basis for 300 fixtures also was too expensive.

MicroCraft’s solution was a fixture with a PCB personality module (Figure 1). The module helped customize the fixture for each of the boards. It allowed a technician to route the stimulus signals from the test system to the correct pin on the board-under-test. A 96 x 4 matrix switch was used to maintain flexibility and provide programmable switching of signals from the test instruments to any of the 96 connector pins.

MicroCraft converted the 300 manual test procedures to the automated system using LabWindows®/CVI software to develop a library of routines to perform the measurements for the PCBs. The routines were put in a sequence using the ExcelR spreadsheet program. MicroCraft also wrote a test executive to allow the customer to convert the manual test procedures to a macro language for automated program.

The spreadsheet enabled the customer to generate test procedures by specifying the instrument to be used, the measurement to be taken and the data acquisition channel to use. By sequencing these macros, nearly one-to-one conversion of manual to automatic procedures was achieved and accuracy and repeatability improved measurably.

MicroCraft Corp., (919) 872-2272.

Switching Off the Noise

Discussions about switching modules in VXI systems often center around problems encountered with high-frequency and noise-free switching. Rarely are applications needed for low-frequency and low signal strength in a low-noise environment.

A company trying to measure low-frequency signals produced by a human heartbeat found it difficult—if not impossible—to find these signals amid the noise. They assumed that a low-frequency switch could perform the measurements because the signals were typically 100 Hz. After integrating the hardware and the software, however, the heartbeat could not be measured. It was lost in waves of overlapping frequencies.

ASCOR was asked to help quiet the noise. The company quickly determined that the noise from the VXI switch module, coupled with the switching power supply, overwhelmed the heartbeat pulse so it could not be measured.

Although a low-frequency switch seemed to be the logical choice to acquire the signal, ASCOR recommended a high-frequency line of switches with electromagnetic shielding, specifically a 35-MHz 120 x 1 switch tree and a 240 x 1 switch tree. Typically, these products are used with DMMs and other scanner-type applications, but the switches provided the needed shielding and allowed the customer to measure the low-power signal of the heartbeat.

This application is a good example of the axiom that a chain is only as strong as its weakest link. Or in this case, a test system is only as good as the switching interface that routes the signals from the UUT.

The switching element of a VXI system is as important as the instruments selected. For example, there is little benefit in selecting a 5 1/2- or 6 1/2-digit voltmeter, a 10-digit counter or >200-MHz digitizer if the switching interface can’t provide the signal quality needed to gather the measurements from the selected equipment. ASCOR Inc., (510) 490-8819.

Counter Module Controls the Flow of Critical Applications

Developing tests for unique, sophisticated products is an unenviable task fraught with problems that can take countless hours to uncover and resolve. This was the type of challenge Sandia Laboratories offered to C&H Technologies.

Sandia Laboratories developed a VXI-based test system to verify products that required major efforts to disassemble and replace components whenever retesting was needed. It incorporated many special testers with critical timing integration needs, including a centrifuge with a 50,000 lbf capacity arm to rotate the product during test.

The various test subsystems used VXI technology with embedded controllers running a real-time operating system. Application code was written in C, developed on a UNIX® workstation, and cross-compiled and downloaded over a network to the VXI controllers of the test subsystem.

The major subsystems included the system test equipment (STE) controller, the radar tester, the high-speed data acquisition subsystem, the low-speed data acquisition subsystem and the calibration subsystem (Figure 1).

The STE controller communicated instructions and commands to other subsystems over a dedicated eight-channel digital fiber optic control bus using a VXI module developed by C&H Technologies. The high-speed data acquisition subsystem acquired signal data above 100 kHz and used a group of multichannel digitizing oscilloscopes with data sampling rates up to 1 GHz.

The low-speed data acquisition subsystem captured data at rates to 100 kHz, using the VXI TTL trigger line for sampling control. The calibration subsystem communicated with the other system to verify and calibrate the signal path.

The critical nature of the testing required tight synchronization between the various test subsystems. The timing could not be so tight, however, that a single subsystem failure caused the complete system to fail. For this reason, a central clock source was deemed inappropriate. Instead, each subsystem contained its own stable time base provided by the VX491C Counter/Clock from C&H Technologies (Figure 2).

To avoid recalibrating the system between successive tests, a clock with accuracy and stability of 1 x 10-8 ppm was required. If the primary time base failed, dropout detection and automatic switch-over to a VXI backplane clock would continue collection of data.

During a test cycle, the STE controller subsystem was used to sequence and coordinate the other subsystems. The test actions occurred at known or predefined times in the sequence. The times were preloaded into the 32-bit registers of the VX491C and allowed to count at a prescribed rate with interrupts, or on match conditions.

The VX491C provided the time, date and alarm-interrupt capability to time-stamp data files. The data acquisition subsystems used the six counters and programmable frequency dividers provided by the VX491C for test data time-stamping and sample frequency control.

Production versions of the counter/clock were incorporated into the test system and are performing successfully. C&H Technologies, Inc., (512) 251-1171.

Smart Test System Configuration

A Racal Instruments customer testing a variety of circuit cards in a depot environment required a versatile and reconfigurable ATE system that met a broad spectrum of stimulus/response requirements. The customer also needed to replace some existing manual test equipment and procedures with ATE, hoping to decrease test time and cost and increase repeatability and accuracy.

LabVIEW® was selected for general test-program development. The Racal Freedom Series includes the required LabVIEW device drivers for every instrument in the system.

Since the customer was moving to ATE, a programmable switch system needed to be defined to satisfy both immediate and future needs. A system was designed using Racal’s Freedom Series Designer (FSD) software. It provides a WindowsTM-based environment for configuring ATE systems. The FSD also automatically generated the system wire list, the paths to and from the instruments to the interface test adapter (ITA) receiver, parts list assembly documentation and the ITA pin configuration necessary for UUT test-adapter design.

The customer’s system included modular UUT power supplies; an interface test-adapter receiver; cabling; precision power supplies; an RF network analyzer; an oscilloscope; system power distribution and cooling; and a VXIbus chassis with a DMM, a counter/timer, a 64-channel digital I/O module and Racal’s VXIbus switching subsystem.

During system configuration, the FSD tabulated the power and cooling requirements of each component. The FSD presented the information numerically and graphically. This enabled Racal to ensure that the system contained sufficient resources during the initial configuration of the system.

The software warned the user if equipment limits, such as pulling too much current from the power distribution controller, were approached. This allowed the customer to select a new piece of equipment that met the requirement.

The system also told the user if the system had too much capability, including excess power and cooling capability in the VXIbus chassis. The data showed that a lower powered and more affordable choice could be substituted. After completing the design, the FSD calculated mean-time-to-repair, mean-time-between-failure and system average maintainability.

The FSD allowed the customer to specify and configure the system on-site. It helped document the system to ANSI standards without imposing the cost of nonrecurring engineering. The system also gave the user an opportunity to design the interface test adapters and test programs immediately by documenting the system during the configuration process. Racal Instruments, (800) RACAL-ATE.

Copyright 1995 Nelson Publishing Inc.

May 1995

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