The VXIbus in a manufacturing test environment - VMEbus Extensions for Instrumentation, Standard Commands for Programmable Instruments - Technical

Hewlett-Packard Journal, April, 1992 by Larry L. Carlson, Wayne H. Willis

Engineers at HP's Loveland Instrument Division have found that using the VXIbus and the SCPI programming language provides benefits such as reduced test development time and system support costs.

The adoption of an industry-standard modular instrument architecture was the natural evolution for both the defense and commercial industries. Now, VXIbus is fast becoming one of the modular architectures of choice and VXIbus vendors are providing users with a broad choice of test tools such as instruments, switches, DUT interfaces, and special-purpose functions. Recently, new products with test capability not available in the HP-IB have emerged. VXIbus users have access to the same compatibility benefits the industry has enjoyed for years integrating HP-IB instrumentation. VXIbus along with the new standard instrument language, SCPI (Standard Commands for Programmable Instruments) promises users greater flexibility for building systems that are much easier and less costly to reconfigure or upgrade to meet growing test requirements.

This article focuses on the benefits of the VXIbus and SCPI in a commercial manufacturing application for functional test. The information is based on the implementation of a new test strategy and experience gained at HP's Loveland Instrument Division where precision digital multimeters and modular instruments are manufactured. This article also explains how the VXIbus and SCPI are implemented to reduce test development time and system support costs. It examines the importance of these new standards in building a standard test platform that is designed to be easily upgraded and configured for a variety of testing applications.

Functional Test on the Manufacturing Floor

The manufacturing profile at HP's Loveland Instrument Division is characterized as low-volume and high product mix. This means that production volumes are on the order of 10 to 100s per month and the total number of different products is high, more than 250. Production characteristics such as these typically mean that the cost of testing can be very high because of the need for a wide variety of test capability. Both in-circuit and functional test are employed in this production situation. Functional test is almost always used because of the need to do calibration and in-circuit test is used when the economics justify it.

Functional Tester Characteristics

Historically, modular instrumentation has been an important part of the test strategy. Test systems were typically built using HP-IB rack-and-stack instruments along with modular instrument products for housing switching and some functional capability such as digital multimeters, counters, and digital I/O. Lack of a large breadth of modular products to choose from was one of the obstacles to taking full advantage of these proprietary modular systems. Furthermore, their obsolescence meant that few, if any, replacements were available for upgrading the system, and long-term system maintenance was a problem. An industry standard architecture such as VXIbus solves the breadth of product offering and obsolescence issues.

Custom engineered products are typically built into test systems to meet special measurement and control needs for functions not commercially available. Custom equipment is expensive to design and build and costly to document and support. Modular instrument systems have provided a vehicle for custom designs because of the availability of breadboard modules that are powered by mainframe power. The package, interface, and documentation are provided by the module supplier leaving the task of designing and documenting the unique circuitry to the test engineering design team. Breadboard modules are usually available for most modular systems including VXIbus. However, VXIbus development tools (not available for the earlier modular systems) facilitate the job of custom design. The real advantage of the VXIbus is that it protects custom design investments because they can be used on other compatible test platforms on the manufacturing floor. On the other hand, because of the broad choice of VXIbus (and compatible VMEbus) functions available on the market, the need to build custom functions is greatly reduced.

Standard Test Platform

In the last ten years a large number of different test systems were developed to test the various products introduced into manufacturing. There is a very high cost associated with maintaining and supporting so many test systems. Some of the older test subsystems have become especially difficult to support because of poor parts availability and equipment obsolescence. Today, products are being developed at a faster pace, and competitive pressures are driving the need for shorter time to market. Therefore, test system development time and cost must be reduced. Also, the number of test platforms required to test a rapidly expanding mix of products must be reduced. This requires devising a test strategy that not only includes an upgrade path, but also allows a sufficiently equipped system to be reconfigured easily to test any one of many DUTS.