New Switching Technology Reduces Size and Cost of ATE

Many different types of ATE are available to help manufacturers improve production quality and throughput. Although these test stations are used in different applications, they share one common denominator—a switching system—that directs the input/output traffic between the test instrumentation and the devices being tested.

The budget allocated to ATE for manufacturing test, or even for service and repair, is considered part of the overhead in bringing the final product to market. As a result, reducing this cost is an important consideration. The goal of meeting the budget can be achieved by decreasing the purchase price of the ATE station or increasing its throughput; that is, increasing the number of units being tested per day.

With advancements in technology and competitive constraints increasing, the need to drive overhead costs down has intensified. Modular, high-density VXIbus instruments have helped improve the size, cost, and throughput of VXIbus systems because virtually every test station now being designed uses modular VXIbus instruments.1 Today, 10 years after the introduction of the first VXI switches, modular signal switching technology has been introduced and is continuing to improve the cost/benefit ratio of the production test cycle.

Hardware Trends in VXI

While the VXIbus standard is only a decade old, it is far from being mature. Manufacturers finally are designing products specifically for the VXIbus.

This trend, started by the VXI Modular Instrumentation Platform (VMIP), takes maximum advantage of the real estate available on VXI cards by placing multiple high-performance instruments in one C-size slot. To gain market share, many other manufacturers are jumping on the modular-design bandwagon as quickly as possible.

Currently, there are modular platforms available for high-end instruments such as 6.5-digit DMMs and high-density DACs (VMIP), platforms to adopt existing standards into VXI such as Industry Pack (IP) and M-module card carriers, and platforms for data acquisition (ProDAQ).

Modular designs also are starting to be used in the area of signal switching, leveraging off the success of modular instrumentation. These are some examples of modular switching systems:

Hewlett-Packard M-module Carriers and VXI Technology VMIP Series, both with switch modules that can be integrated with instruments in the same VXIbus card slot for low-density switch applications of approximately 20 channels.

The Switch Modularity and Interface Platform (SMIP Series) for high-density applications from VXI Technology.

The future will see these trends continue as the market creates competition. Consumers will benefit from a reduction in the cost of instruments and switching systems as well as the capability to perform many more tests in a single VXIbus chassis. This makes VXI even more ideal for portable test applications and commercial product testing.

Advantages of Next-Generation Signal Switching

There are many standards and platforms for ATE, and price and performance are key factors in determining the correct platform for the application. Whenever a large amount of signal switching is required in ATE, VXIbus makes cost-effective sense and, in many cases, helps justify the decision to select a VXIbus system. While VXI products have made measurable improvements in ATE signal switching, the next generation of signal switching systems is vaulting the VXIbus to a higher level in terms of performance, density, and value.

Most first-generation VXIbus switch cards contain one switch configuration per card slot (e.g. 32 SPDT, 64×1 two-wire scanner, 4×64 matrix), very similar to first-generation VXIbus instruments. These switch systems typically are GPIB/VME-based products converted to the VXIbus. For example, the Racal Series 1260 is derived from the company’s 1250 GPIB Series, and HP’s cards are from the GPIB HP3235A System.

Next-generation signal switching systems are designed exclusively for the VXIbus. They take advantage of the recent advancements in relay and driver circuitry technology which provide higher performing switches in a smaller footprint.

An example of a next-generation switching system is the SMIP Series, where up to two different switch configurations can be accommodated in one C-size slot and up to six switch configurations in a double VXI card slot. This provides a level of modularity that is unfamiliar in first-generation VXIbus cards and reduces the number of slots used in a VXIbus chassis.

A recent application, testing engine control modules, called for the following switch requirements and demonstrated the size and cost advantages of next-generation switch systems:

A 12×24 matrix to connect an engine control module I/O to a test station.

A 64-channel mux connecting a DMM to multiple measurement points.

240 Form C relays for high-density, general-purpose switching.

The two proposed VXIbus switch systems are detailed in Figure 1. The first-generation system occupied seven slots; the modular switch system used three. The modular system was selected because an additional four slots were freed up for instrumentation purposes, and the switches were rated for 2 A as opposed to 1 A for the switches in the first-generation system.

This system could have been reduced to a six-slot chassis if portability had been an issue. Subsequent hardware costs were reduced 30% to 40% because the modular system shared resources, such as connectors, a VXIbus interface, and sheet metal.

The VXIbus reduces test cycle times because the instruments and switches share a common backplane. The VXI specification reserves eight lines dedicated for triggering back and forth between instruments. VXI switch cards that use scan lists (a sequence of relay states) then can be triggered to advance through this list by an instrument on the bus.

For example, test specifications of a device often require the verification of continuity or isolation between connector pins. Most often, a DMM is used to verify the results. The continuity/isolation test can involve a large number of pins. As a result, the DMM needs to be switched to a connector pair and take its measurement before moving to the next pair.

This sequence of events can take a significant amount of time, particularly if preprogrammed scan lists are not used due to the overhead in handshaking between the DMM and the switch card. By using a VXI-based card with a scan list, all handshaking can be done across the backplane.

Since the scan list is stored in memory, no software overhead is incurred. Large channel counts can be scanned in a fraction of the time that it takes GPIB-based systems to do the same task.

Traditional VXI-based switch cards implement scan-list capabilities in two ways. A message-based switch card requires an intelligent interpreter that usually takes the form of a plug-in card. Scan lists are stored on this card, and message-based commands are parsed by the interpreter. Traditional register-based switch cards must have a driver downloaded to the Slot-0 controller, and this driver acts as the interpreter. In both cases, ASCII strings are sent via the host controller to set up the switch card and scan list. There also is a relatively substantial amount of latency between the time a trigger is received by the switch card to the time that the card issues a trigger to the backplane indicating that the relays have settled (25 to 40 ms).

Next-generation switches, such as the SMIP Series, build the intelligence into the hardware registers. The time delta between TTL trigger in (command to close the relays) to TTL trigger out (indicating relays have settled) is reduced to the settling time of the relay itself which is about 3 to 5 ms.

An example highlighting the effectiveness of this technique was demonstrated by a defense contractor exercising a missile simulator. As shown in Figure 2, the stimulus was applied to the sequencer via the normally closed contact of a Form C relay on an SM5002 card. The sequencer processed the data and issued an initiate-fire command, sending out a pulse on TTL0 of the VXI backplane.

The time-stamp module recorded the TTL0 pulse as t0. The SM5002 also detected the pulse on TTL0, and the relay changed state. The SM5002 issued a relay-settled pulse on TTL1, which the time stamp recorded at t1. Dt (t1 – t0) needed to be less than 10 ms to meet specifications for this application and was verified to be less than 3 ms. This was compared to a switch card using an intelligent interpreter plug-on card which had a Dt of more than 40 ms.

Conclusion

In the past decade, VXIbus switching systems have become a significant part of ATE because of the capability to fit a large number of switches on a card. But test-instrumentation consumers continually demand product that is faster, smaller, and less expensive. Innovative hardware designs greatly ease the burden placed on production engineers by cutting down on test cycle time and reducing hardware costs.

Modular VXIbus switch products are leading the way to meet the expectations of customers by taking advantage of the latest relay technology yielding higher performing switches in a smaller footprint. Modular switch cards provide the flexibility to design a system that optimizes space and keeps down the overall price.

Reference

1. Dhillon, P., “More Instruments Per Card Fuels VXI Developments,” EE-Evaluation Engineering, January 1996, pp. 56-62.

NOTE: This article can be accessed on EE’s TestSite at www.nelsonpub.com/ee/. Select EE Archives and use the key word search.

About the Author

Paul Dhillon is the executive vice president of VXI Technology. He has been involved in VXI test and measurement since the introduction of the VXI standard in 1987. Mr. Dhillon earned a bachelor’s degree from Kingston University, U.K. VXI Technology, 17912 Mitchell, Irvine, CA 92614, (949) 955-3041.

Copyright 1998 Nelson Publishing Inc.

October 1998

Sponsored Recommendations

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!