Selecting the Right Cable/Harness Test Architecture

All cable/harness testers check for shorts, opens and interconnection integrity—but that’s where their similarity ends. Quantitative differences are obvious, such as the maximum number of test points available or specific measurement ranges; but major architectural differences also exist which distinctly affect applicability, ease of use and price.

Architecture-dependent capabilities include hipot test, capacitance measurement and data-transmission quality-assessment features. For large harnesses or those with many terminations, ease of use is affected by whether a central or a distributed architecture is used.

Hipot Testing

Testing for shorts, opens and insulation resistance with low-voltage stimuli is adequate for many applications, but regulatory, customer or quality-assurance requirements may dictate otherwise. Hipot tests may be required to ensure there is no arcing or corona breakdown at operating voltage levels or higher and to determine insulation resistance and dielectric absorption.

Hipot tests are typically conducted with a current-limited 1,000-V to 1,500-V source. These voltage levels make it impractical to use inexpensive semiconductor switches as is normally done for low-voltage tests. Instead, relays or special high-voltage semiconductor devices must be used.

Along with using high-voltage switching components, the system must prevent misapplication of high voltages, avoid equipment damage due to external misconnections, and protect operating personnel. These architectural differences make it unrealistic to expect low-voltage cable/harness testers to upgrade to include hipot tests.

Signal Transmission and Capacity Tests

Cables and harnesses often contain twisted-pair, shielded or coaxial conductors to facilitate transmission of high-frequency signals. While crosstalk, attenuation and other transmission-quality factors are best measured directly with dedicated instruments, an indirect assessment via conductive and capacitive measurements is more practical for production test applications.

Consequently, some cable/harness testers include capacitive measurement capabilities. These must be used in conjunction with hardware- or software-based compensating features to zero out the capacitances of the tester and the interconnecting cables.

Capacitance measurements not only help find misconnected wires in twisted pairs and shielded wire misterminations, but also may pinpoint the exact physical defect locations. Details on this subject may be found in References 1 and 2.

Switching Topologies

The test system and the cable/harness/backplanes under test may be interconnected using a centralized or a distributed switching topology.

Centralized (Radial) Topology

In a centralized system, the controller, the test instrumentation and all switching facilities needed to route stimuli and measurement resources to each conductor are centrally co-located. This does not mean that they must be contained in one enclosure, but the system may consist of a PC, a central measurement and switching assembly, and expansion switching units. All cables connecting the test system with the UUT ports emanate from this single location.

Distributed Topology

A distributed system also employs a controller and a central measurement and switching assembly, but most of the switching is performed remotely. The remote switches may be in individual breakout boxes mating with each cable/harness or backplane termination or in several portable suitcase-like multiport switching assemblies. Remote units are daisy-chained and communication between them and the main unit is accomplished via a common bus.

Applications

A centralized system is usually appropriate for testing individual cables and compact wiring assemblies. However, when the product to be tested exceeds the size of a work bench, the sheer number and length of the interface cables may become unwieldy and test accuracy may be negatively affected.

“This is where a distributed bus system has advantages,” said Kevin Steidel, Senior Engineer at Eclypse International. “It allows you to place switching modules at each port rather than depending on extremely long interface cables with differing conductance values. The long cables also are more susceptible to EMI, which could be particularly troublesome when low-level test stimuli are used.”

Whether a few suitcase-type switching assemblies or many compact mating switch assemblies should be used depends on the application. “Generally, you use remote suitcases when testing a large product such as aircraft wiring,” said Karl Sweers, Technical Marketing Manager at DIT-MCO International.

The suitcases are placed at a few selected locations adjacent to clusters of UUT ports. “Then you use short adapter cables to connect the suitcase to the UUT,” Mr. Sweers continued. “Distributed suitcases eliminate the need for running lengthy bulky adapter cables from the central console to every individual port.”

Compact mating switch assemblies, rather than switch clusters in suitcases, are more appropriate for high pin-out applications such as interconnection tests of multiple backplanes.

“In telecommunications equipment, for instance, 50,000 or 100,000 terminations may have to be checked out. Interconnecting all these points with centralized switching bays would be difficult and require a complex adapter-cable management system,” said Eugene Sequeira, National Sales Manager at Computer Systems Technical Support. “But distributed switch modules packaged on extender boards and mating directly with UUT connectors require only a single daisy-chain cable for communicating interconnection information.”

Combinations

Both radial and distributed switching architectures have their place. Some systems can be configured for either or both topologies. The Cabletest MPT architecture, for instance, accommodates both and can combine the two topologies within the same test system. “These combination systems minimize the length of some of the interface cables and provide a radial switching capability at the control area of the UUT,” said Mike Mathews, Director of Sales and Marketing at Cabletest.

Many companies offer centralized as well as distributed test-analyzer configurations. “All of the data communications on our test systems are based on distributed bus technology,” explained Mr. Steidel. “With some changes in enclosures and cabling, our systems can be reconfigured between centralized and distributed at any time.”

Pros and Cons

“With distributed switching, the length and number of interface cables are reduced and the switching is placed very close to the product. This allows for less expensive interfacing and better measurement accuracy. On the other hand, radial switching is generally more flexible,” Mr. Mathews noted.

Centralized switching is preferred when high-voltage testing is required. “High-voltage wiring analyzers often determine critical parameters, such as testing for leakage in the gigaohm range or performing dielectric breakdown tests with 1,500 VAC and measuring currents in the microamp region,” said Frank Piracci, General Manager, FACTâ Systems, at Palomar Products. “For such tests, both the repeatability and total measurement accuracy are significantly improved when you use a centralized switching architecture.”

Karl Zimmermann, President of CK Technologies, cited these relative advantages and disadvantages:

Centralized Switching

Advantages: Systems are generally less costly.

Higher test voltage/test current capabilities are available.

Disadvantages: More complex, expensive adaptation to the UUT is required.

Longer adaptation cables can degrade test quality.

Distributed Switching

Advantages: Switch assemblies can be packaged in the same form factor as Line Replaceable Units (LRU) used in aerospace applications.

Suitcase-packaged units can be placed inside an aircraft cockpit, reducing cable length and simplifying hook-up.

Disadvantages: Complex packaging increases cost.

Dense packaging may limit high test-voltage application.

LRU-packaged switching is application-dedicated and inflexible.

High-density packaging may not allow space for guard components needed for measuring small capacitance values.

Build-Aid Features

Most cable/harness testers are used in factory environments during or following the wiring assembly/termination process. Since it is always better to prevent defects, or at least identify them, as early as possible, many systems include in-process test capabilities and provide operator guidance. For instance, the Cablescan Series 90 identifies wires by an operator’s touch and displays work instructions in color graphics. These types of systems test terminations for both continuity and shorts during the build process.

The wire identification feature applies a low voltage to one end of the conductor and, upon the operator touching the other end, a small current develops which triggers an appropriate message. Alternatively, the operator may be prompted to touch or probe a sequence of wires and be advised how each wire should be connected.

After making all connections, the system performs an overall verification test. If hipot tests are required, built-in features must prevent high-voltage application during the build phase.

Additional Capabilities

Since many harnesses and backplanes include passive and active components, some cable/harness testers are equipped with additional instrumentation. “For example, many cables now have resistor-divider signal terminators which may look like shorts to most cable and harness testers,” said Brian Laine of CheckSum. “With the accurate resistance measuring and guarding capabilities usually found in MDAs, individual resistors can be tested for value rather than for the parallel terminator combination.”

Key parameters for today’s test systems are shown in the chart that accompanies this article. The majority of these include menu-type fill-in-the-blank programming. Programs may be generated by learning interconnection information from a known-good cable/harness, transferring CAD information, or manually entering the parameters.

References

1. Stone, J.R., “Test Techniques for Shielded and Twisted Pair Cables—The Importance of Verifying Capacitance,” Application Note, 1993, DIT-MCO International.

2. “Capacitance Testing Using Wiring Analyzers,” Technical Bulletin, May 1992, CK Technologies.

Copyright 1996 Nelson Publishing Inc.


April 1996


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