The Cable Test Problem
Acquiring the Connection Map
Test Data and Match Data
Checking for Intermittent Connections
PC-Based vs. Standalone Testers
Unique Benefits Offered by a PC-Based System
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1 - a missing connection (an "open"),
2 - an extra connection (a "short"), and
3 - a miswired connection (wrong pin).
Consequently, the most important single test applied to a completed cable assembly is an exhaustive continuity check to ascertain the electrical relationship between each pin of every connector in the cable. Without proper continuity, other electrical tests, such as electrical resistance, insulation breakdown, and impedance, are irrelevant. For many cables, if the continuity is correct, the probability is very high that the cable will function properly, and other electrical tests, if necessary at all, may be performed on a sampled basis.
Defects not revealed by a continuity test include improper clearance between connector pins, insulation thinning, foreign particle contamination, and inadequate contact between wire and pin (bad crimp or cold solder joint). In these cases, either a resistance check or a "hipot" (high potential) measurement may reveal the problem.
When a moderate volume of cables must be tested, firms generally purchase a commercial tester from any of about a dozen major suppliers in the industry. A serious test instrument provides continuity tests for opens, shorts, and miswires as the primary test, and, at greater cost, will also test for resistance, insulation breakdown, or other electrical parameters. Serious professional instruments vary in price, ranging from around $1,000 for basic continuity measurements to well over $5,000 when tests for other electrical characteristics are included.
Both test data and match data are considered to be raw data
and require further processing to provide a meaningful display
of wiring or a netlist. Valid test data or match data must be
present for most functions of the tester to operate. In many cases,
once test data has been acquired from a test cable (1 or 2 seconds),
PC-Based Equipment: the equipment requires a personal computer for operation and cannot function without it. To justify its expense and bench footprint, PC-based equipment must make effective use of full-screen high-resolution color graphics, high-speed computation, large data storage capacity, and the full-size keyboard. Other input devices such as a mouse, trackball, and voice control, may also be employed that would otherwise be uneconomical or impractical for a non-PC-based equipment, or would defeat the portability requirement of such equipment. Note that numerous test instruments permit the upload and download of information to PC software for off-line data analysis and storage, but operate on a stand-alone basis. As such, these instruments do not qualify as "PC-based".
Microprocessor-Controlled Equipment: an embedded, on-board microprocessor effects control of the system and executes whatever function that system is intended to perform. Such systems are usually stand-alone and totally self-contained, with the presence of microprocessor control being invisible to the User. Dedicated switches, keyboards, indicators, and numeric display units comprise a non-standardized User interface. "PC-controlled" stand-alone equipment using an embedded single-board PC (Intel 80x86 microprocessor with Microsoft operating system kernel) functions largely as an embedded microprocessor-based system, and offers advantage primarily to the manufacturer of the equipment as follows: (a) software development may employ high-level programming languages and debugging tools for which many skilled programmers are available, and (b) hardware implementation uses high-performance, off-the-shelf computer modules minimizing any custom design. The human interface, however, still relies on dedicated electrical components, although more sophisticated components, such as a touch-screen display, may be employed.
Over the last ten years, a number of relatively inexpensive microprocessor-based testers have been introduced that quickly acquire the connection map of a cable and store it in on-board memory. By saving in RAM the connection map of a model cable known to be good, it is a simple matter to acquire data from a cable under test and compare it to the model. A matching connection map indicates that no opens, shorts, or miswires are present. In accomplishing this, the cable under test becomes certified, and the benchtop tester will have performed its job.
In evaluating the manufacturing process, CAMI Research has found that other steps necessary in the production of cables are currently handled separately from the test process and often do not involve computerized equipment. These steps include cable design, cable assembly, fault location, labeling, hard-copy documentation, and cataloging in a database for future reference. We believe that integrating these functions in a single general-purpose instrument greatly facilitate cable production by saving expensive engineering time and production labor, reducing waste, and minimizing error. Achieving the desired multi-purpose functionality, however, is beyond the ability of the 8-bit microprocessors and rudimentary display devices used in most benchtop units. By moving all computational, storage, and display functions to the PC and leaving behind only data acquisition hardware, CAMI Research has produced a cost-effective PC-based cable test system with new capabilities that address multi-faceted needs.
Unique Benefits Offered by a PC-Based Tester
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Wiring Display - Any tester must convert raw connection data from the continuity matrix to a wire list. For the purpose of wiring analysis or fault location, wire paths may also be displayed as a graphic wiring diagram when a PC is employed, as shown in Figure 3. The direction of view, indicated by a small connector icon, may be changed to show wiring looking into the pins or looking into the termination. Individual wire paths may be highlighted using the cursor control keys.
Descriptive Notes and Labels - When new cable data is acquired, the test technician may annotate it with descriptive notes, part numbers, vendor data, color codes, signal assignments, or other text and store all information in a searchable on-line database. This data may be recalled by matching against wiring, file name, descriptive text, or by manual selection. Database size is limited only by available disk space. Label text may also be entered and stored with each cable.
Comparison Against a Model - In addition to checking test data against a golden cable, the on-line database may be searched automatically for matching wiring. This ability permits cable identification and the automatic association of descriptive text and labels to the measured cable. For a manual comparison, the operator may quickly alternate between the schematic display of test and match wiring. Because the connectors and wiring are geometrically located in exactly the same position on the screen, rapid alternations will instantly reveal subtle differences in the wiring, particularly if the wire paths of interest are highlighted.
Printing - The complete cable schematic, along with the wire list, descriptive text and label text, may be printed on a laserprinter or inexpensive ink jet printer. This high-quality documentation is essential for maintaining accurate wiring and construction records on cables and is based on measured data, not data redrawn by a draftsman from a rough sketch. Thus, engineering time is saved, and another opportunity for error is eliminated. Data produced in this manner may be used directly in equipment manuals or other printed documentation, or it may be enclosed with each cable that a contract assembly house ships to its customers.
Intermittent Connections - When intermittent connections are found, the wiring display is again useful in showing which wires were found to be intermittent. In combination with the audible tone sounded during flexure, the location of the problem may be readily identified.
Automatic Test Sequences - In a production environment where unskilled operators may do the testing, it is unnecessary for the test operator to enter commands at the computer keyboard, or read results on a computer screen. For pass/fail testing, a test engineer can predefine a desired test sequence and store it as a script on disk. CAMI's CableEye® tester is equipped with a single TEST pushbutton, and three LED indicators labeled READY, MATCH, and ERROR. Pressing the TEST pushbutton triggers a predefined Macro, with results indicated on the lamps. The operator's only task, then, is to mount the test cable, push the button, apply a label (optional), and place the tested cable in an appropriate bin.
Cable Design - To design and test a cable, the test engineer simply enters the desired connector types and wire list using a built-in editor. From this, a cable schematic and ordered wire list are generated by software and may be printed or stored in the database with descriptive notes. If cable design and production occur in different facilities, the engineer can modem the design the cable assembly plant. Technicians at that location may then use their CableEye system to load the test data and reproduce the printed wiring design, as well as have test specifications preloaded in the tester when completed saved and the chance for human error greatly reduced.
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PC-based cable-test systems offer a suite of benefits that are neither available nor economically feasible on traditional stand-alone testers. A PC-based system not only tests cables but also provides an integrated software package for cable design, labeling, documentation, cataloging, data logging, on-line assembly checking, and test scripting. The added cost of a PC is easily justified by (a) the reduced cost and increased reliability of the test fixture itself; (b) the elimination of errors in transcription, drawing, and rekeying of wire lists as cables pass through design, test, and documentation phases; and (c) the ongoing savings in engineering time, fault-location time, and documentation. Finally, continuing advances in test software become available to customers at low cost without requiring any change in the test fixture, data-acquisition electronics, or connector cards.
Prepared by CAMI Research Inc.