Cable Assembly Probing Ranges from Easy to Challenging

An experienced engineer told me years ago, and not entirely in jest, that cable assemblies—connectors paired with one or more parallel copper wires, and often referred to simply as a “cable”—were potential sources of problems connecting two other potential sources of problems. While he was right, these cable assemblies were much more than that. They were often convenient windows into what was going on in a circuit or the interaction between two subassemblies.

Consider the once ubiquitous RS-232 interface and its most common connector, the 25-pin D-shaped connector known as a DB-25. Although it is now considered a “throwback” that has been superseded by USB in many cases, and is rarely used on new designs, it served the industry and users well for many years and was the go-to connector for low to moderate data rates and other links.

Even better, due to its physical size, designers could directly probe the connector’s wires with a voltmeter, oscilloscope, or other test instrument, often done by removing the protective shell and accessing the back of the connector. There were even very convenient breakout boxes which made it easy to connect probes to one or more wires of the RS-232 assembly, make/break the signal paths, and even jumper and cross-connect the wires (Figure 1). This open access made it easy, if for example, you needed to create a null modem and transform a DTE (data terminal equipment) device into a DCE (data communications equipment) one. It also allowed you confirm what you really needed, and then you could quickly solder up a new connector/cable with the right wiring configuration.

Figure 1: This easy-to-use, handy RS-232 breakout box lets you attach probes to one or more wires, break signal paths, and even connect a jumper from one contact to another. (Image source: Tecra Tools, Inc.)

What about RJ11 telco lines?

The availability of convenient breakout boxes wasn’t limited to DB-25 connectors. For the standard six-wire RJ11 modular connector used for wired telephones, you could get a breakout box which allowed you to effortlessly tap into the conductors using alligator clips or slide-on connectors (Figure 2). This allowed you to monitor or inject signals when working on products such as standalone answering machines, fax machines, and more.

Figure 2: This simple RJ11 breakout box greatly eases the task of connecting probes, signals, or systems under design to the wired telephone line. (Image source: Bill Schweber)

For cases where a smaller, soldered interface between the six wires and a project prototype was required, the handy SparkFun Electronics RJ11 Breakout Board makes the electrical interconnection reliable and effortless (Figure 3).

Figure 3: This SparkFun Electronics RJ11 breakout board enables simple solder connection to the six wires of the widely used modular connector. (Image source: SparkFun)

Even IDC assemblies could be probed

Higher density assemblies using medium pitch insulation-displacement connectors (IDCs) and flat cable could also be fairly friendly to probing. At the prototype set-up bench, you could crimp an extra connector, such as TE Connectivity AMP Connectors’ 1658623-6, a 26-position, rectangular receptacle connector, anywhere along the cable assembly (Figure 4).

Figure 4: An extra 1658623-6 26-pin IDC from TE Connectivity AMP Connectors can be crimped along the flat cable, and then used as an access port to one or more of the cable wires. (Image source: TE Connectivity AMP Connectors)

Then, just insert a solid 28 AWG wire into one or more contact holes and attach probes to the inserted wire. It may sound kludgey, but it worked. Beyond basic grey, the flat cable was also available as a multicolor rainbow, which made testing and debugging much easier (Figure 5).

Figure 5: The IDC connector can be used with mono-color or multicolored flat cable; the latter makes debugging and wire tracing much easier. (Image source: author)

Multi-gigahertz (GHz) designs change the situation

But times have changed, and so much design work now centers on signals with bandwidths in the multi-GHz range, and corresponding data rates of gigabits per second. Any interconnecting cable assembly is now a precision engineered component, with a coaxial cable that may only be only a millimeter in diameter. These cable assemblies are designed to be used with a surface mount receptacle such as Rosenberger’s 01K80A-40ML5, rated for operation to 110 GHz. Some connectors even come with a torque wrench to ensure the amount of tightening is just right (Figure 6).

Figure 6: The Rosenberger 01K80A-40ML5 RF connector is designed for operation to 110 GHz and mates with a connector terminating a coaxial cable just one millimeter in diameter. (Image source: Rosenberger)

A GHz+ cable assembly has an invisible but important “Do Not Disturb” sign on it, and for a good reason: any obstruction or add-on probe will very adversely affect the cable’s impedance, performance, signal integrity, and bit error rate (BER). Today's high-speed, fast-slewing, small-swing signals, sensitive as they are to capacitance, load, and even sometimes temperature, can't tolerate the relatively heavy hand, figuratively speaking, of just any casual probing you might do. If you need to look at a signal going into or out of that assembly, you need to carefully plan and implement a buffer strategy.

There's not much we can do about this, as the realities of the physics of these signals is not something you can fool; it's the electronic test and measurement version of Heisenberg's uncertainty principle, where the very act of measurement changes that parameter which you are trying to measure. We live in a world of fast-moving signals and their precision connectors, and they don’t like to be touched. Even a benign scope probe or careless finger can upset the careful balance of inductance, capacitance, and other factors which the signal and connector were designed to play with nicely.

But I still think about those basic breakout boxes, and how much good they did while their time lasted; and they had their days of glory. They are still useful in relevant applications, but those are fading fast. I suspect many of these breakout boxes are now stuffed in back of the equipment locker. Perhaps they’ll be valuable collector’s items in the distant future, or even come to the rescue (with the help of an “old timer”) when the malfunction of some ancient but vital system threatens civilization in a futuristic script?

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About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

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