Basics of Design Engineering: Thinking Cables

Tag: ceramic base

True plug-n-play is the holy grail manufacturers strive to attain when systems must work with each other. The ideal scenario lets users take a device out of the box, plug it in, and start using it. But in the real world it’s more like “plug-n-pray” that users don’t experience too many difficulties getting a device connected and operational.

Just because two units have the same communication hardware, like a USB port, doesn’t mean they’ll be able to talk over that connection. Hardware is only half the equation. The other half is making sure each device understands what the other is saying. A telephone isn’t much good for talking if the person at one end only knows English while the other speaks only French.

Fortunately for the sake of international diplomacy and system integration, the wire, cable, and cable-assembly community has embarked on a revolutionary plan to help OEM-equipment designers change the way they deal with interconnectivity via the cables that connect devices. Through the use of thinking cables, system designers can literally think outside the box — data translation services can reside within connecting cables.

Like most communication technologies, there are many forerunners that claim rights and precedence for thinking cables. One inventive thinker rushed to aid a struggling progenitor in the early 1990s. A designer of handheld bar-code scanners had to communicate with electronic cash registers from IBM, NCR, ICL, and others, each with different communications needs. One answer: Embed active components on a PCB directly into the cable assembly. Users merely bought the right cable for their register. The cable translated the scanner data into the required format. Power for the circuit came from the cash-register connector port.

While this solution appears simple and straightforward, the final design had to overcome significant engineering challenges. For example, the circuit pod needed strain relief on both sides to withstand the severe flex and movement of a consumer-oriented, high-volume, point-of-sale (POS) application. The active circuit board inside the pod had to be potted to secure components against that same abuse while the connections to the board demanded enhanced soldering techniques to ensure long-term reliability.

By 1994, shipments of handheld bar-code scanners that used thinking cables reached millions of units per year. One manufacturer of active cable assemblies, C&M Corp., had hundreds of thinking- cable designs in production throughout the 1990s. Some designs eliminated the midcable pod by moving the active circuit board into the connector end.

In the 1980s designers from Digital Equipment Corp., IBM, and others were incorporating resistors, capacitors, inductors, and other components into their cable assemblies to modify the waveform of the signal. Some designers used the electrical properties of the cable alone to affect the modification. While these cables could be called active cables because they did modify signals, most did not need external power to perform their magic. The signal only had to travel from one end of the cable to the other. Today, it is commonplace to find compensating circuits embedded in Fibre Channel and Infiniband copper-cable assemblies. They are just one form of thinking cable.

Thinking cables help exceed the limitations of standard cabling. In fact, there doesn’t appear to be any limits as to what a thinking cable can do. For example, a copper connector with an optical cable hybrid assembly could be 100 longer than a standard multiconductor copper cable while remaining lighter and more flexible. Of course there are still technical limits to consider such as the heat dissipation of the embedded electronics and meeting UL and other regulatory safety requirements. Thinking cables also work better in point-to-point situations, such as machine-to-machine communications, rather than as part of an infrastructure wiring plan.

When datacom fiber optics became commonplace in the early 1990s, OEMs produced either copper or fiber-based equipment. The media converter and pluggable port was created to give customers the option of selecting different media (copper versus fiber) or transceivers for different optical distances. Thus, the same base equipment could serve different connection needs by merely swapping media converters. They were given names such as GBIC (gigabit interface converter), SFP (small form-factor pluggable), and others.

Thinking cables can replace the multitude of media converters and add-on accessories. In place of an external pluggable device, the equipment carries just one interface port that handles the thinking cable. The cable carries all of the electronics to adapt the interface to the medium of choice, whether it is low-loss copper or fiber optic. Changing media is as easy as swapping cables.

Standards committees, notably the Power over Ethernet (PoE) group and Infiniband Trade Association group, recognized that providing power at the connector port could support a variety of uses. For example, it could spur development of multifunction devices, such as rotating security cameras; trickle-charge laptops; run sensors; and power transceivers and equalization circuits in cables. Seeing the benefits of such an option, the Infiniband standards committee changed their specifications to allow power at the connector for active high-speed interconnect applications. Powered emphasis circuits in cables can boost operating distances over noncompensated assemblies. Whether in industrial automation, high-speed communications, or general communications, the idea that power at the connector port can spur innovation has taken hold. All of these changes individually are interesting; but, taken collectively, they point to a monumental change for the cable-assembly industry.