Praise for the Lowly Modular Plug
With praise for the lowly modular plug, Bob Hult examines how its elegant design, simplicity of termination, mechanical ruggedness, excellent electrical characteristics, and compact size make it a keeper even after all these years.
In a world where many products are calibrated to fail days after the warranty period ends, it is nice to see a device that continues to perform way past its expected life. I always considered antiques to be examples of products that were overdesigned and probably cost more than they should have as their original applications had become obsolete long ago. This is not the case with modular plugs and jacks.
In the connector world, there have been interfaces that had their time in the spotlight, but as technology moved on, so did the market. We don’t see many new applications for vacuum tubes or bubble memory sockets anymore.
The modular plug and jack are an example of a connector that combines elegant design, simplicity of termination, mechanical ruggedness, excellent electrical characteristics, and compact size. Not bad for a low-cost interface.
Originally developed by Bell Labs in 1975 for voice-grade signal applications in telephony equipment, they have proven to be incredibly reliable interfaces that feature good signal density at a price that is hard to beat. The design of the mod plug and jack incorporates many features that have made it attractive for applications well beyond the phone.
The plug consists of a one-piece molded plastic housing that includes an effective latch that provides a tactile and audible click when fully mated. Contacts are gold-plated flat blades. One edge is the mating surface, the other is an insulation displacement barb that penetrates unstrapped wire and assures multiple points of contact to solid or stranded wire. The receptacle (jack) consists of a series of gold-plated spring wires designed to provide the normal force to create a high-pressure, gold-to-gold interface that can withstand shock, vibration, moisture, and mechanical abuse in a user-friendly profile.
Termination tooling for the wire-mounted plug is an example of simplicity itself and was an early example of insulation displacement technology. The plug with preloaded contacts is snapped into the hand tool that pre-aligns the connector to the incoming conductors. About a quarter inch of the outer jacket of the cable is stripped off, often with a feature built into the termination tool, exposing the four, six, eight, or ten insulated wires. The stripped cable end is then simply inserted into the rear of the connector body. Internal channels guide the exposed wires into the proper centerlines. Plug connectors are typically molded of clear plastic material to enable visual inspection. The tool is then actuated, which drives the contact blades down to penetrate through the insulation and into the conductors, establishing an electrical connection and at the same time setting the top edges of the contacts to the correct mating height. As the contacts are terminated, a hinged plastic bar in the connector housing is pushed down to simultaneously create an effective strain relief to the cable jacket. A combination of features built into the connector and application tooling ensures reliable termination nearly independent of operator skill.
When mated, the edges of these insulation displacement contacts are pressed against the gold-plated, spring-loaded wires of the receptacle to create a high normal force interface. Inserting the plug into the jack generates a wiping action that can penetrate contamination that may have formed on the mating surfaces.
The plastic latch on the plug also provides positive polarization, making it impossible to plug the connector upside down.
Years of proven reliability and low cost stimulated the development of variations to fit very specific application requirements. The RJ45 connector was adapted for Ethernet applications and has become the industry standard on routers and switches.
In order to survive extremely harsh environments that range from salt water spray to the battlefield, modular plugs and jacks have been packaged in tough metal shells with gaskets that provide IP-67 sealing for environmental resistance.
As data rates continued to rise, sensitivity to radiated noise (EMI) resulted in the development of fully shielded versions of RJ45 connectors. These single and ganged jacks may also feature integrated magnetic components as well as LED activity indicators.
Other versions are outfitted with ferrite or capacitive low-pass filters to ensure electromagnetic compatibility.
The introduction of Power-over-Ethernet (PoE) enabled a device to receive up to 25.5 watts of power as well as data, eliminating the need for a power source at a remote device location such as a video camera or remote sensor. Unused connector pairs on the standard RJ45 connector are used to deliver this DC power terminated to Category 5 cable.
Relatively few enhancements have been made to the basic interface over the years since its introduction. Color-coded plug housings are now available and an elastomeric boot can be added to prevent the plastic latch from hooking with other connectors. The ends of the original spring-wire jack contacts were exposed, creating a cut hazard to a child who poked his finger into the receptacle. The spring tips are now protected behind a plastic barrier.
A combination of great engineering and low cost driven by huge manufacturing capacity has made the modular plug and jack one of the highest-volume connector types in the world. In addition to telecom, modular plugs can be found in networking, medical, industrial, instrumentation, and consumer equipment applications.
A continuing series of adaptations has enabled this interface to stay relevant for nearly 40 years, a remarkable feat in our fast-paced world of digital electronics, where data rates have evolved from Kilobits per second to Gigabits per second.
Robert Hult, Market Director, Bishop & Associates, Inc.
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