They adorn the ends of Cat5 network patch cables and the flat satin cables that come with all-in-one printers that we generally either toss in the scrap bin or throw away altogether. The blocky rectangular plugs, molded of clear plastic and holding gold-plated contacts, are known broadly as modular connectors. They and their socket counterparts have become ubiquitous components of the connected world over the last half-century or so, and unsurprisingly they had their start where so many other innovations began: from the need to manage the growth of the telephone network and reduce costs. Here’s how the modular connector got that way.
Service as a Service
For the first 80 years or so of the US telephone network, the Bell Company called all the shots. They owned absolutely every bit of equipment in the system – the wires on the poles, the switchgear in the central offices, microwave links, and even the phone sets in customer homes. They had complete control over every aspect of the delivery of their services, and used their monopoly to build an incredibly integrated and durable system.
But with great power comes great responsibility, or in the case of a complex technological system prone to breakdown no matter how robust it is, a great support burden. Customers were completely reliant on phone company technicians for everything. Want a phone moved to another room? That’s a service call. Dog chewed the cord between the set and the wall? Service call. Junior got the scissors and decided to clip the handset cord? Most definitely a service call. Some services were charged back to the customer, but even for phones built like a battleship with a design life of 40 years, wear and tear eventually add up.
Sinking ever more money into customer service calls, AT&T decided to look into ways to minimize costs in this area. With mountains of service tickets to provide raw data, they discovered that by far the most common calls were for broken cords between the base and the handset. That’s understandable; the handset cord is twisted and kinked, and despite ample strain reliefs at stress points, eventually the cord frays and breaks. Not only were there a lot of broken cord calls, but each one required a good chunk of the technician’s time to correct, as the phone had to be disassembled for the dodgy cord to be replaced.
To reduce the service burden, in the late 1960s AT&T turned the engineers at Western Electric, its manufacturing arm, on the problem. The team, including Edwin Hardesty and Charles Krumreich, looked at the specs. They knew they needed four conductors, and that the connector had to be easily and rapidly disconnected in the field. The connector would have to have strain relief built in, be easily manufactured, and be cheap. It was also desirable to have the handset connector be generally useful, in particular for replacing the cord between the phone and the wall, another frequent cause for service calls.
An Easy Squeeze
It’s not exactly clear where the inspiration for a rectangular molded plastic connector came from, except that Krumreich had patented a similar connector in the mid-1960s. The earlier design had a lot of familiar elements: multiple parallel conductors, plastic channels to isolate and insulate each circuit, insulation displacement contacts in the plug, and springy wire contacts in the socket. The later design changed a few things, notably replacing the spring wire contacts in the socket with fixed blades, but on the whole the designs were very similar.
The big difference was in the thought given to how the connectors would be used in the field, and how they would be manufactured. Focusing on insulation displacement meant that field technicians would be able to mass terminate the connector with a simple crimp tool, and made the connector easier to manufacture in the huge numbers that would be needed for a system-wide conversion. Molding the plug body also meant that strain relief would be integrated into the plug, in the form of a flexible bar that would be crimped down onto the cord jacket after the electrical connections were made.
The design would soon be refined, returning to spring wire contacts in the socket and replacing the original metal latch that locked the plug into the socket with a molded plastic latch. But the basic design of the modular plug was set. New phone designs like the Trimline phone, where the handset contained the dial or keypad, would be among the first phones to use the new cords, and during the 1970s phone companies across the country retrofitted millions of existing phones.
As the 1980s rolled around and the AT&T monopoly began to break up, the new modular connectors would play an important role in democratizing the phone system. For the first time, customers could go to a retail store and buy whatever phone they wanted outright, rather than leasing one of the “any color as long as it’s black” phones. The modular connector made customer self-connections possible for the first time; unfortunately, it also made cheesy corporate how-to videos inevitable.
Registered Jack of All Trades
The powerful combination of good design, easy extensibility to more or fewer conductors, and the saturation of the market thanks to a large installed base all led to the general acceptance of the modular connector across a wide range of industries. This was helped by the 1976 mandate by the Federal Communications Commission that phone system connections be standardized for interoperability, primarily to allow customers to connect their own equipment to the telephone network.
The mandate termed these specifications registration interfaces, and the modular connectors became known as registered jacks, or RJ for short. Six-conductor RJ11 became the standard for connecting phones to the network, while RJ14 with four conductors was specified for handset to base connections. With roots in the telephone system, the eight-conductor modular connector eventually became the standard for Ethernet connections under a separate set of standards; it’s commonly but incorrectly referred to as RJ45, which is similar but has a keying lug built into the socket and plug.
Modular connectors have stood the test of time for nearly 50 years now, and while the design isn’t perfect — looking at you, little plastic latch who snaps off — its has proven to be remarkably adaptable to a wide range of applications. That it started with a need to make field service easier seems fitting in a way.
[Featured images: University of Vale do Taquari]