Modular connectors – a brief history and what you need to know

The US phone system was dominated from the 1930s to the 1960s by telephone companies that insisted on owning all the hardware in their networks, including the handsets used in private homes. Buildings and individual phones were connected to these networks using bulky connectors whose design had remained unchanged since the 1930s and bundles of wires that had to be installed and serviced by a visiting technician.

Although this approach was robust, it was so inefficient that it began to limit the network’s growth. In response, AT&T started looking for a way to simplify its connection system and cut its service costs. It commissioned Western Electric to find a solution, and in the late 1960s that company came up with a modular system of standard connection components that were smaller, cheaper to produce, and easier to install than the connectors in use at the time. This modular connection system was first used on AT&T’s popular Trimline phone in 1965. By the early 1970s, consumers could buy the connectors and cables for themselves.

At about the same time, America’s Federal Communications Commission (FCC) established a standard for interconnection between phone-company equipment and customer equipment. It also mandated that customers owned the telephone wiring in their homes and offices, separating its ownership from that of the wiring used by phone companies to deliver their service. The new interface was based on a modular connector. In 1976 the FCC created a registration scheme for these connectors ­– which is how we get today’s Registered Jack (RJ) connectors.

The RJ specification covers the physical construction of the connector, the wiring patterns, and which signal each wire should carry. Registered Jacks are labelled with the letters RJ, followed by digits that indicate the connector’s type and variant. The FCC’s purpose, to ensure that confusion over connector details did not undermine the network interoperability it was trying to foster, has been well served. More than 40 years later, the usage of modular connectors continues to grow due to their wide availability and ease of use.

Modular connectors basics

Modular connectors use a molded housing that has stamped metal contacts separated by insulating channels. The contacts in the plug mate with similar contacts in the jack, guided into place by the insulating channels. The cable is locked in place by the plastic housing of the plug, providing strain relief. The plug also has a molded-in spring that latches onto a notch in the jack to retain it. Pressing the spring releases the plug.

Figure 1: A modular plug, showing the plastic housing that locks a cable in place so it can mate with an RJ jack (Image source: Same Sky)

Modular connectors may have two, four, six, eight or ten contact locations. A naming scheme defines the connector’s wiring, so that a 6P2C connector has six positions and two contacts, while the 4P4C connector used on phone handset cords has four positions and four contacts. Although most contacts are used for signals, RJ series connector contacts can be used to supply low-voltage AC or DC power.

Modular connectors have been widely used in voice and data communication devices, such as telephone systems and computer networks, for decades. The development of techniques such as Power over Ethernet, or PoE, which enables small devices to be powered over their network connections, has further expanded their usage. PoE makes it much easier to add equipment to a network, especially in applications such as smart buildings where it would be costly to run power cabling to everywhere that you want to put a networked sensor.

There are a wide variety of application-specific modular connectors, including:

• RJ10: a simple 4P4C rectangular connector for telephone handsets
• RJ11: either a 6P2C or 6P4C square connector for linking modems to phone lines
• RJ12: same size as RJ11, but 6P6C
• DEC MMP/MMJ: smaller versions of the RJ11/12 connector, used in Digital Equipment Corporation equipment
• RJ13: 6P4C, like the RJ12, but with a behind-the-line circuit
• RJ14: 6P4C, with two lines to connect two phones or modems
• RJ21: modular connector with up to 50 contacts, used to connect up to 25 circuits in a large phone system
• RJ22: similar in shape to RJ11, but 4P4C, typically used to connect handsets
• RJ25: 6P6C, like the RJ11, but can connect three devices
• RJ45: 8P8C or 8P10C, used for Ethernet LANs. For more details, read Same Sky’s blog post, The Ultimate Guide to RJ45 Connectors.
• RJ48: 8P8C, like RJ45, but for connection to shielded cables for T1 data lines

Figure 2: Examples of popular modular connector types (Image source: Same Sky)

Key modular connector features and design considerations

The diverse range of modular connectors available today is a testament to their engineering adaptability, with various features tailored for specific electrical and communication applications. Below is an overview of key design options: 

• Shielding: for extended cable runs or environments like factory floors where Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) are prevalent, shielded cables are essential. These cables incorporate grounding to mitigate EMI/RFI effects. However, the connectors and jacks must also be shielded to fully absorb and direct the interference to ground. Examples include shielded RJ45 and RJ48 connectors, which are readily available for such high-noise environments
• Keying: true RJ45 connectors employ a keying mechanism via an attached tab that ensures correct orientation during insertion, preventing miswiring. In contrast, generic 8P8C connectors lack this feature. Enhanced keying and color-coding options provide additional security and help restrict unauthorised access to network ports
• Mounting options: modular connectors used in Original Equipment Manufacturer (OEM) applications offer various mounting configurations, including panel mount, board mount, through-hole, and surface mount. Additionally, modular jacks (sockets) come in both vertical and horizontal orientations
• Display and indicator functions: modern connectors integrate LED indicators of various colors to signal connection status and network activity, improving diagnostics and usability in communication systems
• Hi-Reliability (Hi-Rel) products: high-reliability modular connectors are engineered for demanding environments, such as military, aerospace, and factory automation systems. These connectors often feature:
  • EMI/RFI shielding
  • Specialised keying
  • Protective boots
  • Durable components capable of withstanding frequent insertion and removal cycles

• Integrated magnetics: in Ethernet applications, magnetics are integral to complying with the xBASE-T network specifications. By integrating magnetics directly into the RJ45 jack, engineers achieve enhanced EMI shielding, improved electrical isolation, signal balancing, and impedance matching. These wire-wound components within the jack also protect against faults and transients, ensuring stable and reliable connections

Figure 3: Integrated magnetics deliver better EMI shielding and connection reliability for modular connectors (Image source: Same Sky)

When integrating modular connectors into a system, several critical performance parameters must also be evaluated to ensure compatibility and reliability. These include:

• Pin outs: assignments for each contact based on application needs and future use
• Current rating: maximum current (in amps) the connector can handle safely
• Voltage rating: maximum operating voltage the connector can tolerate
• Contact size: maximum wire diameter (AWG) the connector supports; lower AWG handles higher currents
• Number of contacts: always even; determines capacity for data, power, and grounding

Summary

The modular connector system that makes these applications possible was developed to update an outdated and costly interconnect process for an overly controlling service provider. The FCC’s actions, in mandating a standard interface, controlling the proliferation of connector styles through its Registered Jack scheme, and enabling households and businesses to handle their own wiring, helped strengthen any network’s most important feature – its interoperability. 

The resultant RJ series modular connectors have succeeded beyond expectations, with worldwide uptake in multiple application areas. This proliferation does mean, though, that specifying an RJ connector can become complex. Fortunately, Same Sky is here to help designers understand the RJ naming scheme and specify interconnect devices that fit their current and future needs.