Understanding and Specifying Modular Connectors

By Ryan Smoot, Technical Support Engineer, CUI Devices

Modular connectors, also commonly referred to as RJ connectors, have become ubiquitous in telecom and datacom applications since their early inception in the 1960s by AT&T. The Federal Communications Commission (FCC) worked to further standardize this interconnect solution in the 1970s, which gave way to the registration system known today as Registered Jacks (RJ). This RJ system standardizes the wiring patterns, signal specifics, and physical construction of the connector itself. The naming convention, “RJ” followed by two numbers, is what users still see today to indicate differences in the connector package and intended use.

Basic construction

Utilizing a molded housing to enclose stamped metal contacts separated by insulating channels, modular connectors or RJ connectors were developed to create a more “modular” and cost-effective connection system for consumer and business phone systems. The contacts in the plug mate with similar stamped metal contacts in the jack, guided into place by the insulating channels, while the plastic housing of the plug locks the cable in place to provide strain relief. Further strain relief is provided by molded features in both the plug and the jack: the plug has a molded-in spring that latches onto a notch molded into the jack to retain it. The plug is released from the jack by depressing this spring.

Diagram of CUI Devices modular plug with a plastic housingFigure 1: A modular plug with a plastic housing locks a cable in place and mates with an RJ jack. (Image source: CUI Devices)

Modular connectors may have two, four, six, eight, or ten contact locations, although these positions are not always populated. For example, RJ11 phone connectors usually have four or six connections, while Ethernet (RJ45) connectors have eight. A standard 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 connector contacts can be used to supply low-voltage AC or DC power.

Modular connectors have various benefits. For product makers, their advantages include:

  • Low cost
  • High availability
  • Solderless assembly
  • Easy customization
  • Surface-mountable jacks
  • Ruggedized variants for industrial applications

Modular connectors also work well for field installation and maintenance tasks due to the following advantages:

  • Simple upgrade or replacement of old connections
  • Easy field assembly with simple tooling
  • Ability to customize cable assemblies on site
  • Multiple connector and wiring options
  • Simple training of field-service personnel

In both contexts, the connectors’ simple plug-in/unplug action makes it much easier for users to fix their own issues, reducing service calls.

Common types

There are a wide variety of application-specific modular connectors. The most common types and their specifications are included here:

  • 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. Read CUI Devices' blog, The Ultimate Guide to RJ45 Connectors for more details
  • RJ48 – 8P8C, like RJ45, but for connection to shielded cables for T1 data lines

Image of a sampling of popular CUI Devices modular connector typesFigure 2: A sampling of popular modular connector types. (Image source: CUI Devices)

Key connector specs and features

Modular connectors can be specified with additional features such as:

  • Shielding, to protect longer cable runs from electromagnetic (EMI) or radio frequency interference (RFI). Shielded RJ45 and RJ48 connectors, which continue the grounded connection of the shielding braid on the cable into the panel on which they are mounted, are widely available.
  • Keying, certified RJ45 connectors use a tab on the connector to stop connectors from being inserted into a socket the wrong way. This feature is not normally present on a standard 8P8C connector. Color coding can be used to indicate which cables should be allowed to use particular network connection points.
  • Mounting options, such as panel mount, board mount, through-hole, and surface-mount to PCBs are typically available. Modular jacks are offered with vertical or horizontal orientations.
  • Display and indicator functions, such as colored status LEDs.
  • High-reliability variants, which can include EMI/RFI shielding, special keying, protective boots, and robust parts that enable more insertion/removal cycles.
  • Integrated magnetics, to improve EMI shielding and connection reliability in xBASE-T networks. These are wire-wound components in the jack that protect against transients and provide isolation, signal balancing, and impedance matching.

Diagram of CUI Devices modular connectors with integrated magnetics for EMI shieldingFigure 3: Modular connectors with integrated magnetics for EMI shielding and connection reliability. (Image source: CUI Devices)

Additional parameters

Choosing the right modular connector for an application also involves considering the following parameters:

  • Pin outs – the assignment of each contact within the connector
  • Current rating – the maximum current that a device can tolerate
  • Voltage rating – the maximum operating voltage that a device can tolerate
  • Contact size – defines the maximum diameter wire that a connector can accommodate
  • Number of contacts – this is always an even number


Although modular connectors were standardized over forty years ago, they continue to remain popular due to their wide adoption, availability, and ease of use. While the most common uses are in phone systems, data networks, and low-speed serial connections, the development of Power over Ethernet (PoE) as well as their overall flexibility have further expanded their footprint. Applications such as industrial controls, motion control, smart lighting, development boards, and more are just a small sampling of designs incorporating modular connectors today.

RJ series modular connectors may have become ubiquitous, but they are not all the same. Working with suppliers to consider the factors described above can help ensure that engineers specify the right RJ plugs and jacks for their application. CUI Devices' line of modular connectors offers RJ10, RJ11, RJ12, RJ13, and RJ45 jack types with a range of position, contact, and mounting options.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Ryan Smoot, Technical Support Engineer, CUI Devices

With an extensive knowledge of CUI Devices' products, Ryan Smoot provides customers with a wide range of technical and application support capabilities in the field. His management of CUI Devices' robust CAD model library further offers engineers with an invaluable resource for streamlining their product designs.