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What are modular connectors?

​​​​​​​Modular connectors are commonly used for telephone systems, data networks, and low-speed serial connections. These connectors are inexpensive, relatively simple to terminate, and easy to plug and unplug. A modular connector typically has a clear, plastic body, with a tab that locks the plug and jack into place when connected. In the vernacular used by the technology industry, they are called "RJ" connectors. This is technically inaccurate, but the naming convention is widely used. RJ is an acronym for Registered Jack, which is part of a coding system developed in the 1970s by AT&T to classify telephone services and equipment. The system, called the Universal Service Order Code (USOC), used designations that began with the letters RJ to denote the capabilities of jacks in a building, and how they should be wired in order to connect to the public phone network. RJ plug connections are typically terminated by using a special tool to crimp the connector onto the conductors of a cable. These connections are often made in the field for convenience; however, a factory-assembled RJ cable typically offers the best performance.

Select the modular connector that you want to learn more about:

The RJ-12 connector has 6-position, 6-conductor arranged in a single row. This connector is used for voice/data applications: telephone (two-line), networking, extended-distance peripherals.

The wiring scheme for a single line phone was identified as "RJ11", and the name stuck to describe the connector used on a phone cord. Today, RJ-11 is widely used in the connectivity industry to denote a 6-position, 4-conductor modular connector or jack.

An 8-position, 8-conductor modular connector that is most often used for data networks such as Ethernet. RJ-45 connectors are physically wider than the RJ-11/12 connectors used for telephone. In network applications, RJ-45 cable assemblies are used to connect from a patch panel to a network switch, and also to connect a computer's NIC to a data port.

This is a 10-position, 10-conductor modular connector. This is a relatively uncommon connector. The typical application for this connector is on a T1 cable.

A smaller version of the RJ-11 modular plug, the RJ-22 has only 4 pin positions. It is typically used on telephone handsets. RJ-22 is an "unofficial" designation, not part of the official USOC recognized by the FCC.