Modular connectors and RJ connectors are electrical connectors used for telephone and Ethernet connections. Modular connectors and RJ connectors are two common names for the same connection device and do not imply two different types of connectors. 

Basic Information

Modular connectors are typically clear, crimp-on connections with anywhere from two to eight contacts. For example, telephone connectors usually have four to six connections whereas Ethernet typically uses eight. This type of connector was originally used with the registered jack system. This system describes how the connectors are wired for telecommunications with specifications defining the wiring patterns of the jacks, not the physical dimensions or geometry of the connectors of either gender. These latter aspects are covered by ISO standard 8877.

Telephone connections are commonly referred to as RJ11 connections and Ethernet connections are commonly known as RJ45. Other naming conventions include names such as 6P8C or 8P8C where the number before the C indicates the number of contacts in the connector. The number before the P is the maximum number of positions possible.

Specifications

Selecting a modular/RJ connector requires understanding of the intended application as well as several specifications including connector type, gender, and terminal options.

Connector Type

The terms modular connector and RJ connector refers to a family of electrical connectors used for many purposes. They are found most commonly in telecommunications applications.

• RJ-11 - Registered Jack 11 - A four- or six- wire connector used primarily to connect telephone equipment in the United States.
• RJ-45 - Registered Jack 45 - An eight-wire connector used to connect computers, printers and other devices into a network. 
• Telecom / Data / Network Connectors - Connectors are used in the telephone, data, and network industry. 

Gender

Modular connectors and RJ connectors have gender: male connectors are called plugs, while female connectors are called jacks or sockets.

• Male / plug - Male connectors or plugs are often referred to as header or free connectors. In coaxial RF connectors, the plug is normally the movable portion and is usually attached to a cable or removable sub-assembly. In shell type multiple-contact connectors, the plug usually contains the socket contacts and is mounted on the "hot" (rack) side of the system. Plugs mate with receptacles, jacks, outlets, etc.
• Female / jack - Female connectors or jacks are connecting devices into which plugs are inserted to make circuit connections. 

Terminal Options

Terminals are devices designed to terminate a conductor to be affixed to a post, stud, chassis, or other conductor in order to establish electrical connection.

• Through hole / pin - Through-hole technology (THT) mounts components on printed circuit boards by inserting component leads through holes in the board and then soldering. In pin termination, components are mounted on PCBs without soldering.
• PCB solder/ solder pin - In PCB solder or solder pin termination, electrical connection is made by soldering wires or pins onto a printed circuit board (PCB). 

Performance Specifications

In order to select the best modular/RJ connector for the intended application, there are several performance specifications that need to be considered.

• Current rating - Current rating is the maximum, recommended flow of continious electrical current.
• Voltage rating - Voltage rating is the maximum operating voltage.
• Number of contacts - The number of contacts is the number of conductive elements that mate with a corresponding element to provide an electrical path.
• Contact size (AWG) - An electrical contact is the termination of an electrical wire in the connector's pin or socket. Contact size, also referred to as "termination size," is the maximum diameter wire the connector can accommodate. In North America, wire area is measured by the American Wire Gauge (AWG) to indicate conductor size. The AWG is used to measure certain conductors including copper. The higher the AWG number the thinner the wire. This is because AWG stems from a measurement that represented the number of times the copper wire was run through a wire machine that reduced the diameter of the wire. Thus 24-gauge wire went through the machine 6 more times than 18-gauge wire.
• Operating temperature - Operating temperature is the full-required range of ambient operating temperature. 

Rating Systems & Standards

NEMA ratings indicate whether modular connectors are suitable for hazardous or non-hazardous locations and designed for indoor or outdoor use. All NEMA enclosures protect personnel against incidental contact with the enclosed equipment. 
 

Additional standards for modular connectors and RJ connectors can be found at the IHS standards store.

• ASD- STAN PREN 4165-023 - Aerospace series connectors, electrical, rectangular, modular operating temperature 175 Celsius continuous part 023: tooling for assembly of receptacle coding component product standard
• MIL-C-28754/46 - This specification is approved for use by all Departments and Agencies of the US Department of Defense. The complete requirements for acquiring the connector described herein shall consist of this specification and the latest issue of MIL-C-28754 

Resources

RJ (Registered Jack) style modular plugs 

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