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RJ connector
Modular connector — registered jack

A modular connector is a type of electrical connector for cords and cables of electronic devices and appliences, such as telephones, handsets, headsets and computer networking interfaces. The connector was originally developed in the 1960s by Bell Labs, for use on specific telephone sets. In 1976 they were adopted by the FCC as a legal standard in the United States for all telephone sets, after which they became knows as Registered Jack (RJ) [1][2]. Here are some examples:

Although the Registered Jack is officially an American standard, it has meanwhile been adopted worldwide. Many types and variants exist, each identified by the letters RJ followed by two digits that express the type. This defines the interface however, and not the actual number of contacts.

To describe the connector more accurately, it is better to use the original Modular Connector designator, as it specifies the number of positions (P) and the actual number of contacts (C). In the above examples these designators are listed below the RJ-designator. Below are a number of popular wiring schemes for modular connectors, in which A = ring and B = tip.

The most common convention for connecting an analogue telephone set to a POTS network, is by using the middle two contacts (3 and 4) of an RJ11 connector, This connector is also known as 6P2C, as it has 6 positions for contacts, of which only 2 are populated. This connector type is commonly also used on the equipment itself. Different confurations are used for multi-line subscriptions. In the specifications below, the two wires of a single line are denoted (A) and (B).

Most countries in the world have adopted the US standard, but there are a few exceptions, notably the United Kingdom (UK) and Belgium.  More on this topic

POTS 1-line   RJ11 (6P2C) — US/EU standard
  1. not present
  2. not present
  3. Line (A)
  4. Line (B)
  5. not present
  6. not present
    RJ11 (6P2C) receptacle used for a single POTS telephone line (US)
POTS 1-line   RJ14 (6P4C) — UK standard
  1. not present
  2. Line (A)
  3. not present
  4. not present
  5. Line (B)
  6. not present
    RJ14 (6P4C) receptacle used for a single POTS telephone line (US)
POTS 1-line   RJ14 (6P4C) — Belgian standard
  1. not present
  2. not present
  3. not present
  4. Line (A)
  5. Line (B)
  6. not present
    RJ14 (6P4C) receptacle used for a single POTS telephone line (US)
POTS 2-line   RJ14 (6P4C) — US standard
  1. not present
  2. Line 2 (B)
  3. Line 1 (A)
  4. Line 1 (B)
  5. Line 2 (A)
  6. not present
    RJ14 (6P4C) receptacle used for a double POTS telephone line (US)
POTS 3-line   RJ25 (6P6C) — US standard
  1. Line 3 (B)
  2. Line 2 (B)
  3. Line 1 (A)
  4. Line 1 (B)
  5. Line 2 (A)
  6. Line 3 (A)
    RJ25 (6P6C) receptacle used for a triple POTS telephone line (US)
POTS 4-line   RJ45 (8P8C) — Crypto Museum standard
  1. Line 3 (A)
  2. Line 3 (B)
  3. Line 2 (A)
  4. Line 1 (A)
  5. Line 1 (B)
  6. Line 2 (B)
  7. Line 4 (A)
  8. Line 4 (B)
 Other telephone connection standards

Handset / headset   RJ-9 (4P4C)
  1. Microphone (1)
  2. Speaker (1)
  3. Speaker (2)
  4. Microphone (2)
    RJ9 (4P4C) receptacle used for connection of a telephone handset or headset

Ethernet   RJ-45 (8P8C)
Below is the common wiring scheme for Cat 3 and Cat 5 unshielded twisted pair ethernet cabling. Note that for speeds up to 100 Mb/s (10BaseT and 100BaseT) only two pairs (2 and 3) of the 4-pair cable are used for receive (RX) and transmit (TX) respectively. For higher speeds (gigabit and beyond), all four pairs are used in both directions, using special modulation techniques [3][4].

  1. Pair 3 (B)
  2. Pair 3 (A)
  3. pair 2 (B)
  4. Pair 1 (A)
  5. Pair 1 (B)
  6. Pair 2 (A)
  7. Pair 4 (B)
  8. Pair 4 (A)
    RJ45 (8P8C) receptacle on a PC used for Ethernet networking

Serial port (RS232)   RJ-45 (8P8C)
An RJ-45 (8P8C) connector is sometimes used on network equipment, such as internet switches and routers, for connection of a 'dump' terminal. It allows its configuration to be changed from a regular PC or laptop that is equipped with terminal emulation software (e.g. VT102) and an RS232 interface (COM port). On modern computers the COM port can be emulated with an USB adapter.

  1. RTS
    Request To Send
  2. DTR
    Data Terminal Ready
  3. TXD
    Transmit Data
  4. GND
  5. GND
  6. RXD
    Receive Data
  7. DSR
    Data Srt Ready
  8. CTS
    Clear To Send
    RJ45 (8P8C) receptacle on an ethernet device's Comm Port or V24 Port
 Other RS232 serial port connectors

  • Modular connector
  • Modular jack
  • Registered jack
  • RJ-connector
  1. Wikipedia, Modular connector
    Retrieved April 2021

  2. Wikipedia, Registered jack
    Retrieved April 2021

  3. Wikipedia, Ethernet over twisted pair
    Retrieved April 2021

  4. Wikipedia, Gigabit Ethernet
    Retrieved April 2021
Further information
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Crypto Museum. Created: Sunday 04 April 2021. Last changed: Monday, 15 April 2024 - 08:48 CET.
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