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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.


Telephone
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. Different confurations are used for multi-line subscriptions. 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   US
  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 2-line   US
  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   US
  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   Crypto Museum
  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)
    RJ45 (8P8C) receptacle used for four POTS telephone lines (Crypto Museum Standard)
 Other telephone connection standards

Handset / headset
  1. Microphone (1)
    black
  2. Speaker (1)
    red
  3. Speaker (2)
    green
  4. Microphone (2)
    yellow
    RJ9 (4P4C) receptacle used for connection of a telephone handset or headset

Ethernet
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)
    TX
    orange/white
  2. Pair 3 (A)
    TX
    orange
  3. pair 2 (B)
    RX
    green/white
  4. Pair 1 (A)
    -
    blue
  5. Pair 1 (B)
    -
    blue/white
  6. Pair 2 (A)
    RX
    green
  7. Pair 4 (B)
    -
    brown/white
  8. Pair 4 (A)
    -
    brown
    RJ45 (8P8C) receptacle on a PC used for Ethernet networking


Nomenclature
  • Modular connector
  • Modular jack
  • Registered jack
  • RJ-connector
References
  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: Tuesday, 31 August 2021 - 07:35 CET.
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