Registered jack
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RJ stands for registered jack. RJXX is a general term for electrical connector designs used for telephony and data, including the RJ11, RJ14, RJ25, RJ48, RJ61 and RJ45 connectors.
Numbering and pinouts were set forth by the Bell System as the Universal Service Order Code (or Universal Service Ordering Code) (USOC), and was introduced in the 1970s by AT&T. They are also registered with the U.S. Federal Communications Commission (FCC), under 47 CFR §68.502.
It is important to note that the USOC does not necessarily define exactly which connector is used. Unofficial designations for this often use P for positions (even if they do not have an actual pin in them), and C for conductors (that is, actual wires or pins connected). These are given in the format #P#C, such as 6P4C or 4P2C, or just as #x# or even #/#.
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Common usage
The most familiar registered jacks are the 6-position connectors known variously as RJ11 (2 conductor/1 pair), RJ12 or RJ25 (6 conductor/3 pair), and RJ14 (4 conductor/2 pair); and the RJ45 (8 conductor/4 pair), all sometimes simply called RJ connectors or modular phone plugs/jacks. These are commonly used in building wiring for telephone and local area networks. They were originally invented and patented by Bell Telephone Laboratories (patent filed 6 July 1973; Template:US patent issued 14 January 1975), and replaced the hard-wired connections on most Western Electric telephones around 1976. Thus, they are also sometimes called Western jacks and Western plugs.
Naming confusion
There is much confusion over these connection standards. The six-position plug and jack commonly used for telephone line connections may be called an RJ11, RJ14 or even RJ12 (RJ25), all of which properly define interfaces that use the same physical connector. The four-position RJ11 standard dictates a 2-wire connection, while RJ14 spells out a 4-wire configuration, and RJ12 or RJ25 uses all six wires. These six-position plugs are often called modular connectors to distinguish them from older telephone connectors, which were very bulky or wired directly to the wall and therefore not very modular.
Physical compatibility
There is also confusion over the physical compatibility. As designed, they are physically compatible in that a four-pin plug will fit a six- or eight-pin socket, connecting to the center four of the conductors, and a six-pin plug will fit an eight-pin socket, connecting to the center six conductors. However, plugs from different manufacturers may not have this compatibility, and some manufacturers of eight-pin jacks now explicitly warn that they are not designed to accept smaller plugs without damage.
RJXX jacks are also available keyed, with an extra side tab to fit in an appropriate keyed jack, to make it intentionally incompatible with standard jacks.
RJ11 is found commonly on almost all phone and fax equipment.
Twisted pair
All these connectors are normally used with twisted pair wiring. Wiring conventions were designed to take full advantage of this physical compatibility, but here again there has been a problem. The original concept was that the centre two pins would be one pair, the next two out the second pair, and so on until the outer pins of an eight-pin connector would be the fourth twisted pair. Additionally, signal shielding was optimised by alternating the “live” (hot) and “earthy” (ground) pins of each pair. This standard for the eight-pin connector is the USOC-defined pinout, but the outermost pair are then too far apart to meet the electrical requirements of high-speed LAN protocols. Two variations known as T568A and T568B overcome this by using adjacent pairs of the outer four pins for the third and fourth pairs. The inner four pins are wired identically to RJ14. (See: Category 5 cable.)
List of types
Common types
- RJ11C/RJ11W: 6P2C, for one telephone line (6P4C with power on second pair)
- RJ14C/RJ14W: 6P4C, for two telephone lines
- RJ25C/RJ25W: 6P6C, for three telephone lines
- RJ45S: 8P8C keyed, for one data line with programming resistor
Unofficial types
Uncommon types
- RJ12C/RJ12W: 6P4C, for one telephone line ahead of the key system
- RJ13C/RJ13W: 6P4C, for one telephone line behind the key system
- RJ15C: 3-pin weatherproof, for one telephone line
- RJ18C/RJ18W : 6P6C, for one telephone line with make-busy arrangement
- RJ61X: 8P8C, for four telephone lines
- RJ21X: 50-pin miniature ribbon connector, for up to 25 lines
- RJ2MB: 50-pin miniature ribbon connector, 2-12 telephone lines with make-busy
- RJ31X: 8P8C (although usually only 4C are used), allows an alarm system to seize the telephone line to make an outgoing call during an alarm
- RJ38X: 8P8C, similar to RJ31X, with continuity circuit
- RJ41S: 8P8C keyed, for one data line, universal
- RJ26X: 50-pin miniature ribbon connector, for multiple data lines, universal
- RJ27X: 50-pin miniature ribbon connector, for multiple data lines, programmed
- RJ48S: 8P8C, for four-wire data line (DDS)
- RJ48C: 8P8C, for four-wire data line (DSX-1)
- RJ48X: 8P8C with shorting bar, for four-wire data line (DS1)
Suffixes
- C: flush-mount
- W: wall-mount
- S: single-line
- M: multi-line
- X: complex jack
External links
- RJ glossary
- RJ types
- TIA-968-A - Contains dimensions for jacks and plugs.