On Wiring Modular Telephone Connectors by John R. Carlsen 8P8C plug connector and its pin position numbers The modular connectors commonly used today for telephony and twisted-pair Ethernet data communication are types of insulation-displacement connectors often associated with the A.T.&T. Bell System Universal Service Ordering Code (USOC) registered jack (R.J.) configurations for which they are used. Having four, six or eight contact positions, these connectors accommodate from two to four twisted pairs of wires. The six-position bodies may be populated with two, four or six contacts. The most common varieties include: Common Types of A.T.&T. USOC Modular Registered Jacks # positions, contacts application RJ9 4P4C telephone handsets RJ11 6P2C 1-line telephone RJ14 6P4C 2-line telephone RJ25 6P6C 3-line telephone RJ45 8P8C twisted-pair Ethernet RJ48 8P8C T1, ISDN RJ50 10P10C (proprietary) RJ61 8P8C 4-line telephone When assembling cables using these connectors, wires are inserted into the connectors with their insulators intact; the connectors are then crimped (squeezed inside a tool), causing each contact to pierce the insulation of its wire and causing a strain relief to secure the cable's exterior sheath. Using the colors of their insulators, wires are matched with contacts in the orders listed in the table below. (When holding the connector facing the cable side with the retaining tab facing the floor, position 1 will be on the left side, as illustrated.)

Common A.T.&T. USOC Modular Registered Jack Wiring Configurations

4 positions (4P4C)	6 positions					8 positions (8P8C)
RJ9 *	RJ11 (6P2C)	RJ14 (6P4C)	RJ25 (6P6C)	color †		pin	RJ45 *		RJ61
				obs. (old)	current (new)		T568A (obsolete)	T568B
pin: pair pole: color	position: pair pole						pair pole: color
						1	3 tip: /green ‡	2 tip: /orange ‡	4 tip: /brown
	none	none	1: 3 tip	orange	/green	2	3 ring: green/ ‡	2 ring: orange/ ‡	3 tip: /green
1: 2 tip: black	none	2: 2 tip	2: 2 tip	black	/orange	3	2 tip: /orange ‡	3 tip: /green ‡	2 tip: /orange
2: 1 ring: red	3: 1 ring	3: 1 ring	3: 1 ring	red	blue/	4	1 ring: blue/	1 ring: blue/	1 ring: blue/
3: 1 tip: green	4: 1 tip	4: 1 tip	4: 1 tip	green	/blue	5	1 tip: /blue	1 tip: /blue	1 tip: /blue
4: 2 ring: yellow	none	5: 2 ring	5: 2 ring	yellow	orange/	6	2 ring: orange/ ‡	3 ring: green/ ‡	2 ring: orange/
	none	none	6: 3 ring	blue	green/	7	4 tip /brown	4 tip /brown	3 ring green/
						8	4 ring brown/	4 ring brown/	4 ring brown/

* RJ9 and RJ45 are not registered jacks for customer interfaces

† RJ11 and RJ14 use the same colors as RJ25, but fewer pairs

‡ 10-base-T and 100-base-TX twisted-pair Ethernet use only pairs on pins 1 and 2 (for "transmit across", TX) and 3 and 6 (for "receive across", RX) pairs

Cable Handling and Storage

Use care when pulling cables with these small connectors, as their retention and release tabs will snag and break easily. To avoid tangling stored cables, I coil and stored them in resealable transparent plastic bags.

Flush Wall Plate Orientations

According to a former Pacific Bell engineering vice president (and former neighbor of mine), flush wall-mounted modular telephone connectors (such as RJ11W) were intended to be mounted with the release tab on top so that field technicians could – with the aid of an instrument such as a ruler – release connectors from behind desks or other furniture. It is now common practice to orient these connectors with the release tab on the bottom and contacts on top, apparently to protect the contacts from deterioration due to environmental factors. (Similarly, it is now apparently common practice to mount three-conductor Edison power connectors with the ground pin at the top.)

parts of quarter-inch diameter cylindrical tip ring-sleeve telephone plugs: sleeve (1), ring (2), tip (3), insulator (4)

Polarity: Origin of "Tip" and "Ring" Designations

The terms tip and ring refer to the electrical polarity associated with the parts of the quarter-inch diameter cylindrical tip ring-sleeve plugs used by telephone operators. For safety – because the the tip is most likely to touch something other than intended when a connector is handled – the tip is connected to the pair's wire nearest ground potential. To protect copper wires from corrosion caused by electrolysis, the ring's polarity was changed from positive to negative (nominally -48V).

An Ethernet crossover cable, for example, may be constructed by swaping the "transmit across" (TX) and "receive across" (RX) pairs at one end of a cable, but the same polarity must be maintained for each. (Note that crossover cables for 1000base-TX interfaces – "gigabit Ethernet" – must also swap pair 1 with pair 4, and that some 1000base-TX interfaces include automatic MDI/MDI-X configuration that therefore do not require crossover cables.)

Related Reading

See also the A.T.&T. Monopoly Bumper Sticker.