Modular connector

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Other systems exist for assigning signals to modular connectors; physical interchangeability of plugs and jacks does not ensure interoperation, nor protection from electrical damage to circuits. For example, modular cables and connectors have been used to supply low-voltage AC or DC power and no clear standard exists for this application.


Modular connectors also go by the names "modular phone jack/plug", "RJ connector" and "Western jack/plug". The term "modular connector" arose from its original use in a novel system of cabling designed to make telephone equipment more modular. This includes the 4P4C handset connector.

It is very common to use a registered jack number to refer to the physical connector itself; for instance, the 8P8C modular connector type is often called RJ45 because the Registered Jack standard of that name was an early user of 8P8C modular connectors. A very popular use of 8P8C today is Ethernet over twisted pair, and that may be the most well known context in which the name RJ45 is known, even though it has nothing to do with the RJ45 standard. Likewise, the 4P4C connector is sometimes called RJ9 or RJ22 and various 6P connectors are called RJ11.


Modular connectors were originally developed and patented by General Cable Corp in 1974.[a] They replaced the hard-wired connections on most Western Electric telephones around 1976. At the same time, they began to replace screw terminals and larger 3 and 4 pin telephone jacks in buildings.


Modular connectors have gender: plugs are considered to be "male," while jacks or sockets are considered to be "female," a reference to reproductive organs.


Latching tab and orientation[edit]

Modular connectors are designed to latch together. A spring-loaded tab on the plug snaps into a jack so that the plug cannot be pulled out. To remove the plug, the latching tab must be depressed. The standard and most common way to install a jack in a wall or panel is with the tab side down. This usually makes it easier to operate the tab when removing the plug, because the person grabs the plug with thumb on top and presses the tab up with the index finger. This orientation is recommended by manufacturers because any dust or fine abrasive or conductive particles that may enter an unused jack will tend to fall away from the electrical contacts, rather than settling onto the contact surfaces.

The modular connector suffers from a design flaw or weakness however, as the fragile latching tab easily snags on other cables and breaks off. When this happens, the connector is still functional, but the crucial latching feature is lost. Some higher quality cables have a flexible sleeve called a boot over the plug, or a special tab design, to prevent this. These cables are often marketed as snagless. Boots are seen mainly on 8P8C data cables, but are also used on other sizes of connectors.

Most protective boots must be installed onto a cable before the modular plug is crimped on. This means that field retrofits of these types of boots are not possible, except by cutting off the existing plug, and then replacing it with a new one. However, protective boots or rigid protective "ramp" adapters are available which can be snapped over an installed unprotected modular plug, to add a measure of protection to the latch tab.

Sizes and contacts[edit]

8P8C modular plug pin numbering
8P8C plug with contacts for solid wire (left) and stranded wire (right)
Contacts for solid wire (top left) and stranded wire (bottom right)

Modular connectors come in four sizes: 4-, 6-, 8-, and 10-position, where a position is a location for a contact. Not all of the positions may have contacts installed. When contacts are omitted, they are typically done so from the outermost pair of contacts inward, such that the number of contacts is almost always an even number. The insulating plastic bodies of 4P and 6P connectors have different widths, whereas 8P or 10P connectors share an even larger body width. The connector body positions with omitted contacts or contacts unattached to wires are unused for the electrical connection, but ensure that the plug fits correctly. For instance, RJ11 cables often have connectors with six positions and four contacts, to which are attached just two wires.

The connectors are designated with two numbers that represent the quantity of positions and contacts, with each number followed by a "P" and "C", respectively: for example, "6P2C" for a connector having six positions and two contacts. Alternate designations omit the "P" and "C" while separating the position and contact quantities with either an "x" ("6x2") or a slash ("6/2").

Internally, the contacts have sharp prongs that when crimped, pierce the insulation and connect with the wire conductor, a mechanism known as insulation displacement. Ethernet cables, in particular, may have solid or stranded wire conductors and the sharp prongs are different in the 8P8C connectors made for each type of wire. A modular plug for solid (single strand) wire often has three slightly splayed prongs on each contact to securely surround and grip the conductor. Modular plugs for stranded or tinsel wire have prongs that are designed to connect to multiple wire strands. Connector plugs are designed for either solid or stranded wire; a plug for one wire type might not make reliable contact when crimped to a cable with wires of the other type.



Some modular connectors are indexed: their dimensions are intentionally non-standard, preventing connections with connectors of standard dimensions. The means of indexing may be non-standard cross-section dimensions or shapes, retention mechanism dimensions, or retention mechanism quantity. For example, a Modified Modular Jack (MMJ) using an offset latching tab was developed by Digital Equipment Corporation (DEC) to prevent accidental interchange of data and telephone cables.



The special plugs are preferred for test equipment and adapters, which may be rapidly connected to a large number of corresponding connectors in quick succession for testing purposes. Use of the special plugs avoids inadvertent damage to the equipment under test, even when a narrower plug is inserted into a nominally incompatible wider jack.


Modular connector dimensions (millimeters)[3]
Connector Length Width Height
4P4C (RJ9) 7.7
6P4C (RJ11) 12.34 9.65 6.60
8P8C (RJ45) 21.46 11.68 8.30


A modular plug crimper

Termination of modular connector cables is very similar, regardless of the number of positions and contacts in the plug. To prevent damage to the plug (and the expensive crimp die-set), the crimp tool must be carefully matched to the plug being attached. For example, termination of a cable with an 8P8C plug involves using a hand crimper or crimp machine containing an 8P8C die-set or an A67T[citation needed] standard die-set. An 8P8C crimp die-set usually looks similar to an 8P8C jack, except for the eight teeth lining the top portion of the die. When the tool is operated, the die compresses around the 8P8C plug. As the die compresses, these teeth force the plug contacts down into the conductors of the cable being terminated, permanently attaching the plug to the cable.

There are two sub-types of plug, that differ only in the type of contacts used. One contact is suitable for solid (single strand) copper conductors and the other is suitable for stranded or tinsel wire copper conductors. The crimper may also permanently deform part of the plastic plug body in such a way that it grips the outer sheath of the cable. This helps to keep the plug securely fastened to the end of the cable, by providing strain relief.


Contact assignments, or pinouts, vary by application. Telephone network connections are standardized by registered jack numbers, and Ethernet over twisted pair is specified by the TIA/EIA-568 standard. Other applications have no standardization; for example, there are multiple conventions for use of 8P8C connectors for RS-232.

For this reason, D-sub-to-modular adapters are typically shipped with the D-sub contacts (pins or sockets) terminated but not inserted into the connector body, so that the D-sub-to-modular contact pairing can—and must—be performed by the end-user.


4P4C modular connector on a handset cord
Wired telephone that uses 4P4C connectors for the coiled handset cord


This handset connector is not a registered jack, because it was not intended to connect directly to the telephone lines. However it is often referred to as RJ9, RJ10, or RJ22.

Handset wiring[edit]

Wiring diagram of 4P4C/RJ9 connector in telephone handset cord.

Handsets and often headsets for use with telephones commonly use a 4P4C connector. The two center pins are commonly used for the receiver, and the outer pins connect the transmitter, so that a reversal of pin connection is unaffected. Standard handset receivers functions normally when their polarity is reversed, but the electret microphone transmitter used in most modern handsets may not. Many telephones include polarity protection, so that the polarity may be reversed without affecting operation. Old handsets manufactured before 1985 using a carbon microphone transmitter are not sensitive to polarity. Some hands-free headsets also may have a 4P4C connector, but the wiring may differ from the above diagram.

Data port[edit]

The Macintosh 128K, Macintosh 512K and Macintosh Plus from Apple used 4P4C connectors to connect the keyboard to the main computer housing. The connector provided power to the keyboard on the outer two contacts and received data signals on the inner pair. The cable between the computer and the keyboard was a coiled cord with an appearance very similar to a telephone handset cable.[5] The connector wiring, however, required a polarized straight through pinout. Using a telephone handset cable instead of the supplied cable could short out the +5 volt DC supply and damage the computer or the keyboard.[6]

Some consumer equipment such as DirecTV set top boxes include a 4P4C low-speed data port connector.[7] Such connectors can be adapted for use with a computer's serial port so that control commands can be sent from the computer to the set top box.

6P6C [edit]

6P4C crimp-on style connector

Modular plugs are described as containing a number of potential contact "positions" and the actual number of contacts installed within these positions. RJ11, RJ14, and RJ25 all use the same six-position modular connector, thus are physically identical except for the different number of contacts (two, four and six respectively).

The 6P2C, 6P4C, and 6P6C modular connectors are probably most well known for their use as RJ11, RJ14, and RJ25 registered jacks respectively.



RJ11 wiring[edit]

6P6C connector showing the location of pin 1

Cables sold as RJ11 often actually use 6P4C RJ14 connectors (six position, four contacts), with four wires running to a central junction box. Two of its six possible contact positions connect tip and ring, and the other two contact positions are then unused. 6P2C and 6P6C can also be found in stores.

The contacts other than the two central tip and ring contacts are in practice used for various things such as a ground for selective ringers, low voltage power for a dial light, or for 'anti-tinkle' circuitry to prevent pulse dialing phones from ringing the bell on other extensions. With tone dialing, anti-tinkle measures are not required.


The pins of the 6P6C connector are numbered 1–6, counting left to right when holding the connector tab side down with the cable opening toward the viewer.

Position Pair T/R ± RJ11 RJ14 RJ25 Twisted pair colors 25-pair colors Old colors[a] German colors[b] Australian colors
1 3 T + T3 Pair 3 Wire 1 Cat 5e/6
Pair 4 Wire 1
Pair 3 wire 1
Pair 3 wire 1
Pair 3 wire 1
2 2 T + T2 T2 Pair 2 Wire 1 Cat 5e/6
Pair 2 Wire 1
Pair 2 Wire 1 Old
Pair 2 ext. bell
Pair 2 ext. bell
3 1 R R1 R1 R1 Pair 3 Wire 2 Cat 5e/6
Pair 1 Wire 2
Pair 1 Wire 2 Old
Pair 1 wire A
Pair 1 wire B
4 1 T + T1 T1 T1 Pair 1 Wire 1 Cat 5e/6
Pair 1 Wire 1
Pair 1 Wire 1 Old
Pair 1 wire B
Pair 1 wire A
5 2 R R2 R2 Pair 3 Wire 1 Cat 5e
Pair 2 Wire 2
Pair 2 Wire 2 Old
Pair 2 ground
Pair 2 ground
6 3 R R3 Pair 1 Wire 1 Cat 5e
Pair 3 Wire 2
Pair 3 Wire 2
Pair 3 wire 2
Pair 1 Wire 1 Old

However, with German domestic telephone equipment (and that in some neighbouring countries), 6P4C plugs and sockets are typically only used to connect the telephone cable to the phone base unit, whereas the mechanically different TAE plug is used at the other end of the cable. Older base units may accommodate the additional connectors of TAE (E, W, a2, b2) and may feature non-RJ standard sockets that can be connected "straight" to TAE plugs. Further, flat DIN 47100 cables typically place the wires in ascending order. When used directly with 6P4C plugs, the colors will be scrambled.

Powered version of RJ11[edit]

In the powered version, Pins 2 and 5 (black and yellow) may carry low voltage AC or DC power. While the phone line itself (tip and ring) supplies enough power for most telephone terminals, old telephone terminals with incandescent lights in them (such as the classic Western Electric Princess and Trimline telephones) need more power than the phone line can supply. Typically, the power on Pins 2 and 5 comes from a transformer plugged into a wall near one jack, supplying power to all of the jacks in the house. Trimline and Princess phone dial lights are rated at 6.3 volts and the transformer output is typically around 5 volts, providing a long service life for the incandescent lamps.

Compatibility with structured cabling[edit]


Both the third and fourth pairs of RJ61 are similarly split. Because of this incompatibility, and because they were never very common to begin with, the TIA/EIA-568-B conventions are displacing RJ25 and RJ61 for telephones with more than two lines.

8P8C (8 position 8 contact)[edit]

This article is about the generic 8P8C modular connector. For the registered jack (RJ) wiring standard, see registered jack.
An 8P8C modular plug before being crimped onto a cable
Connector and cable

The 8 position 8 contact (8P8C) connector is a modular connector commonly used to terminate twisted pair and multi-conductor flat cable. These connectors are commonly used for Ethernet over twisted pair, registered jacks and other telephone applications, RS-232 serial using the EIA/TIA-561 and Yost standards, and other applications involving unshielded twisted pair, shielded twisted pair, and multi-conductor flat cable.

An 8P8C modular connection has two paired components: the male plug and the female jack, each with eight equally-spaced conductors. On the plug, these conductors are flat contacts positioned parallel with the connector body. Inside the jack, the contacts are suspended diagonally toward the insertion interface. When an 8P8C plug is mated with an 8P8C jack, the contacts meet and create an electrical connection. Spring tension in the jack's contacts ensures a good interface with the plug and allows for slight travel during insertion and removal.

Although commonly referred to as an RJ45 in the context of Ethernet and category 5 cables, it is incorrect to refer to a generic 8P8C connector as an RJ45.[8][9][10] A telephone-system-standard RJ45 plug has a key which excludes insertion in an un-keyed 8P8C socket.[11] The registered jack (RJ) standard specifies a different mechanical interface and wiring scheme for an RJ45S from TIA/EIA-568-B which is often used for modular connectors used in Ethernet and telephone applications. 8P8C modular plugs and jacks look very similar to the plugs and jacks used for FCC's registered jack RJ45 variants, although the RJ45S is not compatible with 8P8C modular connectors.

For more details on the registered jack naming confusion, see Registered jack § Unofficial plug names.

The original RJ45S jack mates with a keyed 8P2C modular plug,[12][13] and has pins 4 and 5 (the middle positions) wired for ring and tip of a single telephone line and pins 7 and 8 shorting a programming resistor. It was intended for high speed modems, and is obsolete.

Telephone installers who wired RJ45S modem jacks or RJ61X telephone jacks were familiar with the pin assignments that were part of the standard. However, the standard un-keyed modular connectors became ubiquitous for computer networking, and informally inherited the name RJ45. While RJ45S uses a keyed variety of the 8P body, meaning it may have an extra tab that a common modular connector cannot mate with, the visual difference is subtle and likely caused the confusion.






T568A wiring
T568B wiring


Types and incompatibility[edit]

Two types of 8P8C plugs and installation tools (used for crimping the plug onto a cable) are commonly available: Western Electric/Stewart Stamping (WE/SS) and Tyco/AMP. While both types look remarkably similar, the tooling used to install the two different plug types is mutually exclusive and cannot be interchanged between the two types. WE/SS compatible plugs are available from a large number of manufacturers, whereas Tyco/AMP plugs are produced exclusively by Tyco Electronics. Both types of modular plugs will plug into the same standard 8P8C modular jack.

WE/SS and Tyco/AMP 8P8C plugs have different spacing for the cable strain relief.[14][15] Thus, using a WE/SS 8P8C crimp die set on a Tyco/AMP 8P8C plug will crush the top of the connector and damage the crimp die set, and vice versa. While the WE/SS compatible plug is produced by a larger number of manufacturers than the Tyco/AMP plug, it is still important to know what style is being used to avoid damaging the plug or tool during crimping.

Both types of 8P8C plugs are available in shielded and unshielded varieties, depending on the attenuation tolerance needed. Shielded plugs are more expensive and require shielded cable, but have a lower attenuation and can reduce signal noise.

Although a narrower 4-pin and 6-pin connector will fit into the wider 8-pin jack, the smaller connector can potentially damage the contacts of the larger, because the outside edges of the smaller connector press onto the contacts of the larger. The outside edges of an RJ11/RJ45 plug typically project out by about 0.5 to 1.0 mm further than the contact surfaces, and these edges press the outermost contacts of the larger connector further than if a full-size connector were plugged in. The smaller connector may therefore permanently bend pins 1, 8 or 2, 7 of the larger connector.



The 8P8C modular connector is also used for RS-232 serial interfaces according to the EIA/TIA-561 standard.[16] This application is common as a console interface on network equipment such as switches and routers. Other applications include other networking services such as ISDN and T1.

In floodwired[b] environments the center (blue) pair is often used to carry telephony signals. Where so wired, the physical layout of the 8P8C modular jack allows for the insertion of an RJ11 plug in the center of the jack, provided the RJ11 plug is wired in true compliance with the US telephony standards (RJ11) using the center pair. The formal approach to connect telephony equipment is the insertion of a type-approved converter.




In landline telephony, an 8P8C jack is used at the point a line enters the building to allow the line to be broken to insert automatic dialling equipment, including intrusion alarm panels. In analogue mobile telephony, the 8P8C connector was used to connect an AMPS cellular handset to its (separate) base unit; this usage is now obsolete.




Pin T568A pair T568B pair 10/100BASE-T signal id. 1G/10GBASE-T signal id. Wire T568A color T568B color Diagram
1 3 2 DA+ DA+ tip Pair 3 Tip
white/green stripe
Pair 2 Tip
white/orange stripe


2 3 2 DA− DA− ring Pair 3 Ring
green solid
Pair 2 Ring
orange solid
3 2 3 DB+ DB+ tip Pair 2 Tip
white/orange stripe
Pair 3 Tip
white/green stripe
4 1 1 NC DC+ ring Pair 1 Ring
blue solid
Pair 1 Ring
blue solid
5 1 1 NC DC− tip Pair 1 Tip
white/blue stripe
Pair 1 Tip
white/blue stripe
6 2 3 DB− DB− ring Pair 2 Ring
orange solid
Pair 3 Ring
green solid
7 4 4 NC DD+ tip Pair 4 Tip
white/brown stripe
Pair 4 Tip
white/brown stripe
8 4 4 NC DD− ring Pair 4 Ring
brown solid
Pair 4 Ring
brown solid

Crossover cables[edit]



The pin arrangement for a 10P10C socket
A 10P10C plug


The most common uses of the 10P10C connector are in proprietary data transfer systems,[19] such as the Digiboard[20] and Equinox Super-Serial multi-port TIA-232 adapters.[20] 10P10C connectors are also used to implement RS-485 interfaces, and for data link connections in APC and Eaton uninterruptible power supplies. In the latter case, a keyed 10P10C plug with a protrusion on the pin 1 side near the back is used.

This connector is also used by some vendors, such as BOCA, for expansion modules of their multi-port RS-232C serial boards. For example, Cyclades (later absorbed by Equinox) used pin 1 as an "RI" (ring indicator) signal, which is seldom used, allowing an 8P8C plug to be inserted to their 10P10C socket for most applications. The Cisco Systems STS-10X terminal server features this connector. FordNet, a five-pair communications networking medium, also used the 10P10C between terminals.


Polycom utilizes this connector to interconnect multiple SoundStructure audio mixers.

The 10-pin connector is also used by Demag Cranes AG[21] in some pendant connections. National Instruments is also using the 10p10c connector for their NI 9237.[22]


See also[edit]


  1. ^ Patent filed 12 Nov 1971; U.S. Patent 3,789,344 issued 29 January 1974.[1]


  1. ^ "Patent US3789344 - Electrical connector for telephone cords". 
  2. ^ "Universal Modular Plug". 
  3. ^ "Modular Connectors" (PDF). Cambridge connectors. Retrieved 2012-09-28. 
  5. ^ "Apple Macintosh Plus", My Old Computers .
  7. ^ Direc TV Channel Control (wiki), GBPVR, Each end of a handset cord is wired opposite the other... 
  12. ^ Modular jack wiring, Ontario, CA: HVS 
  13. ^ Modular wiring reference, Siemon 
  14. ^ "Stewart Connector 937-SP-3088 – Eight conductor/eight position line cord module" (PDF). Glen Rock, PA: Bel Stewart Connector. 2006-02-01. Retrieved 2009-09-10. 
  15. ^ "Tyco/AMP 5-554739-2 – Modular plug assembly, 8 position, flat oval cable" (PDF). Harrisburg, PA: Tyco Electronics. 2008-03-31. Retrieved 2009-09-10. 
  16. ^ "RJ45", Layer 1, Zytrax .
  17. ^ "Modular Jack Wiring". Ontario, CA: HVS. Retrieved 2010-10-14. 
  21. ^ Demag cranes AG .





External links[edit]