Modular connector

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

Internally, the contacts in the plugs have sharp prongs that, when crimped, displace the wire insulation and connect with the conductors inside—a mechanism known as insulation displacement. Cables have either solid or stranded (tinsel wire) conductors, and a given plug is designed for only one type. The sharp prongs are different in the connectors made for each type of wire, and a mismatch between plug type and wire type results in unreliable connections. A modular plug for solid (single-strand) wire often has three slightly splayed prongs on each contact to securely surround and grip the conductor while scraping along the outside, and a plug for stranded wire has prongs that are designed to pierce the insulation and go straight through to contact multiple wire strands.

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

The dimensions of modular connectors are such that a narrower plug can be inserted into a wider jack that has more positions than the plug, leaving the jack's outermost contacts unconnected. The height of the plug's insertion area is 0.260 inches (6.60 mm) and the contacts are 0.040 inches (1.02 mm) apart (contact pitch), so the width is dependent on the number of pin positions.[7][8] However, not all plugs from all manufacturers have this capability, and some jack manufacturers warn that their jacks are not designed to accept smaller plugs without damage. If an inserted plug lacks slots to accommodate the jack's contacts at the outermost extremes, it may permanently deform those outermost contacts of an incompatible jack. Excessive resistance may be encountered when inserting an incompatible plug, as the outermost contacts in the jack are forcibly deformed.

Special modular plugs have been manufactured (for example, the Siemon UP-2468[9]) which have extra slots beyond their standard contacts, to accommodate the wider jack's outermost contacts without damage. These special plug connectors can be visually identified by carefully looking for the extra slots molded into the plug. The molded plastic bodies of the special plugs may also be colored with a light blueish tinge, to aid in quick recognition. 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. The 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.

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 conductors between the ends of a cord does not affect the signal routing. This may differ for other equipment, including hands-free headsets.

The Macintosh 128K, Macintosh 512K and Macintosh Plus from Apple as well as the Amiga 1000 from Commodore 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.[11] The connector on the Amiga 1000 used crossover wiring, similar to a telephone handset. The connector wiring on the Apple computers, 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 Apple computer or the keyboard.[12]

Modular connectors are often used for data links, such as serial line connections, because of their compact dimensions. For example, some DirecTV set-top boxes include a 4P4C data port connector with an adapter cord to a computer serial port so that remote control is possible from the computer.[13]

Cables sold as RJ11 often actually use 6P4C connectors (six positions, four contacts) and RJ14 wiring – 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 may be unused or provide low-voltage power for night light or other features on the telephone set. In some installations, an extra contact was also required for the ground connection for selective ringers.

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

Position	Pair	T/R	±	RJ11	RJ14	RJ25	Twisted pair colors	25-pair colors[upper-alpha 1]	Old colors[upper-alpha 2]	German colors[upper-alpha 3]	Australian colors	Dutch colors[upper-alpha 4]	Diagram
1	3	T	+	Does not appear	Does not appear	T3	white/green	white/green	white	pink	orange	Does not appear	6P6C connector showing the location of pin 1
2	2	T	+	Does not appear	T2	T2	white/orange	white/orange	black	green	red	orange
3	1	R	−	R1	R1	R1	blue	blue/white	red	white	blue	red
4	1	T	+	T1	T1	T1	white/blue	white/blue	green	brown	white	blue
5	2	R	−	Does not appear	R2	R2	orange	orange/white	yellow	yellow	black	white
6	3	R	−	Does not appear	Does not appear	R3	green	green/white	blue	gray	green	Does not appear

However, with German domestic telephone equipment, and that in some neighboring countries, 6P4C plugs and sockets are typically only used to connect the telephone cord to the phone base unit, whereas the mechanically different TAE connector is used at the other end to connect to a service provider interface. 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 directly to TAE plugs. Further, flat DIN 47100 cables typically place the wires in ascending order. When used directly with 6P4C plugs, the color coding may be undetermined.

In the powered version of the RJ11 interface, pins 2 and 5 (black and yellow) may carry low-voltage AC or DC power. While the telephone line on pins 3 and 4 (red and green) supplies enough power for most telephone terminals, old telephone terminals with incandescent lights, such as the Western Electric Princess and Trimline telephones, need more power than the phone line can supply. Typically, the power on pins 2 and 5 is supplied by an AC adapter plugged into a nearby power outlet which potentially even supplies power to all of the jacks in the house.

Structured cabling networks adhering to ANSI/TIA-568, ISO/IEC 11801 (or ISO/IEC 15018 for home networks) are widely used for both computer networking and analog telephony. These standards specify the T568A or T568B wiring arrangements compatible with Ethernet. The 8P8C jack used by structured cabling physically accepts the 6-position connector used by RJ11, RJ14 and RJ25. Only RJ11 and RJ14 have full electrical compatibility because Ethernet-compatible pin-outs split the third pair of RJ25 across two separate cable pairs, rendering that pair unusable by an analog phone. Both the third and fourth pairs of RJ61 are similarly split. The incompatible T568A and T568B layouts were necessary to preserve the electrical properties of the third and fourth pairs for Ethernet, which operates at much higher frequencies than analog telephony. Because of these incompatibilities, and because RJ25 and RJ61 were never very common, the T568A and T568B conventions have largely displaced RJ25 and RJ61 for telephones with more than two lines.

The shape and dimensions of an 8P8C modular connector are specified for US telephone applications by the Administrative Council for Terminal Attachment (ACTA) in national standard ANSI/TIA-1096-A and international standard ISO-8877. This standard does not use the short term 8P8C and covers more than just 8P8C modular connectors, but the 8P8C modular connector type is the eight-position connector type described therein, with eight contacts installed.

For data communication applications (LAN, structured cabling), International Standard IEC 60603 specifies in parts 7-1, 7-2, 7-4, 7-5, and 7-7 not only the same physical dimensions but also high-frequency performance requirements for shielded and unshielded versions of this connector for carrying frequencies up to 100, 250 and 600 MHz.

T568A wiring, defined in TIA-568

T568B wiring, defined in TIA-568

8P8C connectors are frequently terminated using the T568A or T568B assignments that are defined in ANSI/TIA-568. The drawings to the right show that the copper connections and pairing are the same, the only difference is that the orange and green pairs (colors) are swapped. A cable wired as T568A at one end and wired as T568B at the other end (Tx and Rx pairs reversed) is an Ethernet crossover cable. Before the widespread acceptance of auto MDI-X capabilities, a crossover cable was needed to interconnect similar network equipment (such as Ethernet hubs to Ethernet hubs). Crossover cables are sometimes still used to connect two computers together without a switch or hub, however, most network interface cards (NIC) in use today implement auto MDI-X to automatically configure themselves based on the type of cable plugged into them. A cable wired the same at both ends is called a patch or straight-through cable, because no pin/pair assignments are swapped. If a patch or straight cable is used to connect two computers with auto MDI-X capable NICs, one NIC will configure itself to swap the functions of its Tx and Rx wire pairs.

Pin	T568A pair	T568A color	T568B pair	T568B color	10BASE-T/100BASE-TX signal[20]	1000BASE-T/10GBASE-T signal	Wire	Diagram
1	3	white/green stripe	2	white/orange stripe	TD+	DA+	tip	Pin numbering on plug. Connected pins on plug and jack have the same number.
2	3	green solid	2	orange solid	TD−	DA−	ring
3	2	white/orange stripe	3	white/green stripe	RD+	DB+	tip
4	1	blue solid	1	blue solid	not used	DC+	ring
5	1	white/blue stripe	1	white/blue stripe	not used	DC−	tip
6	2	orange solid	3	green solid	RD−	DB−	ring
7	4	white/brown stripe	4	white/brown stripe	not used	DD+	tip
8	4	brown solid	4	brown solid	not used	DD−	ring

Two types of 8P8C plugs and crimping tools for installing the plug onto a cable are commonly available: Western Electric/Stewart Stamping (WE/SS) and Tyco/AMP. While the two types are similar, the tooling and plug types cannot be interchanged.[lower-alpha 2] 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 can be mated with a standard 8P8C modular jack.

Both types of 8P8C plugs are available in shielded and unshielded varieties for different attenuation tolerances as needed. Shielded plugs are more expensive and require shielded cable, but have a lower attenuation, and may reduce electromagnetic interference.

Although a narrower 4-pin and 6-pin plug fits into the wider 8-pin jack and makes a connection with the available contacts on the plug, because the body of the smaller connector may stress the remaining contacts,[lower-alpha 3] the smaller connector can potentially damage the springs of the larger jack.

8P8C connectors are commonly used in computer networking applications, where interconnecting cables are terminated at each end with an 8P8C modular plug wired according to TIA/EIA standards. Most wired Ethernet communications are carried over Category 5e or Category 6 cable terminated with 8P8C modular plugs. The connector is also used in other telecommunications connections, including ISDN and T1.

Where building network and telephone wiring is pre-installed, the center (blue) pair is often used to carry telephony signals. While this allows an RJ11 plug to connect, it may damage the modular jack; an approved converter prevents damage. 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 dialing equipment, including intrusion alarm panels.

The EIA/TIA-561 standard describes the use of 8P8C connectors for RS-232 serial interfaces.[23] This application is common as a console interface for network equipment, such as switches, routers, and headless computers.

8P8C modular connectors are also commonly used as a microphone connectors for PMR, LMR, and amateur radio transceivers. Frequently the pinout is different, usually mirrored (i.e. what would be pins 1 to 8 in the ANSI/TIA-568 standard might be pins 8 to 1 in the radio and its manual).

In analog mobile telephony, the 8P8C connector was used to connect an AMPS cellular handset to its (separate) base unit; this usage is now obsolete.

The physical connector is standardized as the IEC 60603-7 8P8C modular connector with different categories of performance. The physical dimensions of the male and female connectors are specified in ANSI/TIA-1096-A and ISO-8877 standards and normally wired to the T568A and T568B pinouts specified in the ANSI/TIA-568 standard to be compatible with both telephone and Ethernet.

A similar standard jack once used for modem and data connections, the RJ45S, used a keyed variety of the 8P8C body with an extra tab that prevents it from mating with other connectors; the visual difference compared to the more common 8P8C is subtle, but it is a different connector. The original RJ45S[18][24] keyed 8P2C modular connector, obsolete today, had pins 5 and 4 wired for tip and ring of a single telephone line and pins 7 and 8 shorting a programming resistor.

Electronics catalogs commonly advertise 8P8C modular connectors as RJ45. An installer can wire the jack to any pin-out or use it as part of a generic structured cabling system such as ISO/IEC 15018 or ISO/IEC 11801 using 8P8C patch panels for both phone and data.

A router-to-router crossover cable uses two 8-position connectors and a unshielded twisted pair (UTP) cable with differently wired connectors at each end.