Left to right, modular connectors: 8P8C plug, 6P6C plug, 6P4C plug, 4P4C plug, 6P6C jack.
An 8P8C modular plug. This is the common crimp type plug, of the same kind pictured above crimped onto a cable (with moulded sleeve)
Modular connector is the name given to a family of electrical connectors originally used in telephone wiring and now used for many other purposes. Many applications that originally used a bulkier, more expensive connector have now migrated to modular connectors. Probably the most well known applications of modular connectors are for telephone jacks and for Ethernet jacks, both of which are nearly always modular connectors.
Modular connectors were originally used with the registered jack system, which precisely describes how the connectors are wired for telecommunications. The registered jack specifications define the wiring patterns of the jacks, not the physical dimensions or geometry of the connectors of either gender. Instead, these latter aspects are covered by ISO standard 8877, first used in ISDN systems. TIA/EIA-568is a standard for data circuits wired on modular connectors.
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.
Main article: Registered jack § Unofficial plug names
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 Bell Telephone Laboratories in 1975. They replaced the hard-wired connections on most Western Electrictelephones 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: male connectors are called plugs, while female connectors are called jacks or, sometimes, sockets.
Plugs are used to terminate loose cables and cords, while jacks are used for fixed locations on surfaces such as walls and panels, and on equipment. Other than telephone extension cables, cables with a modular plug on one end and a jack on the other are rare. Instead, cables are connected using a male-to-male adapter, which consists of two female jacks wired back-to-back.
Latching tab and orientation
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
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 6 positions and 4 contacts, to which are attached just 2 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 6 positions and 2 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.
The contact positions are numbered sequentially starting from 1. When viewed head-on with the retention mechanism on the bottom, jacks will have contact position number 1 on the left and plugs will have it on the right. Contacts are numbered by the contact position. For example, on a 6-position, 2-contact plug, where the outermost four positions do not have contacts, the innermost two contacts are numbered 3 and 4.
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 byDigital Equipment Corporation (DEC) 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. 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) 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. 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.
The contact spacing is always 1.02 mm ( center to center ).
Modular connector dimensions (millimeters)
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 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-B standard. Other applications have no standardization; for example, there are multiple conventions for use of 8P8C connectors for RS-232.