Modular connector

modular connector is the name given to a family of electrical connectors an example of which is pictured. These connectors were originally used in telephone wiring and are still prevalent for that, but are used for a variety of other things. A modular connector's advantage over many other kinds of connectors is its small size and ease of plugging and unplugging. Many uses that originally used a bulkier connector have migrated to modular connectors. Probably the most well known applications of modular connectors is for telephone jacks and for Ethernet jacks, which are nearly always modular connectors.

The modular connector was first used in the Registered Jack system, so there are precise specifications of them within the Registered Jack specifications. Those are the specifications to which all practical modular connectors are built. However, the Registered Jack specifications don't name the physical connectors and there is in fact no recognized standard for the connectors themselves.

Modular connectors have gender. The male connector is called a plug, while the female connector is called a jack or sometimes a socket.

Modular connectors come in 4 sizes: 4 position, 6 position, and 8 position. A position is a place that can hold a conductor (pin). The positions need not all be used; a connector can have any even number of conductors. Unused positions are always the outermost positions. The connectors are designed so that a plug can fit into any jack that has at least the number of positions as the plug.

The members of the family are named like "6P2C", which means 6 positions, 2 conductors. Alternate namings are like "6x2" or "6/2".

Modular connectors lock together. A spring loaded tab called a "hook" on the plug snaps into a jack so that the plug cannot be pulled out. To remove the plug, you have to press the hook. The most common way to install a jack in a wall or panel is with the hook side down. This usually makes it easier to operate the hook when removing the plug, because the person grabs the plug with thumb on top and presses the hook with the index finger.

The positions of a jack are numbered left to right, looking into the receiving side of the jack with the hook side down, starting at 1. The positions of plug are numbered the same as the jack positions with which they mate. The number of a conductor is the same as the number of the position it's in. So for example in a 6P2C plug, only conductors 3 and 4 exist.

The application of jack versus plug is generally based on physical installation only. Jacks go in walls and panels, while plugs go on wires. This is in contrast to some other connector systems where the gender is related to the function of the connector (e.g. in a power cabling system, a female connector delivers power while a male connector receives power). Extension cables (male on one end, female on the other) are rarely seen; instead, people use male-male cables and female-female couplers.

Modular connectors also go by the names "modular phone jack/plug", "RJ connector," and "Western jack/plug." The 8P8C modular connector type is often called RJ45, but the name "RJ45" is also used for something quite different. The "modular connector" moniker is not terribly descriptive, but arose from its original use in a novel system of cabling designed to make telephone systems more modular.

Modular connectors were originally invented and patented by Bell Telephone Laboratories (patent filed 6 July 1973; U.S. patent 3,860,316 issued 14 January 1975), and replaced the hard-wired connections on most Western Electric telephones around 1976. At the same time, they began to replace screw terminals and large 3 and 4 pin jacks in buildings.