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Modular connector
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Modular connector is the name given to a family of electrical connectors examples of which are pictured.
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Modular connector is the name given to a family of electrical connectors examples of which are pictured. These connectors were originally used in telephone wiring. Even though they are still used for that purpose they are used for a variety of other things as well. A modular connector's advantage over many other kinds include small size and ease of plugging and unplugging. However the plastic retaining spring clip tends to get broken off when cables are pulled from storage for use. If that happens, the plug can easily fall out of the wall jack. 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.
Modular connectors were first used in the Registered Jack system, so Registered Jack specifications describe them precisely. Those are the specifications to which all practical modular connectors are built. However, the Registered Jack specifications don't name the physical connectors. Instead, these are covered by ISO standard 8877, first used in ISDN systems.
Sizes and contacts
Modular connectors have positions, some or all of which have contacts in them, some or all of which have wires attached; positions with omitted contacts or contacts unattached to wires are unused for the electrical connection, but ensure that the plug fit correctly. For instance, RJ11 wires usually have connectors with 6 positions and 4 contacts, to which are attached 2 wires.
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: when contacts are omitted, they are typically done so from the outermost two contacts inward, such that the number of contacts is always an even number.
The connectors are typically designated using 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").
The dimensions of the connectors are such that a plug can be inserted into a jack that has fewer positions than the plug, leaving the jack's outermost contacts unterminated. 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.
When all of the positions have contacts, the contacts are numbered sequentially starting from 1. When viewed head-on with the retention mechanism on the bottom, jacks will have contact number 1 on the left and plugs will have it on the right. If the connector has positions that do not have contacts, the remaining contacts are numbered as if all of the positions had contacts: 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.
Pinouts
Contact assignments, or pinouts, vary by application. While standardized for telephone applications by registered jack standard, and Ethernet over twisted pair by the TIA/EIA-568-B standard, there may be no standardization within an application: 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 without the D-sub contacts (pins or sockets) terminated, so that the D-sub-to-modular contact pairing can — and must — be performed by the end-user.
Nomenclature
Modular connectors also go by the names "modular phone jack/plug", "RJ connector," and "Western jack/plug."
One should distinguish the physical connector itself, which does not specify the wiring/pinout, from various standards such as registered jack (RJ) which specify the connector and the pinout; for instance, the 8P8C modular connector type is often loosely called RJ45 (for a standard that uses this connector), but 8P8C connectors are also used with different wiring, such as for Ethernet over twisted pair.
The term "modular connector" is not terribly descriptive, but arose from its original use in a novel system of cabling designed to make telephone systems more modular. This includes the 4P4C handset connector which is not part of the Registered Jack system.
Gender
Modular connectors have gender: male connectors are called plugs, while female connectors are called jacks or, sometimes, sockets.
The choice of gender is generally based on the location of the connector: plugs are used to terminate loose cables and cords, while jacks are used for fixed locations on surfaces such as walls and panels. (This is in contrast to some other connector systems, where the choice of gender is based on the function of the connector: for example, in power cabling systems, female connectors deliver power, while male connectors receive power.) Cables with a plug on one end and a jack on the other are rare; instead, cables that have the more common arrangement of plugs on both ends are connected using a male-to-male adapter, which has two jacks.
History
Modular connectors were originally invented and patented by Bell Telephone Laboratories (patent filed 6 July 1973; 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.
Locking & Hook
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.
A disadvantage of modular connectors is that the fragile hook on a plug easily snags and often breaks when you try to pull a cord through other cords and other obstructions. Some higher quality cables have a flexible sleeve called a "boot" over the plug, or a special hook design, to prevent this. Boots are seen mainly on 8P8C data cables.
See also
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