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The XLR connector is a rugged electrical connector design. XLR plugs and sockets are used mostly in professional audio and video electronics cabling applications. Home audio and video electronics normally use RCA connectors.
In reference to its original manufacturer, Cannon (now part of ITT), the connector is colloquially known as a cannon plug or canon. Originally the "Cannon X" series, subsequent versions added a Latch ("Cannon XL") and then a Rubber compound surrounding the contacts, which led to the acronym XLR. Many companies now make XLRs. The initials "XLR" have nothing to do with the pinout of the connector. XLR connectors can have other numbers of pins besides three.
They are superficially similar to the older smaller and less rugged DIN connector range, but are not physically compatible with them.
The most common is the 3-pin XLR3, used almost universally as a balanced audio connector for high quality microphones and connections between equipment. XLR4s with four pins are used for ClearCom, Tecpro intercom systems, 12 volt power, and the older AMX analog lighting control. The XLR5 is the standard connector for DMX digital lighting control.
Many other types exist, with various pin numbers. Most notable are two now obsolete 3-pin patterns manufactured by ITT Cannon. The power Cannon (also called the XLR-LNE connector) had shrouded pins and red insulation, it was intended as a mains power connector, but has been superseded by the IEC mains connector and increasingly, more recently, the PowerCon connector developed by Neutrik.
The loudspeaker Cannon had blue or white insulation (depending on its gender), was intended for connections between audio power amplifiers and loudspeakers. At one time XLR3 connectors were also used extensively on loudspeaker cables, as when first introduced they represented a new standard of ruggedness, and economic alternatives were not readily available. The convention was that a 2-conductor loudspeaker cable had XLR3F connectors on both ends, to distinguish it from a 3-conductor shielded signal level cable which has an XLR3F at one end and an XLR3M at the other. Either pin 2 or 3 was live, depending on the manufacturer, with pin 1 always the 'earthy' return. This usage is now both obsolete and dangerous to equipment but is still sometimes encountered, especially on older equipment. For example, some loudspeakers have a built-in XLR3M as an input connector. This use was superseded in professional audio applications by the Neutrik Speakon connector.
The female XLR connectors are designed to first connect pin 1 (the earth pin), before the other pins make contact, when a male XLR connector is inserted. With the ground connection established before the signal lines are connected, the insertion (and removal) of XLR connectors in live equipment is possible without picking up external signals (as it usually happens with, for example, RCA connectors).
Five pin XLR connectors are now used almost exclusively in lighting control for entertainment applications. While only three pins are used to carry the DMX signal, the design allows expansion with the remaining two pins (now considered for use with Remote Device Management (RDM) and Architecture for Control Networks (ACN) and also prevents users from using lower-grade microphone cable for transmission of signals. Although, many manufacturers of DJ lighting and professional lighting are opting to use three-pin connectors. Some manufacturers such as Leviton and Lightronics have even established new protocols non-compatible with DMX that use three pin XLR to control lighting devices (primarily dimmers made by the same manufacturer). These protocols are not generally accepted in the industry and are used primarily to promote consumers to buy multiple products from the same company.
EIA Standard RS-297-A describes the use of the XLR3 for balanced audio signal level applications:
Some audio equipment manufacturers reverse the use of pin 2 (properly the normal input) and pin 3 (inverting input). This reflects their own previous usage before any standard existed. Pin 1 is always ground, and many connectors connect it internally to the connector shell or case.
Note that neither the standards nor manufacturers agree on the best way to handle the usage of pin 1 at both ends of a cable, particularly with respect to the cable shield, the connector's shell, signal ground, and a third cable wire connected to pin 1 -- which may (or may not) be connected to the shield. Comments on AES48
An XLR3M (male) connector is used for an output and an XLR3F (female) for an input. Thus a microphone will have a built-in XLR3M connector, and signal cables such as microphone cables will each have an XLR3F at one end and an XLR3M at the other. At the stage box end of a multicore cable, the inputs to the mixing desk will be XLR3F connectors, while the returns to the stage will be XLR3M connectors. Similarly, on a mixing desk, the microphone inputs will be XLR3F connectors, and any balanced outputs XLR3M connectors.
Neutrik also offers a "combo" jack that accepts both XLR and 1/4" TRS plugs.