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.
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.
RJ11 is a physical interface often used for terminating single telephone lines. RJ14 is similar, but for two lines, and RJ25 is for three lines. RJ61 is a similar registered jack for four lines, but uses an 8P8C connector.
Modular connectors are designated using two numbers that represent the maximum number of contact positions and the number of installed contacts, with each number followed by P and C, respectively. For example, 6P2C is a connector having six positions and two installed contacts. Alternate designations omit the letters while separating the position and contact quantities with either an x (6×2) or a slash (6/2).
The most common uses of the 10P10C connector are in proprietary data transfer systems, such as the Digiboard and Equinox Super-Serial multi-port TIA-232 adapters. 10P10C connectors are also used to implement RS-485 interfaces, and for data link connections in APC and Eaton uninterruptible power supplies. In the latter case, a keyed 10P10C plug with a protrusion on the pin 1 side near the back is used.
The contact assignments (pinout) of modular connectors vary by application. Telephone network connections are standardized by registered jack designations, and Ethernet over twisted pair is specified by the TIA/EIA-568 standard. For other applications, standardization may be lacking; for example, multiple conventions exist for the use of 8P8C connectors in RS-232 applications. For this reason, D-sub-to-modular adapters.
Modular connectors are designed to latch together. As a plug is inserted into a jack, a plastic tab on the plug locks so that the plug cannot be pulled out. To remove the plug, the latching tab must be depressed against the plug to clear the locking edge. The standard orientation for installing a jack in a wall or panel is with the tab down.
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.