Most people will recognize modular connectors as the plug at the end of a telephone wire. It has more uses than to plug a land-line into your home, though. The most popular uses for modular connectors are for phone jacks, data networks, low-speed serial connections and Ethernet jacks.
Modular connectors were invented in 1974 and replaced the hard-wired connections in most telephones by 1976. The jacks are male while the sockets are female. The jack and socket are designed to latch together with the help of a spring-loaded tab. Once the jack is inserted into the socket it will not release until the tab is pressed down. The jack is generally inserted tab side down.
The tab is a weak part of the design and can break off. When this happens, the jack can still be inserted into the socket but there is no guarantee that the electrical contacts will remain touching.
Higher quality cables will have a flexible plastic sleeve over the plug to keep the tab from breaking off. The sleeve must be put on before the plug is crimped onto the cable, so older connectors cannot be retrofitted.
Modular connectors have either four, six, eight or ten positions, or contact locations.
Although modular connectors are physically interchangeable, they may not be interoperable and there could be electrical damage to the circuits if it is plugged into the wrong system. For this reason, some modular jacks are purposely manufactured to be a non-standard size.
Modular connector plugs can be attached to cables in the field using a hand-held crimper. The crimper presses the plug contacts into cable, permanently attaching the two. It is important to use the appropriate crimping tool, otherwise the cable and jack will be damaged. Factory-assembled modular connectors generally perform better than those assembled in the field.