The 4P4C connector is a standardize module connector used at ends of telephone handset cords, and is often called a handset connector. Handset connector is typically thought as a registered jack but it is not true, as it was not supposed to connect directly to telephone lines. Conversely it is most often stated as RJ9, RJ10 and RJ22.
Receivers and often headsets for use with telephones typically use a 4P4C connector. The two central pins are mostly used for the receiver while the outer pins connect to the transmitter, so that a reverse of pin connection remains unaffected. Ordinary handset receivers perform functions normally whenever the polarity is inverted, but the electronic microphone transmitter utilized in most modern handsets might not. Many of the telephones includes polarity guard, so that the polarity can be reversed devoid of affecting the operation. Telephones and Handsets manufactured before 1985 with carbon microphone transmitter were not sensitive to its polarity. Some hands free receivers may also have a 4P4C connector, but the wiring is different from the diagram.
Macintosh 128K, 512K and Macintosh Plus from Apple as well as the Amiga 1000 of Commodore used 4P4C connectors to connect keyboard to computer housing. This connector provides power to keyboard on the two outer contacts and data signals is received on the inner pair. The cables between computer and keyboard was a twisted cord with exterior similar to the telephone handset cable and connector on the Amiga 1000 had crossover wiring system, identical to telephone handset while connector wiring on earlier Apple computers, nonetheless required a polarized straight-through pinout. By using a telephone handset cable as an alternative of the supplied cable might short out +5 volt DC supply and can damage a computer or the keyboard.
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