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RJ31X Pin Layout (Fire Alarm and Security Wiring)
The RJ31X jacks are designed to provide a fire or security alarm panel preference in dialing out in case of emergency. If any other terminal device (phone, fax, modem) is using the line when the alarm panel needs to call, the panel's internal circuitry breaks that connection and establishes its own. Do not plug any other device into the RJ31X jack: the jumpers between pins 1-4 and 5-8 are removed upon a 8P8C plug insertion, and the dial tone is not going to be delivered to house phones (trunk ports in case of aa KSU or PBX). The RJ31X jack always has to be located "upstream" from any other telephone equipment on the premise.
When a plug is removed, the house wiring gets connected to the phone company wiring through the 1-4, 5-8 jumpers, therefore it is not necessary to remove RJ31X jack if the fire alarm/security equipment has been uninstalled.