Which of the following jacks is used to interface the digital communicator with the telephone line?

• A. RJ11
• B. RJ31X
• C. RJ45
• D. RJ12

Answer: (B)

The interface between an alarm system’s digital communicator and the phone network is a dedicated alarm jack. The jack used is RJ31X because it’s specifically designed for alarm systems to connect to the telephone line and to seize the line when the panel dials out to report an alarm. This ensures the alarm signal can be sent even if the line is in use for voice calls, while still allowing normal phone use when the system isn’t sending signals.

RJ11 is just a standard residential phone plug, RJ12 is a six-position version used for some multi-line setups, and RJ45 is the Ethernet jack for networks. None of these are the specialized interface required for alarm communications, which is why RJ31X is the correct choice.