In a digital communicator system, the voltage across RJ31X terminals when active typically falls within which range?

• A. 0V to 2V
• B. 7V to 9V
• C. 12V to 14V
• D. 15V to 18V

Answer: (B)

When a digital communicator seizes the telephone line to dial out, it loads the line with a controlled, limited current. The two-wire phone line in a building is normally around 48 V DC when idle, but once the alarm panel goes off-hook to place a call, the line voltage drops to a low level due to the load it presents. The standard, expected reading across the RJ31X terminals during this active dialing state is in the 7 to 9-volt range. This low voltage is enough to send dial pulses and digits while preventing excessive current from flowing through the line.

The other ranges don’t fit this active-dialing behavior. They would imply either too high a load (keeping the voltage high) or an almost shorted condition (near 0 V), neither of which matches how the alarm’s dialer interacts with the phone line.