Final answer:
The voltage across terminals 1 and 6 of an active digital communicator's RJ31X connector is typically around 48 volts, which is standard for line seizure by an alarm system.
Step-by-step explanation:
When a digital communicator is active on an alarm system, the voltage across terminals 1 and 6 of a RJ31X connector is typically designed to be around 48 volts. This is because the RJ31X jack is meant to allow an alarm system to seize the phone line for emergency communication. However, it is essential to take into account that voltage readings can slightly vary based on the specific installation or the telephone company's line voltage.