The Digitize Radio Boxes include interior and exterior transmitter boxes. In typical interior radio boxes, each zone is end-of-line resistor type with 4.7K Ohms as the standard value.  Ground detection is built-in for both sides of each zone connection.  The unit consists of 12VDC battery, AC transformer, microprocessor control board, and AM or FM radio transmitter.  Built in features include battery charger, watchdog timer, auto restart, clock circuit, alarm/trouble visual and audible indicators, zone bypass, local FORM C trouble relay, AC power fail and low battery detection, fuse blow circuit, and a processor stop circuit should battery voltage drop below 8.5 VDC. NFPA 1221 compliant.