been detected, and 0 mA would produce a trouble indication, telling the monitoring site the alarm circuit had failed. Some devices, such as two-way radio remote control consoles, can reverse the polarity of currents and can multiplex audio onto a DC current. These devices can be employed for any remote control need a designer might imagine. For example, a current loop could actuate an evacuation siren or command synchronized traffic signals. Two-way radio use Current loop circuits are one possible way used to control radio base stations at distant sites. The two-way radio industry calls this type of remote control DC remote. This name comes from the need for DC circuit continuity between the control point and the radio base station. A current loop remote control saves the cost of extra pairs of wires between the operating point and the radio transceiver. Some equipment, such as the Motorola MSF-5000 base station, uses currents below 4 mA for some functions. An alternative type, the tone remote, is more complex but requires only an audio path between control point and base station. For example, a taxi dispatch base station might be physically located on the rooftop of an eight-story building. The taxi company office might be in the basement of a different building nearby. The office would have a remote control unit that would operate the taxi company base station over a current loop circuit. The circuit would normally be over a telephone line or similar wiring. Control function currents come from the remote control console at the dispatch office end of a circuit. In two-way radio use, an idle circuit would normally have no current present. In two-way radio use, radio manufacturers use different currents for specific functions. Polarities are changed to get more possible functions over a single circuit. For example, imagine one possible scheme where the presence of these currents cause the base station to change state: no current means receive on channel 1, (the default). +6 mA might mean transmit on channel 1 −6 mA might mean stay in receive mode but switch to channel 2. So long as the −6 mA current were present, the remote base station would continue to receive on channel 2. −12 mA might command the base station to transmit on channel 2. This circuit is polarity-sensitive. If a telephone company cable splicer accidentally reversed the conductors, selecting channel 2 would lock the