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The 2-Wire system is foolproof, simple & functional. The heart of the traffic control system is the I.S. power supply / controller (ALSUP_IS TRAFFIC). The controller is mounted in the safe area and powers a parallel connected network (intrinsically safe) of alarm enunciators (lights) and operating switches via a two-wire supply bus. There are two enunciator types, a 'Cross / Arrow' LED display (BLITE_IS) and a LED 'Tower' light (BLLED_IS) - each of which may be mounted in the hazardous zone. Operating switches are 'simple devices' that connect a resistor across the two-wire when operated. A switch is positioned wherever an operator (driver) needs to request a change of system status. The principle of operation is that the output (I.S. field bus) is dual state - Red or Green. The polarity of the two-wire determines the status of the lights and is changed by the controller on detecting a change of state request (increased current due to switch operation) over the two-wire. The controller and lights are set up to control a traffic section or 'block' that may be an intersection, series of intersections, single access tunnel or any combination of these. The lights are set up at the entrance/s to the 'block' under control to either allow access (green light / green arrow) or bar it (red light / red arrow). There are 3 x 24V digital inputs to the controller that are used for calibration, manual operation and a remote alarm state. There are three voltage-free output contacts that are used (if required) to indicate the current output state and to confirm the health of the unit.