The 2-Wire I.S. alarm system is foolproof, simple & functional. Being down-line powered, it requires no batteries or repeaters. The heart of the alarm system is the I.S. power supply / controller (ALSUP_IS). The controller is mounted in the safe area and powers alarm enunciators via a two-wire supply bus. There are several enunciator types, each of which may be mounted in the hazardous zone. The principle of operation is that there are two output states, which are 'safe' (green) & 'alarm' (red). The polarity of the two-wire determines the output 'state' and is set by the controller on command. Either output state can be maintained indefinitely. There are three voltage-free output contacts that are used (if required) to indicate the current output state and to confirm the operation of all connected units.