The heart of this alarm system is the power supply / controller (ALSUP_24 Prestart). The controller is DIN-rail or foot mounted in a control cabinet and powers alarm enunciators via a two-wire supply bus. The system is totally down-line powered with no batteries or repeaters. There are several enunciator types, depending on the application and price point. 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 according to an input command. In the Red or Alarm State the enunciators sound a two-tone piezo alarm while flashing a red light (if fitted). In the Green or 'Safe' state the light is maintained solid green with no audio.

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.

Overlapping alarm performance is achieved with 100mtr centres between alarm units.