Other Circuits:

ALARM SET
When an alarm comes in, turn on the corresponding Annunciator light and then sound an audio alarm that will stay on until you press one of the two ‘Alarm Reset’ buttons. You could do this with software – turn an I/O on and then monitor another line to see when the reset button is pressed or… push this to hardware.

I pulse one I/O line (low/high/low). This triggers an opto-coupled SCR. The SCR keeps the audio alarm on until power is interrupted (The two alarm reset buttons).

ALARM RESET
Conversely, when an alarm clears, turn off the Annunciator light and turn on another alarm for about 0.5 seconds. Again, you could do this in software – raise an I/O, wait 0.5 seconds and then lower the line. Or… push this to hardware.

Pulse an I/O line – if this case, set the I/O line low and immediately raise it back to high. The pulse triggers a 555 timer set for 0.5 seconds. You get a brief tone alerting the operator an alarm has cleared.