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Re: Fire Alarm Cut Off

You might have to consider that the normally on/normally closed condition means that it will be the loss of the 24VDC rather than the application of it that should cause the system to mute.  If you use a device that relies on the application of DC voltage or a contact closure to mute the audio then that may require an intermediate device to modify the control signal type and/or action.  That is why you want to be sure to coordinate the control form being provided.

For example, the Radio Design Lab devices that Jason noted appear to require a TTL logic input to mute, thus with a 24VDC normally on control signal you may require something like a 24VDC operated, nomally closed relay wired between Trigger and Ground on the RDL devices.  That realy would then be held open by the alram system control signal until an alarm, at which time the 24VDC control signal would stop and the added relay would provide a dry contact closure to trigger the audio mute.

Do the processors have external control inputs?  If they do that would seem likely to avoid having to externally mute 21 audio lines.