The CX4 is a 19" 1RU unit, and so fits well with other standard 19" units. It connects between the mixer (or pre amp) and the power amplifiers. It can be connected to four channels of audio, usually two stereo pairs.
The priority input, which can be a microphone or line level source, connects to the "priority input" socket. This can be a microphone that is in general use, which then becomes the priority mic. for any safety announcements if the fire alarm is triggered.
The "control input" is connected to the fire alarm or other triggering device. This input can be either voltage (active high or low) or a switch (normally open or normally closed).
CX4 - Fire Alarm Priority & Sound Level Limiter.