Apparatus for monitoring fire and effecting indication and control operations comprises a matrix monitoring signal line network comprising vertical lines and horizontal lines. A plurality of sensing blocks are connected between vertical lines and horizontal lines of the network. Each of the sensing blocks comprises a sensor connected to a power supply. A monitoring circuit having a switch controlled by the sensor and a diode of forward direction in series with the switch is connected between the respective vertical line and horizontal line of the network. An operating circuit connected in parallel with the monitoring circuit comprises a relay connected in series with a second diode of reverse direction. The horizontal lines and vertical lines of the network are sequentially scanned by a circuit including a counter and demultiplexer. When the scanning detects a short circuit produced by operation of a sensor and respective monitoring circuit, the polarity of the vertical and horizontal lines of the network are reversed and this results in operation of the operating circuit to activate an indicator and any accessory controls such as door closers and the like. After reversal of polarity, the network is again scanned and normal polarity of the network is thereafter restored.