Patent US 3,934,237

An alarm device comprising a first program mechanism at a monitored station and a second program mechanism at a central monitoring station. The program mechanisms have identical shift registers and identical feedback networks and generate binary signals according to a predetermined arbitrary law. A sensing device is connected to the first program mechanism to cause a departure of the binary signals from the predetermined arbitrary law in the presence of a disturbance. The binary signals from the first program mechanism are compared with the output of the second program mechanism, and an alarm is produced if the outputs are different.