Disaster prevention monitoring and control facility

A disaster prevention monitoring and control facility comprising:

• a plurality of terminal units;
• a central unit for connection to said plurality of terminal units, and having means for transmitting and receiving data information signals to and from each of said plurality of terminal units;
• sum checking means, disposed in each of said terminal units and central means, for summing selected portions of each of said data information signals transmitted and received by said central unit and by each of said terminal units, and for determining whether each of said data information signals contains an error on the basis of the thus summed selected portions of each of said data information signals;
• a polling means, disposed in said central unit, for polling each of said terminal units, said polling means conducting P polling attempts during a first polling cycle, P being an integer greater than 1, wherein said central unit initiates a transfer to another process when a terminal unit fails to properly respond to said P polling attempts, said polling means further conducting Q polling attempts during a second polling cycle after said transfer to another process, Q being a positive integer less than P;
• wherein, during said first and second polling cycles, each subsequent polling attempt is conducted when said sum checking means indicates that said data information signal received in response to a previous polling attempt from one of said terminal units contains an error and when an absence of a data information signal is detected in response to a previous polling attempt from one of said terminal units.