The SSI is programmed by logic, it is mainly based on two types of architectures, as well as hardware and software procedures in the cabin. The first type is based on an architecture multiprocessors (triple modular redundancy) that ensures security, inter alia, by means of the correlation obtained by the three computers in parallel that process the same functions. A control device (watchdog) performs the selection on the correlation (at least two computers on 3) the results obtained; The watchdog degrades the system in the event of a discrepancy, because the system can continue to function, but is blocked only if there is a further discrepancy. The second type of system is based on an architecture with 2 processors in Normal function (computers 2 of 2) and other 2 in function of reserve (2 processors of 2); there is a check to see that the 2 processors in Normal function are in agreement, if they are not brought into use the second function of reserve; Also in this case it degrades the system in the event of a discrepancy, because the system can continue to function, but is blocked only if there is a further discrepancy. In Yard are the same equipment present in traditional interlocking
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