Solid State Interlocking
More on solid state Electronics
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Implementations use combination of the methods also. The feed back from the output to the comparator for ensuring that multiple processes lead to the same result before command is given is a very important part of the solid state interlocking system and forms the crux of the equipment. Large part of the safety is dependant on this part of the implementation and the design of the comparator is critical to the system. The comparator has to ensure that by itself it is fail safe and then it can keep the total system is failsafe.
Failsafe in Railway signaling parlance means it will not cause clearance of a signal, or it will not move any switch. Fail safety is required to be ensured in a Solid State Interlocking system in several levels:
a) Solid State Hardware : The design is done so that a single failure of the hardware will be detected and cause the system to give suitable alarms and also shut down if needed for maintaining safety.
b) Other hardware: The fail safety in the connected system is maintained by feedback and comparison using fail safe comparator
c) Communication: If any communication is involved as in the connecting link between outdoor and indoor then the communication requires to be made fail safe by protecting it against errors in the transmission links and other such conditions of fault of the communication link. Protection against such conditions is done by coding of the data and use of error correcting techniques as used in telecommunication links.
d) Software: By using different logic with the same input to protect against design defects and using watch dog timer for protection against drifts during execution of the software.
The watch dog arrangement is a piece of software which ensures that the software for the interlocking is performing its required functions properly and the software is not drifting from the proper paths. This piece of software is put in the software for solid state interlocking with the logic that as the main software progresses normally it triggers this software in its path. In case the main software drifts then this software will not be triggered and the watch dog circuit will trigger a alarm or cause shut down of the system as under such condition it can cause unsafe side failure.
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