Hot Standby Operation in Railway Signalling
At least two units of the same type will be powered up, receiving the same set of inputs, performing identical computations and producing identical outputs in a nearly-synchronous manner. The outputs are typically physical outputs (individual ON/OFF type digital signals, or analog signals), or serial data messages wrapped in suitable protocols depending upon the nature of their intended use. Outputs from only one unit (designated as the master or on-line unit, via application logic) are used to control external devices (such as switches, signals, on-board propulsion/braking control devices, etc.) or simply to provide displays. The other unit is a hot-standby or a hot spare unit, ready to take over if the master unit fails. When the master unit fails, an automatic failover to the hot spare occurs within a very short time and the outputs from the hot spare, now the master unit, are delivered to the controlled devices and displays. The controlled devices and displays may experience a short blip or disturbance during the failover time. However, they can be designed to tolerate/ignore the disturbances so that the overall system operation is not affected.
Read more about this topic: Hot Spare
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