Detail Explanation of Hard Real Time System with example | Real Time System

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hard real time systemHard real time system with examples,
Real Time System Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Hard Real Time System
Those real time systems in which timing constraint is hard and the failure to meet it, is considered to be a fatal fault. Hard deadline is composed on a job because a late result produced by the job after the deadline may have disastrous consequences. For example: Late command to stop the train may cause collision.

The usefulness of a result produced by hard real time job falls off abruptly and even become negative when the tardiness of the job becomes larger than zero. If the user wants the temporal quality of service provided by a task guarantee and the satisfaction of the timing constraints defining the temporal quality validated then the timing constraints are hard.

For many hard real time systems, all task and jobs executed in every operation mode of the system are known as priori. The approach to building and validating such systems is to avoid hardware and software features that lead to non-determinism. Therefore, it is possible to verify the satisfaction of all hard timing constraint by exhaustive simulation and testing. When an application creates a new task with hard timing constraint, it submits an admission request to the scheduler. Upon receipt of such request, scheduler does an acceptance test to determine whether the system can meet the timing constraints of the new task while meeting all the hard timing constraints of task previously admitted into system. The scheduler accepts and admits the new task to system only when task passes acceptance test.

Example: Consider an automatically controlled train. It can’t stop instantaneously. When the signal is red, its braking action must be activated a certain distance away from the signal post at which it must stop. The braking distance depends on the speed of train and safe value of deceleration. From the speed and safe deceleration of the train, the controller can compute the time for the train to travel the braking distance. This time in turn imposes a constraint on response time of jobs which sense and process the stop signal and activate brake.

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