Real time systems
Real time: It is the time span taken by the system to complete all its tasks and provides an output for an input. This time span should be the same for computation of all its tasks.
Real time system: Real time systems are those which must produce the correct response within the specified or defined time limit. If it exceeds these time bonds it results in performance degradation and/or malfunction of system.
For example in aircraft engine control system, the real time control system should perform its task within a specified time as the operator/pilot intended and failure of this can cause the loss of control and possibly the loss of many lives.
Real time program: A program for which the correctness of operation depends upon the logical output of the computation and the time at which the results are produced. Every real time system must be having real time clock which specifies the time of the execution of the task or interruption of the task.
Types of real time system:
As per the clock and execution procedure of task the real time systems are divided as follows
- Clock based systems
- Event based systems
- Interactive systems
Clock based real time system:
In this system the computation of its task has to be completed in the specified time interval called real time clock. Most of plant control systems are in this category. The clock can be in hours for some chemical process or it may be in milli seconds for some control systems. For example of feedback control of tank level, the real time system should read the level of the tank, process it with control algorithm and actuate the valve accordingly to maintain the level. These three tasks should perform in the specified time interval i.e sampling of input, processing and output response.
This clock can be continuous or discrete. In continuous the system will perform the task continuously within a specified time. This is same as above tank level controller where it is a continuous control process. In some chemical industries, The chemicals should be added with some specified intervals these are called discrete control systems.
Event based real time system:
In plants there are some systems where actions have to be performed in response of some events instead of some particular time intervals. For example the control system has to close the value if the liquid level in the tank reaches its high level. Here this action is not time based, its an event based and these are used extensively to indicate the alarm conditions and initiate alarm actions, for example indicating the liquid level in the tank high or temperature of the liquid high etc. The specification of event based systems usually indicates that the system must respond within specified maximum time to a particular event. These systems uses interrupts to indicate the real time system that the action is required. Some small system uses Polling i.e the system periodically asks the various sensors to see whether the action is required. These systems are basically aperiodic tasks and may have deadlines expressed in terms of start up time or finish time. For example after sensing of level of liquid the the valve closer should start after some interval.
The combination of Clock based system and Event based system which gives the importance of average execution time of the task is called interactive systems. This covers the systems like Automatic teller machine, reservation system for hotels, Airlines booking etc. This systems receive the input from the plant or operator and initiate the task and executes within the average response time. For an example if you want draw cash from ATM when u put your card then it process the task of giving the money out. In this case the response time depends on the network traffic and internal processing time and it does not bother about other atmospheric changes.