2. Soft real-time
Systems where will still function correctly if deadlines are occasionally missed.
The characteristics of a soft real-time system are:
- rising cost for lateness of results
- acceptance of lower performance for lateness
Ex.: - data acquisition system.
- a vending machine
- network interface subsystem, we can recovered from a missed thing by using one or another network protocol asking to resend the missed thing
3. Real real-time
Systems which are hard real-time and which the response times are very short.
Ex. missile guidance system.
-
Firm real-time
Systems which are soft real-time but in which there is no benefit from late delivery of service.
REAL_TIME FAILURE DETECTION, ISOLATION AND RECONFIGURATION (FDIR)
REAL TIME BACK UP SYSTEM
CHARACTERISTIC OF REAL TIME SYSTEM
A real time system possesses many special characteristics either inherent or imposed which are identified in the following sections. Clearly not all real time systems will exhibit all these characteristics, however, any general purpose language which is to he used for the effective programming of real time systems must have facilities which support these characteristics.
The basic characteristics of a real time system are:
- Large and complex
It is often said that most of the problems associated with developing software are those related to size and complexity. Largeness is variety. The variety is that of needs and activities in the real world and their reflection in a program, but real world is continuously changing. It is evolving, so all complex systems must continuously evolve by redesigning or rewriting software to respond to the continuously changing requirements of the real world.
Real-time systems undergo constant maintenance and enhancements during their lifetimes. They must be extensible. Although real-time software often complex, features provided by real-time languages and environments enable these complex systems to broken down into smaller components, which can be managed effectively.
- Manipulation of real number
Many real-time systems involve the control of some engineering activity.
A simple computerized controller
Most controllers are implemented as computerized to prevent the complexity of the model, and the number of distinct (but not independent) input and output since linear first order equations are only an approximation to the actual characteristics of the system also presents complications.
Items marked with a * are now discrete values; the sample and hold operations is carried out by an analog-to-digital converter, both converters being under the direct control of the computer.
Within the computer, the differential equations can be solved by numerical techniques, although the algorithms themselves need to be adapted to take into account the fact that plant outputs are now being sampled.
The implementation of this algorithm is, however, of direct concern. They can be mathematically complex and require a high degree of precision. A fundamental requirement of a real-time programming language therefore is the ability to manipulate real, fixed or floating-point numbers.
- Extremely reliable and safe
The more society relinquishes control of its vital functions to computers, the more imperative it becomes that those computers do not fail.
The sheer size and complexity of real-time system exacerbates the reliability problem; not only must executed difficulties inherent in the application be taken into account but also those introduced by fault software design.
- Concurrent control of separate system components
A system will tend to consist of computers and several coexisting external elements with which the computer programs must interact simultaneously and exist in parallel.
The example is like the program has to interact with an engineering system and the computer’s display devices, the operator’s console, the database and the real time clock and it is necessary to consider distributed sites and multiprocessor systems where the data is to be collected and processed at various geographically distributed sites, or where the response time of the individual components cannot be met by a single computer.
5. Real time facilities
Real-time system are usually constructed using processors with considerable space capacity, thereby ensuring that “worst case behavior” doesn’t produce any unwelcome delays during critical periods of the system’s operation and generates the appropriate output at the appropriate times under all possible conditions by make full use of all computing resources at all times.
Given adequate processing power, language and run-time support is required to enable the programmer to:
- specify times at which actions to be performed and completed
- respond to situations where all the timing requirements cannot be met and where the timing requirements are changed dynamically (mode changes).
These are called real time control facilities for enable the program to synchronize with time itself.
For example:
- with direct digital control algorithms, it is necessary to sample readings from sensors at certain periods of the day and as the results of these readings, some action will be performed like the supply of the electricity to domestic customers need to be increased at 5 p.m. on Monday to Friday due to families return back from work, turning all lights, cooking for dinner, etc.
- for mode changes, in air flight control systems, all computing resources were needed to handling the emergency if an aeroplane has experienced depressurization.
In order to meet response times, it is necessary for a system’s behavior to be predictable.
6. Interaction with hardware interfaces
The systems require the computer components to interact with the external world. They need to monitor sensors and control actuators for a wide variety of real-world devices and these devices interfaces to the computer via input and output registers, and their operational requirements are device and computer dependent.
Because of the variety of devices and the time-critical nature of their associated interactions, their control must be often direct, and not through a layer of operation system functions.
- Efficient implementation and the execution environment
Since real time systems are time-critical, efficiency of implementation will be more important than in other systems.
One of the main benefits of using a high level language is that it enables the programmer to abstract away from implementation details and to concentrate on solving the problem ant hand
Example of real time system products.
a)
Real-Time Monitoring screen in Weather
b)
Time Recorder/Access Control
c)
Kontakt digital
Appendix