Download Characteristics of Real-Time Systems (cont.) - O6U E

An Introduction
to
Real time Systems
by
Dr. Amin Danial Asham
References
Real-time Systems Theory and
Practice. By Rajib mall
Simplified Hardware Architecture
System Buses
There are Three Buses:
1. Power Bus: This bus feeds the power to the different
hardware components in the system
2. Address Bus: Is the media used to exchange the addresses of
the memory. To read or write from or to a certain memory
location, an address is used to refer to that location. This
address is carried by the address bus.
3. Data Bus: The data is exchanged between the memory and
CPU via the address bus.
Note: When referring to the system bus, the address and data
buses collectively are generally what are meant.
A Functional Model of A Real Time
System
System Components
A. Sensor: A sensor converts the physical
characteristics of an influencer in its environment in to
electrical signal.
Examples:
1.
2.
3.
4.
5.
Temperature sensors: such as Thermocouples (produces mV related the
measured temperature), PT100 (the electrical resistance changes with
the measured temperature)
Pressure Sensor
Magnetic Sensor: Used in Electronic Compass in many devices such as
smart phones.
Accelerometer Sensor: Measures the acceleration, which used in many
commercial systems as game console joysticks and smart phones.
Gyro sensors: Measures the angular velocity. Used in robotics, game
console joysticks, smartphones……etc.
System Components (cont.)
B. Actuator : Any device that tacks an electrical signal
and converts it to a physical action. This physical action
may be in the form of motion, thermal, or even the
physical characteristics of an object.
Examples:
1. Motor
2. Motorized Valve.
3. Electrical Solenoid
4. Heater
5. Hydraulic and Pneumatic actuators
System Components (cont.)
C. Signal Conditioner Units:
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The electrical signals produced by computers are usually not
suitable to directly drive actuators. Hence, output conditioning is
needed to generate the proper signals to drive the actuators.
Similarly, the level and type of electrical signal generated by sensors
are usually different from the signals to the interface unit, especially
for industrial applications where there may be a big distance
between the sensor (on site) and the interface unit (in electrical
stations). Hence input conditioning is needed.
Example: the voltage generated by photo voltaic cell is in millivolts
and hence need to be conditioned before it can be processed by a
computer.
System Components (cont.)
Some types of conditioning:
1. Voltage Amplifications: signals generated by the sensors may be in order of mV
whereas the signals received by the interface units of order of volts. In this case
amplification is needed.
2. Voltage Level Shifting: Shifting is needed to match the signal generated by a sensor
with the range received by an interface unit.
Ex. If the sensor generates signal in the range from -0.5Vdc to 0.5Vdc while the
interface unit receives signal from 0Vdc to 1Vdc.
3. Frequency Range Shifting and Filtering: Frequency range shifting is used to reduce
the noise. Many types of noise occur in narrow and the signal must be shifted from
the noise bands, so that noise can be filtered out.
4. Signal mode conversion: the signal may be converted from direct current to
alternating current and vise versa. Sometimes the analog signal is converted to a
train of pulses with a rate or pulse width proportional to the signal value. Pulse
train is usually used with transformer coupled circuits where direct current can not
be used.
Ex. In industrial fields signals are sent to the interface units (I/O modules) which are
located far from the sensors as mA signals to avoid voltage drop in case of voltage
signals
System Components (cont.)
D. Interface unit:
1. Input Interface unit (Analog to Digital Converter) Analogto-digital (A/D or ADC) conversion converts continuous (analog) signals from
various transducers and devices into discrete (digital) ones. Conversion from
analogue to digital binary value is carried out through sampling and
quantization.
Sampling
Quantization
Data Register n binary
digits (bits)
............ .
2n-1
20
System Components (cont.)
D. Interface unit (cont.):
2. Output Interface
Converter)
unit
(Digital
to
Analogue
•Digital-to-analog (D/A or DAC) conversion performs the inverse function
of A/D circuitry; that is, it converts a discrete quantity to a continuous one
D/A devices are used to allow the computer to output analog voltages
based on the digital version stored internally
System Components (cont.)
E. Real-Time Computer:
This is the processing unit which collects the
data from the input interface units (input
Modules digital or analog) and runs the control
program to produce the outputs and send them
to the output interface units (digital or analog).
Ex. PLC CPU.
System Components (cont.)
F. Human Computer Interface:
It is called in the industrial field “Human Machine Interface”
(HMI). HMI is the apparatus which presents process data to a
human operator, and through which the human operator
controls the process.
Characteristics of Real-Time Systems
The following characteristics may not be applicable to every real time
system
1. Time Constraint: the response of the system must meet the
determined deadlines. That is the a certain task must be completed
and produce the result before the deadline. Real Time Operating
System (RTOS) is responsible of ensuring that all tasks meet the
deadlines.
2. Correctness Criteria: A real-time system is one whose logical
correctness is based on both the correctness of the outputs and
their timeliness.
3. Embedded: An embedded computer system is physically embedded
in its environment and often controls it. A vast Majority of real time
systems are embedded systems, which are designed for specific
control functions within a larger system often with real-time
computing constraints. Ex. ABS, MPFI, Printers…….etc.
Characteristics of Real-Time Systems (cont.)
4. Safety-Criticality: Š

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

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
A Safe system is the one that does not cause damage in case of failure.
A reliable system is the one that can operate for long duration of time
without any failures.
For non-real time systems safety and reliability are independent issues.
o A hand gun is an example of a reliable system but unsafe since in case of
failure it can cause a severe injury.
o On the other hand, Word processing program is not a reliable system but
safe since in case of failure it does not usually cause any significant damage
or financial loss. .
A safety critical system is one whose failure can cause a severe damage.
Therefore, a safety critical system requires to be reliable since any failure
may cause severe damages.
o An example of a safety-critical system is a the computer on-board of an
aircraft. In case of the computer fails the aircraft fails.
In Real time systems safety and reliability are strongly related.
Characteristics of Real-Time
Systems (cont.)
5. Concurrency: Real time systems usually respond to several events in a
short time intervals according to time constraints.
Ex. in industrial control, sensors generate data asynchronously at different
rates. The real time system collects data from the different types of
sensors concurrently since in case of data loss the system may
malfunction.
6. Distributes and Feedback Structure: In many real time systems the
different components are distributed over a large geographical area.
Typical case is the DCS in Industrial plants
Characteristics of Real-Time Systems (cont.)
7. Task Criticality: is a measure of the cost of the failure of a task. Different
tasks have different criticality based on the application and damage in a task
failure. Task criticality is a different concept from task priority. Task priority is
more related to its timing.
8.Custom Hardware: In real time systems usually depend on a customized
hard ware specially design for a specific application. Ex. ABS, MPFI, Mobile
phones….etc.
9. Reactive: Real time systems react continually according to time
constraints imposed by the environment.
10. Stability: real time system need to meet the deadlines of the most critical
tasks even under overload conditions. On the other hand the deadlines of
non critical tasks may not be met.
11. Exception Handling: Many Real Time systems work round-the-clock
without human operators. Any failure must be automatically detected and
continue to operate in a gracefully degraded operated mode rather than shut
off abruptly. Ex. Embedded automotive real time systems.
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