Classification of RTS
Dr. Hugh Melvin, Dept. of IT, NUI,G
1
RTS Definitions
• Precise definition of RTS?
– Difficult due to the extent and scope of RTS
– System where a substantial fraction of the design
effort goes into making sure that deadlines are met
(Krishna/Shin)
• Response Time is an important parameter:
– The time between the presentation of a set of inputs
to a system (stimulus) and the realisation of the
required behaviour (response) including the
availability of all associated outputs, is called the
response time of the system (Laplante)
Dr. Hugh Melvin, Dept. of IT, NUI,G
2
RTS Definitions
• Alternative definitions
– System that must satisfy explicit (bounded) responsetime constraints or risk severe consequences,
including failure (Laplante)
– System whose logical correctness is based on both
the correctness of the outputs and their timeliness
(Laplante)
• Failed System
– A system that cannot satisfy one or more of the
requirements stipulated in the formal system
specification (Laplante)
• Applies only to Hard RTS
Dr. Hugh Melvin, Dept. of IT, NUI,G
3
RTS Definitions
• Failed Systems
– RTS software rather than hardware usually at fault
– Emergence of Software Engineering
• Attempts by US DoD (SEI) to improve on the
dismal record of software in military systems
• Has extended to general software industry
• All practical system are RTS?
– Time constraints are always bounded?
• Need for Classification of RTS
– Hard – Firm – Soft
– Differ by the consequences of missed deadlines
Dr. Hugh Melvin, Dept. of IT, NUI,G
4
RTS Definitions
• A Hard RTS is one in which failure to meet
a single deadline may lead to complete
and catastrophic system failure (Laplante)
• Eg.
– Power Plant
• Turbine Overspeed Protection
• Fuel Shutdown
– Flight Controller
– ABS (Antilock Braking System)
Dr. Hugh Melvin, Dept. of IT, NUI,G
5
Dr. Hugh Melvin, Dept. of IT, NUI,G
6
Hard RTS
Boiler
Turbine
R30
Operators Keyboard 1
VDU 2
Binary Field Inputs
Binary Outputs
Analogue Inputs
Analogue Outputs
Binary Field Inputs
Binary Outputs
Analogue Inputs
Analogue Outputs
Binary Field Inputs
Binary Outputs
Analogue Inputs
Analogue Outputs
VDU 1
BMS
Operators Keyboard 2
VDU S\E’s
Office
S\E’s Keyboard 1
VDU
Prog. Room
AS101
AS112
AS013
AS017
AS131
AS133
I/O
I/O
I/O
I/O
I/O
I/O
Programmers Keyboard
HW Analog Inputs
Annunciator
I/O
HW Binary Inputs
Printer 1
Tape Drive
Storage Unit
CPU
CPU
CPU
CPU
CPU
Printer 2
CPU
CPU
Printer Prog. Room
MEC-RT
Data Storage Unit
Bus Coupler
Bus 1
Bus 0
CS 275 Bus
VDU 1
VDU 1
VDU 1
VDU 2
Alarms Printer
Local Printer
VDU 2
VDU 2
VDU 3
VDU 3
CPU
Annunciator
PG750
AS231
Maintenance
Computer
Unit Alarms System
Operator’s
Keyboard
Operator’s
Keyboard
Fault Log Printer
Operator’s
Keyboard
CPU
I/O
I/O
I/O
Turbine
Boiler
Local Terminals
5 1/4”FDD
Storage Unit
CPU
Control Room
12:45:01
23:01:97
Control Room
Master Clock
LPA0
MEC-RT
S\E’s VDU &
Keyboard
Data Storage Unit
LPA1
OS254
MEC-RT
Data Storage Unit
Programming Room Printer
Operations
Computer
Teleperm
System
Overview
Dr. Hugh
Melvin,ME
Dept.
of IT,
NUI,G
Programmers VDU & Keyboard
VAX
Maintenance Management
System
7
Unit 2
Unit 1
CS275
Bus 1
Bus A
Bus B
CS275
Bus 1
CS275
Bus 0
Bus A
Bus B
Bus
Coupler
U/I
U/I
Unit 3
CS275
Bus 0
Bus A
Bus B
Bus
Coupler
U/I
U/I
Synogate Interface
to CS275
Synogate Interface
to CS275
Remote Operator
Station
CS275
Bus 1
CS275
Bus 0
Bus
Coupler
U/I
U/I
Synogate Interface
to CS275
OIS Server
IBM NetFinity 5510
MP001
Gateway
R30 Room
Gateway
Communications Room
Station LAN
MP002
Control Room Unit 1
Operator Station
Control Room Unit 2
Operator Station
Dr. Hugh Melvin, Dept. of IT, NUI,G
Control Room Unit 3
Operator Station
8
Burner Management System
Bus A
Bus B
CS275
CPU 1
..
..
U/I
U/I
N8
N8
CPU 2
..
CPU 3
..
..
..
2 out of 3
Non-Redundant
Control Circuits
Double Redundant
Safety Circuits
Dr. Hugh Melvin, Dept. of IT, NUI,G
Triple Redundant Safety Circuits
9
Hard RTS
• Emergence of Fly-by-wire control systems
• Military Aircraft
• Year
Aircraft
%Fns supported by S/W
1960
F-4
8
1982
F-16
45
2000
F-22
80
(Source: W.S Humphreys “Winning with S/W, An Exec. Stgy”, Add-Wes. 2002)
• Civilian aircraft similar
• Also
– Robotics
– Medical Devices
– ABS / Airbag Protection
• Most Hard RTS are embedded devices with limited and very
specialised software designed for specific hardware
Dr. Hugh Melvin, Dept. of IT, NUI,G
10
RTS Definitions
• A Soft RTS is one in which performance is
degraded but not destroyed by failure to
meet response time constraints (Laplante)
• Eg.
– Multitasking PC
– Internet-based Multimedia
• VoIP
– Note: For MM data, requirement for logical
correctness of output can be relaxed somewhat (See
G.1010)
Dr. Hugh Melvin, Dept. of IT, NUI,G
11
Soft RTS: Multimedia ?
Dr. Hugh Melvin, Dept. of IT, NUI,G
12
Soft RTS : VoIP
Dr. Hugh Melvin, Dept. of IT, NUI,G
13
RTS Definitions
• A Firm RTS is one in which a few missed
deadlines will not lead to total failure, but
missing more than a few may lead to
complete and catastrophic system failure
failure (Laplante)
• Difficult to find examples!
Dr. Hugh Melvin, Dept. of IT, NUI,G
14
Firm RTS
• Somewhere in the middle
– Eg. Private IP Network governed by SLA (Service
Level Agreement)
• SLA specifies jitter/delay/loss/availability
– 99.999% Availability
• Lack of adherence results in :
– Irate customers  loss of business
– Penalties imposed on provider
– Consumer Devices
• Mobile Phone / Cameras etc…
– Reservation Systems
Dr. Hugh Melvin, Dept. of IT, NUI,G
15
Sample Time and Response Time
• Sample time refers to the rate at which a
parameter is monitored
• Sample time and Response time are both
related to the underlying physical phenomena
– Eg. Power System control/protection systems for
– Steam Pressure / Turbine Speed / Overvoltage
– 3 phenomena have very different natural characteristics
– Require very different Sample Times and Response Times to
react safely to changing/dangerous conditions
– Eg. Flight Control / Car ABS
– Sample and Response Times depend on Maximum velocity
– 2 aircraft @ 600 mph = relative velocity of 1200 mph
» 1760 feet/sec (Min Vertical flight separation = 1000 ft = 0.6
sec)
– 2 cars at 120 kph = 66 m/sec .. Response time of msec reqd
Dr. Hugh Melvin, Dept. of IT, NUI,G
16
Sample Time and Response Time
• Hard RTS
– No point in sampling more frequently than necessary
• Wasteful of CPU/Memory
• Eg. Steam Pressure versus Voltage
– Response time must be guaranteed : Good Average
performance is of little use
• At 35000 ft when cabin pressure is lost
• When a Power station at full output is suddenly disconnected from
the National Grid
• When ABS is required to work
• Soft RTS
– Conventional PC OS designed for timesharing and multitasking
– Complex timesharing scheduler
– Good Average Performance acceptable
Dr. Hugh Melvin, Dept. of IT, NUI,G
17
Analog Inputs via ADC Modules
CPU
ASE 6DS1714-8AA
2 Wire
Transmitter
4 … 20 mA
Ch 1
0 - 200 bar
Ch 2
2 Wire
Transmitter
0 … 10V
Spare
0 - 3000 rpm
Ch 3
spare
Ch 4
4 Wire
Transmitter
0 … 20 mA
Voltage
Dr. Hugh Melvin, Dept. of IT, NUI,G
0
%
6600
18
RTS Definitions
• Deterministic System
– System that for each possible state and set of
inputs, a unique set of outputs and the next
state of the system are known
 Importance of thorough system specification and
testing
2OI – 2nd Order Ignorance- “You cannot test for
things you don’t know you don’t know”
• Temporal Determinism
• Response Time is also known
• Critical and extra reqd for RTS
Dr. Hugh Melvin, Dept. of IT, NUI,G
21
RTS Definitions
• CPU Utilisation (U)
– Measure of the percentage of non-idle processing
– 70% is useful rule of thumb based on scheduling
theory (cf later)
– Consider task 1 n where
• task i has period p i ,freq of 1/ pi and worst case
execution time ei
• ei can be very difficult to quantify
• Utilisation factor (worst case) for task i ; ui = ei / pi
n
– Overall CPU U=
u
i 1
i
Dr. Hugh Melvin, Dept. of IT, NUI,G
22
RTS Components
• Hardware
– Specific to application
– Tend to be I/O intensive
• Power Systems
• Fly-by-wire
• MM: VoIP
• Software
– Specific to Application
• Assembly language / C widely used
• Interface directly with hardware
– RT Programming Languages
• Power Systems: Siemens OEM Programming Language
• Ada is most recognised Real Time Lang. (RealTime Java?)
– Boeing 777 control written in Ada
Dr. Hugh Melvin, Dept. of IT, NUI,G
23
RTS Components
• RTOS
– Many Hard RTS do not have an OS as such
• Cyclic Executive approach used
• OS introduces too much complexity
• OS not needed if embedded device is simple enough or can
be very well defined
– Soft-Firm RTS often utilise RTOS
• Facilitates multiple concurrent processes
• Requires Scheduler
• Memory Management
Process Priorities ensure determinism
Eg. Mobile Phones / PDA / Network Switches
• Increasing use in Hard RTS : Mars Pathfinder
– Note: Conventional OS can be adequate for Soft/Firm
RTS
Dr. Hugh Melvin, Dept. of IT, NUI,G
24
Simple Control System
Process
Field Inputs / Outputs
Conveyor Belt
Taco-generator
Drive Unit
E
Signal Conditioning
Y
Process Control
Processor
(incl ADC/DAC)
Thyristor Drive Unit
Belt Running
Drive Temp High
Speed Setpoint
Operator Interface
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Less simple control System :
Boeing 777
Dr. Hugh Melvin, Dept. of IT, NUI,G
26
RTS Components
• Fault Tolerant Techniques
– Full/Partial Fault Tolerance
• Hardware Redundancy
– Voting schemes
• Software Redundancy
– N-version Programming
• Time Redundancy
– Build in time slack or roll back
• Information Redundancy
– Error Detection + Correction
» FEC & PLC techniques
– Note: TCP-IP based Error Detection via CRC relies on Time
Redundancy for TCP traffic. Not acceptable for UDP based MM
traffic
– Graceful degradation
– Failsafe operation
Dr. Hugh Melvin, Dept. of IT, NUI,G
27
RTS Components
– Common hydraulic
system for Steering,
Brakes and
Suspension
– Failsafe operation
• Suspension
• Braking
• Steering
Dr. Hugh Melvin, Dept. of IT, NUI,G
28
Fault Tolerant Techniques
CPU 1
DB-Out
DB-In
DB-In
2-out-of-3
CPU 3
DB-Out
DB-Out
DB-In
2-out-of-3
2-out-of-3
ZV3
ZV2
2-out-of-3
ZV1
2-out-of-3
ZV2
2-out-of-3
ZV3
RAM
EPROM
1
RAM
EPROM
2
RAM
EPROM
3
2-out-of-3
Central Unit
ZV1
I/O Level
CPU 2
EAVn
Extension Units
Inputs
Dr. Hugh Melvin, Dept. of IT, NUI,G
Outputs
29
Fault Tolerant Techniques: Airbus
• 3 Main Flight
Controllers
• 2 Backup Flight
Controllers
• Software developed
by different teams
and on different
platforms
• Seamless transfer
• ..See video !
Dr. Hugh Melvin, Dept. of IT, NUI,G
30
RTS: Closing Remarks
• RTS are more concerned with predictability of
response times rather than absolute response
times
– Providing faster processors will convert a PC (Soft
RTS) to a faster PC (Soft RTS), not a Hard RTS.
• Many Hard RTS are embedded devices
–
–
–
–
Specific hardware
Customised and limited software
No OS
Guarantees are provided through simplicity, precise
definition and overprovisioning
• Distinction between Soft-Firm RTS vague
Dr. Hugh Melvin, Dept. of IT, NUI,G
32