1 CT420 Realtime Systems Dr. Hugh Melvin, Discipline of IT, NUI,G 1 Dr. Hugh Melvin/Dr. Michael Schukat Room 406/402 Ext 3716/2031 [email protected][email protected]See Web resources Course Structure • 2 hrs lectures / week for both semesters – Wed 9-11 IT304 Semester 1 • Structured and unstructured labs Dr. Hugh Melvin, Discipline of IT, NUI,G 2 • Assignments – Lab based work • Technical Reports – Linux/Windows based Course Structure • Content updated – Course development in realtime ! Subject to change • Lecture notes on web in advance of lectures A t Dr. Hugh Melvin, Discipline of IT, NUI,G 3 • Assessment – Exam + Assignments • Typically 70/30 split • Site Visit ? – Moneypoint (ESB) / Shannon ATC / Aer Lingus
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CT420 Realtime Systems
Dr. Hugh Melvin, Discipline of IT, NUI,G 1
Dr. Hugh Melvin/Dr. Michael SchukatRoom 406/402 Ext 3716/2031
RT Programming Languages– Consider the following Fortran Loop statement
DO 100 I=1,5<block of text>
100 Continue
– but the following is also validDO 100 I=1 5
Dr. Hugh Melvin, Discipline of IT, NUI,G 27
DO 100 I=1.5<block of text>
100 Continue
– Fortran interpreted DO 100 I as a previously undeclared float variable assigned value 1.5 NASA mishap
– Ada is most recognised RT Prog. Lang.• Boeing 777 control written in Ada
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RTOS
• Windows / Unix / Linux – Designed for time sharing of resources rather than meeting
hard deadlines– Designed for good average performance– Not suited to Hard RTS typically
• Can be used after careful design considerations
Dr. Hugh Melvin, Discipline of IT, NUI,G 28
– Examples• M’pt : User Interface provided by Linux/Win 2000
» BUT not core control system
• Shannon ATC– Unix powers main Thales ATC systems
– More suited to Soft RT functionality – POSIX.4 improves RT performance of some OS
RTOS• Hard RTS
– 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
• RTOS– Can better meet RTS requirements
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q• Different design to conventional OS• Scheduling/Process priorities are key issues
– VxWorks, QNX, Lynx, RTLinux,
– Cyclic Executive approach still widely used • Software runs ‘bare’ on hardware• No OS !
– Used a lot with Soft RTS and some hard RTS• Mobile phone OS: Symbian is market leader
Fault Tolerant TechniquesKey Characteristic of Hard RTS1. Fault-tolerance via Redundancy
• Full/Partial Fault Tolerance
2. Graceful degradation– Eg.1: Citreon CX
• Common hydraulic system for Steering, Brakes and Suspension• Order of failure??
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• Order of failure??– Eg. 2: Power Plant
• Faulty sensor Alarm Overload AS station switch to manualbut holds its state and will still respond to safety events/manual control
3. Failsafe operation• Eg. Power Plant: Steam inlet valves spring loaded to shut on
loss of power
Many Hard RTS combine all 3 techniques
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Citreon CX
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Fault Tolerant Techniques
• Redundancy– Hardware Redundancy
• Voting schemes
– Software Redundancy• Code developed by different teams
Oft bi d ith h d d d
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• Often combined with hardware redundancy
– Time Redundancy• Build in time slack or roll back
– Information Redundancy • Error Detection + Correction • Note: TCP-IP based Error Detection via CRC relies on Time
Redundancy for TCP traffic. Not acceptable for UDP based MM traffic
Fault Tolerant Techniques: Airbus
• 3 Main Flight Controllers
• 2 Backup Flight Controllers
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• Software developed by different teams and on different platforms