Dr. Hugh Melvin, Dept. of IT, NUI,G1 Realtime Systems Dr.Hugh Melvin Room 406 Ext 3716 [email protected] See Web resources

Dr. Hugh Melvin, Dept. of IT, NUI,G 1

Realtime Systems

Dr.Hugh MelvinRoom 406 Ext 3716

[email protected] Web resources

mailto:[email protected]


Main Textbooks

• Real-Time Systems , Krisha & Shin, McGraw Hill

• Real-Time Systems Design & Analysis. Laplante, Wiley

• Programming for the Real World POSIX.4, Gallmeister, O Reilly

• Real-Time Systems & Programming Languages, Burns & Wellings, Pearson


Course Outline

• Classification of RTS• Time Synchronisation for RTS• Scheduling for RTS• RT Programming Languages• POSIX.4 RT Extensions• RTOS• Fault Tolerance for RTS• Soft RTS

– Multimedia & Networks


RTS

• 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)

• Classification of RTS?– Hard – Firm – Soft

• Differ by the consequences of missed deadlines



Hard RTS

Teleperm ME System Overview

VAXMaintenance Management

System

I/O

CPU

I/O

CPU

I/O I/O

CPU CPU

Bin

ary

Fie

ld I

nput

sB

inar

y O

utpu

tsA

nalo

gue

Inpu

tsA

nalo

gue

Out

puts

Bin

ary

Fie

ld I

nput

sB

inar

y O

utpu

tsA

nalo

gue

Inpu

tsA

nalo

gue

Out

puts

Bin

ary

Fie

ld I

nput

sB

inar

y O

utpu

tsA

nalo

gue

Inpu

tsA

nalo

gue

Out

puts

I/O

CPU

AS101 AS133AS112 AS131AS017AS013

Boiler BMSTurbine

I/O

CPU

R30

I/O

CPU

I/O

CPU

I/O

CPU

AS231Unit Alarms System

PG750Maintenance

Computer

OS254OperationsComputer

Turbine Boiler

VDU 1 VDU 1

VDU 2

VDU 3

Operator’sKeyboard

MEC-RTData Storage Unit

Programmers VDU & Keyboard

VDU 2

VDU 3



Programming Room Printer

Fault Log Printer

Con

trol

Roo

m

Control R

oom

LPA0

LPA1

Local Printer

VDU 1

VDU 2


5 1/4”FDDStorage Unit

Alarms Printer

Annunciator

I/O

CPU

Local T

erminals

CPU

I/O


VDU 1

VDU 2

VDU S\E’sOffice

VDU Prog. Room

Tape DriveStorage Unit

Operators Keyboard 1

Operators Keyboard 2

S\E’s Keyboard 1

Programmers Keyboard

Annunciator

Printer 1

Printer 2

Printer Prog. Room

CS 275 Bus Bus 0 Bus 1

HW Analog Inputs

HW Binary Inputs

S\E’s VDU & Keyboard

Bus Coupler

12:45:01 23:01:97

Master Clock


Remote OperatorStation

Control Room Unit 3 Operator Station

OIS Server IBM NetFinity 5510



Station LAN

MP002

GatewayR30 Room Gateway

Communications Room

MP001

Bus ABus B

Bus Coupler

U/I

CS275Bus 0

Synogate Interfaceto CS275

CS275Bus 1

U/I

Bus ABus B

Bus Coupler

U/I

CS275Bus 0


CS275Bus 1

U/I

Bus ABus B

Bus Coupler

U/I

CS275Bus 0


CS275Bus 1

U/I

Unit 1 Unit 2 Unit 3


Burner Management System

CPU 2CPU 1 CPU 3

.. .. ........

2 out of 3

Non-Redundant Control Circuits

Double RedundantSafety Circuits Triple Redundant Safety Circuits

N8 N8

U/I U/I

Bus ABus B

CS275


Other Hard RTS

• Fly-by-wire control systems

• Robotics

• Medical Devices

• ABS / Airbag Protection


Soft RTS: Multimedia


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 :– Poor customer satisfaction– Penalties imposed on provider

– Consumer Devices• Mobile Phone Technology / Cameras etc…


Time Synchronisation for RTS• Particularly critical for RTS

– Synchronising 2v3 systems– Precise data logging for distributed control system

• Clock Terminology– Accuracy , Reliability, Skew, Drift

• Synchronisation Algorithms– Network Time Protocol (NTP)

• Limitations of conventional OS Timing• Timer granularity & Time Services



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<block of text>

100 Continue– Fortran interpreted DO 100 I as a previously

undeclared variable assigned value 1.5 NASA mishap #1

– Ada is most recognised RT Prog. Lang.• Boeing 777 control written in Ada


Boeing 777


RTOS

Realtime Operating Systems– Windows / Unix / Linux

• Designed for time sharing of resources rather than meeting hard deadlines

• Designed for good average performance not used in Hard RTS typically

– ..but have improved Soft RT functionality

– 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 : VxWorks, QNX, Lynx, RTLinux


Fault Tolerant TechniquesKey Characteristic of Hard RTS1. Full/Partial Fault Tolerance

– Achieved mainly through redundancy

2. Graceful degradation– Eg.1: Citreon CX

• Common hydraulic system for Steering, Brakes and Suspension• Order of failure??

– Eg. 2: Power Plant • Faulty sensor Alarm Overload AS station switch to manual

but 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


Citreon CX


Fault Tolerant Techniques

• Redundancy– Hardware Redundancy

• Voting schemes

– Software Redundancy• Eg. Code developed by different teams

– 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

• Software developed by different teams and on different platforms

• Seamless transfer• ..See video !


Fault Tolerant Techniques

CPU 1 CPU 2 CPU 3

2-out-of-3 2-out-of-32-out-of-3

2-out-of-3 2-out-of-3 2-out-of-3

2-ou

t-of

-3

Inputs Outputs

Extension Units

EAVn

ZV1 ZV2 ZV3

ZV1 ZV2 ZV3

DB-In DB-In DB-InDB-Out DB-Out DB-Out

Cen

tral

Uni

tI/

O L

evel

RAMEPROM

1

RAMEPROM

2

RAMEPROM

3


Soft RTS: MM and Networks

Real Time Communications– Best Effort Internet default service– Recent QoS Initiatives to differentiate

between traffic streams• Eg. Voice over IP

– Requires M2E delay of < 150 msec– Can tolerate higher delays if echo cancellation used– Can cope with small degree of packet loss

» More Relaxed definition of RTS

• See ITU G.1010


Mouth-to-Ear Delays


Real Time Communications• LAN based developments

– Switched Networks (Embedded RTOS)• Also Fast/Gigabit Ethernet : Overprovisioning?

– New difficulties for wireless environment • Access Protocols CSMA

• WAN based developments– IntServ– DiffServ – MPLS : Layer 2.5

• Also development of RTP/RTCP/RTSP Protocols for MM traffic


Real Time Communications

• Endpoint Issues– Increased complexity in terminals– OS / Driver / Application Software / Hardware

issues • Hardware: Skew Issues = lack of synchronisation

• Other options– Token Based Networks

• Moneypoint Token Bus IEEE 802.4


CS 275 Bus: IEEE 802.4

U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IBU/IA U/IB U/IA U/IBU/IA U/IB

101N8

AS220E

102N8

AS220E

103N8

AS220E

104N8

AS220E

105N8

AS220E

106N8

AS220E

107N8

AS220E

108N8

AS220E

U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IBU/IA U/IB

126N-BKBus 0

123N-UHRM-Clock

121N16

OS254

112N8

AS220E

111N8

AS220E

110N8

AS220E

109N8

AS220E

U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IBU/IA U/IB U/IA U/IB

160NS5NAT

PG750

133N8

AS EHF

132N8

AS EHF

131N8

AS EHF

128N8

AS231

125N16R30

141NAT-24Synogate

U/IA U/IB

127 N-BK Bus 1



Clock Skew : Buffer Issues


Audio-System Clock Skew

Documents

Dr. Hugh Melvin, Dept. of IT, NUI,G1 Realtime Systems Dr.Hugh Melvin Room 406 Ext 3716 [email protected] See Web resources