REAL TIME SYSTEMS

UNIT 1: INTRODUCTION TO REAL-TIME SYSTEMS: Historical background,

RTS Definition, Classification of Real-time Systems, Time constraints, Classification of

Programs. 6 Hours

Historical background

Brown and Campbell – 1950 – Earliest proposal - Computer operating in real time

(as a part of the control system)

Consists of feedback and feed-forward loops

Assumption was that, the analog computers can be used

Digital computer elements did not were excluded

First digital computer – developed for real time control – for airborne operation

Digitrac digital computer – 1954 – used to provide an automatic flight and

weapons control system

Industrial control – in digital computers usage – late 1950s

Computer and electronic system manufacturers – for extending markets – been

out from military uses of – initiated the computer control industries

1958, September – Louisiana Power and Light Company - Daystrom computer

system’s installation – for plant monitoring – in their power station – at Sterling,

Louisiana

It was not the control system

First industrial computer installation -

Texaco Company –Ramo-Wooldridge Company - RW-300 system’s installation -

at Port Arthur refinery in Texas

1959, March 15th – Refinery were using – closed loop control system

(Anon, 1959)

1957-58 - Monsanto Chemical Company – with the cooperation of Ramo-

Wooldridge Company – studied the control by computer

1958, October – it planned to have computer control for ammonia plant – at

Luling, Louisiana

1960, July 20th – Commissioning of computer control for ammonia plant at

Luling, Louisiana – began

1960, April 4th – Closed loop control was achieved – after rewriting – the control

of the program - Noise problems – were faced – on the measurement of signals

1959-60, B. F. Goodrich Company – Acrylanite plant, at Calvert city, Kentucky –

had the same installation scheme – as above – and also –40 systems of RW-300

based – were in for supervisory control systems for using – for steady-state

optimization calculations – for determining the set-points – for standard analog

controllers – Here, computer – not controlling directly – movement of the valves

or other plant actuators

1959 – Plan to had the following installation were begun (Burkitt,1965)

1962, November – Ferranti Argus 200 systems - First DDC (Direct Digital

Control) system – installed – at ICI ammonia-soda plant at Fleetwood,

Lancashire, UK

It was the large system – 120 control loops (94 of were used actually) – 256

measurements (224 of were used actually) – in Fleetwood system

1961 – Monsanto Company – Texas city plant – and, a hierarchical control

scheme for petrochemical companies, at Chocolate Bayou – DDC projects –

began

RW-300 computer – used the rotating drum store – to hold the control program

Ferranti Argus 200 – Used the ferrite core store – to hold the control program -

PROM – program was held in it

Loaded by – physically inserting pegs into a plug board – each peg representing –

one bit in the memory word

Was laborious to set up initially

Was very reliable –since, destruction of the memory contents - can be done only

by physical dislodgement of the pegs

Security was enhanced – using special power supplies – and, switch-over

mechanisms – to protect information held in the main core store

The information – was as follows –

1. Set points – Loss most undesirable

2. Valve demand – Presence after controlled stoppage allows computer to gain

control of plant immediately and without disturbing the plant (referred to as

bumpless transfer)

3. Memory calculations – Loss is tolerable, soon will be updated and only slight

disturbance to plant

4. Future development – Extension to allow for optimization may require

information to be maintained for long periods of time

5. More rapid memory access – were in Ferranti Argus 200 – that of RW-300 and

similar machines

Began the second phase of application of computers – to real time control

1960s, Early – Computers ware using – combined magnetic core memories and

drum stores – drum eventually giving way to hard disk drives

Ex.: for early 1960s computers –

General Electric 4000 series

BM 1800

CDC 1700

Foxboro FOX 1, and Foxboro 1A

SDS

Xerox SIGMA series

Ferranti Argus series

Elliot Automation 900 series

Cost of earlier computer increased – in attempts to resolve some problems – and

only one computer – for supervising control and DDC – can be justified only – it

was with further problems in development of softwares

Softwares – written by specialists programmers – in machine code – it was

manageable earlier – since, tasks were defined clearly and the programs’ length

were less

Combining of DDC and supervisory control – increased –

1. the code length, for a given application

2. the complexity of the programming

DDC and supervisory control – were with very different time-scales

DDC control programs – have to interrupt the supervisory control programs’

increase in code length – made all the code could be stored in core memory and

also swapping of code between the drum memory and core – to be to done were

also - Solutions to increased code length problems above – were by -

Development of general purpose real-time operating systems and high-level

languages

Late 1960s – RTOS were developed – PROCESS FORTRAN compilers had their

appearance

Problems and costs of involving in having one computer only for use – made the

users to retreat – the smaller systems – for which newly developing

microcomputers (like DEC PDP-8, PDP-11, Data General Nova, Honeywell 316,

etc.,) were ideally suited

Microcomputers less cost – made them suitable – to load the large number of

tasks onto one machine, (by using more than one microcomputer, instead a single

computer)

1970 – two computers on the systems were using – in which one computer acting

simply stand-by – to function in the event of failure of the working computer

Throughout 1970 – Developments in ICs and construction techniques in circuit

boards – led to - increment in reliability of the systems – reduction in the cost –

increment in the processor power – increase in the fast memory amount – made

more correct and dependable softwares to be to write

1974 – microprocessors’ advent – made it economical to use distributed computer

control system

RTS Definition

RTS definition (in as The Oxford dictionary of Computing):

Any system in which the time at which the output is produced is significant. This

is usually because the input corresponds to some movement in the physical world,

and the output has to relate to that same movement. The lag from input time to output

time must be sufficiently small for acceptable timelines

RTS definition (Cooling – 1991):

Real-time systems are those which must produce correct responses within a

definite time limit. Should computer responses exceed these time bounds then

performance degradation and/or malfunction results

RTS definition (alternate definition):

A real-time system reads inputs from the plant and sends control signals to the

plant at times determined by plant operational considerations – not at times limited by

the capabilities of the computer system

RTS definition:

A program for which the correctness of operations depends both on the logical

results of the computations and the time at which the results are produced

Ex.: Aircraft engine control system – sending response to UNIX workstations

Classification of Real-time Systems

Computer is connected to the environment within which it is working by a wide

range of interface devices and receives an sends a variety of stimuli – in the real time

systems and embedded computers

Ex.: Plant input, plant output, and communication tasks shown in Fig.

They have one common feature – that they are connected by physical devices to

processes which are external to the computer. These external processes all operate in their

own time scales and the computer is said to operated I real time if actions carried out in

the computer relate to the time scales of the external processes

Synchronization between the external processes and the internal actions (tasks) –

carried by the computer:

1. Clock based: Synchronization between the above two - if in depends on passages of

time, actual time of day (clock based RTS)

2. Event based: Synchronization between the above two – if in depends events

Ex.: Closure of a switch

3. Interactive Systems: Relation between the actions in the computer and the system – is

much more loosely defined

Requirement, typically – in the form that – a set of operations in the computer should be

completed within a predetermined time

Majority of the communication tasks – are of this category

Control tasks: Even though – not obviously and directly connected to the external

environment – they need to operated in real time – since, time is usually involved in

determining the parameters of the algorithms used. It is useful to divide tasks to be

carried out by embedded computers into the interactive categories and characteristics of

each class are to be recognized

Types of tasks in real time systems: The three types of tasks are

1. Clock-based (Cyclic, Periodic)

2. Event-based (Aperiodic)

3. Interactive systems

1. Clock-based Tasks (Cyclic, Periodic)

Process of the plant - operate in real time

Plant time constant: it’s the measure of the time taken by a plant to respond to a change in

input or load and is used as a characteristic of the plant

May be measured – in hours for some chemical processes or

In ms for an aircraft system

Feedback control: It involves the feedback control

It requires the sampling rate to be dependent on the time constant of the process to be

controlled

Sampling rate increases – as the time constant value decreases

Synchronization in real time – between the two is – required

All the required operations (like measurement, control and actuation – within each

sampling interval) – can be able to carry out

Completion of the operation – within the specified time: Dependent on

1. Number of operations to be performed

2. Speed of the computer

Real-time clock: It’s the clock – added to the computer - to have the synchronization. It’s

signal – is used to interrupt the operations of the computer – at some predetermined fixed

time interval

Computers carry out – plant input, plant output and control tasks in response to the clock

interrupt. If the clock interrupt is at a faster rate than the sampling rate – count of each

interrupt to be to – until it’s the time to run the tasks.

Different sampling rates: used in larger plants – where tasks are subdivided into groups –

for controlling different parts of the plant

Clock interrupt – used frequently – to keep a clock and a calendar – and keep the

computer – aware of both the time and the date

Clock based tasks (cyclic or periodic tasks) – here, task is run once per time period T

(cycle time, T) –or run at exactly T unit intervals

2. Event-based Tasks (Aperiodic)

Systems in which actions are performed in response to some events and not

performed at response at particular times or time intervals

Ex.:

1. Turing off a pump - Closing a valve when the level of a liquid tank reaches

predetermined value – like the one in - level of fuel in the vehicle fuel tank – reaching

the pump nozzle

2. Switching a motor off – in response to the closure of a micro-switch

indicating - that some desired position had reached

Used extensively – to indicate – alarm conditions and initiate alarm actions

Ex.: Indication of too high a temperature or too great a pressure

Includes the requirement– that the system must respond – within a given maximum time

to a particular event

Uses the interrupts – to inform the computer system – that action is required. Smaller

systems – use polling - where the computer periodically asks (polls) various sensors – to

see if action is required

Aperiodic tasks: Events usually occur at non-deterministic intervals

Aperiodic tasks – may have deadlines – expressed in terms of having start times or finish

times or even both.

Ex.: Task may be required to start – within 0.5s of an event occurring

Task may have to produce an output – within 0.5s of an event

3. Interactive systems

Probably represent – the largest class of real time systems

Ex.: 1. Automatic bank tellers

2. Reservation systems for hotels, airlines and car rental companies

3. Computerized tills

Requirement of interactive systems: can be expressed in terms such as ‘the average

response tome must not exceed…’

Ex.: Automatic bank teller system might – require an average response time – not

exceeding 20s

Event-based systems Vs Interactive systems

The two are

1. Same: since, it apparently response to a signal from the plant (in this case usually

a person)

2. Different: Since, it responds a a time determined by the internal state of the

computer and without any reference to the environment

Ex.: Automatic bank teller - does not know that – you will miss a train, or that it’s

raining hard and you are getting wet, but it’s response – depends on how busy the

communication lines and central computers are and also your amount

Clock-based systems Vs Interactive systems

The two are

1. Same: since, are capable of displaying the date and time, and also they have a real

time clock – which enables them to keep track of time

2. Different: when the test is done whether or not the answer for the question – ‘Can

the system be tightly synchronized to an external process?’

If answer is ‘yes’ – they are clock-based

If answer is ‘no’ – they are event-based