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A computer is an electronic device, which canaccept and process data by carrying out a set ofstored instructions in sequence.

This sequence of mathematical and logicoperations is known as a Program.

The computer is constructed from electronic

circuits, which operate on an ON/OFF principle.

The data and instructions, used in the computer,must therefore be in logical form.

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All data and program information must, beconverted into binary form, before being fed into

the computer circuitry.

One of the most important characteristics of acomputer is that it is a general-purpose device,

capable of being used in a number of differentapplications.

By changing the stored program, the same machine

can be used to implement totally different tasks.

In general, aircraft computers only have to performone particular task so that fixed programs can be

used.

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A computer is basically a problem-solving device.

In aircraft radio systems the problem to be solvedis concerned with navigation, in that given certain

information, such as range and bearing to a fixedknown point, steering commands need to becomputed to fly the aircraft to the same, or someother fixed point.

Since the input and output information iscontinuously changing during flight, analoguecomputation provides an obvious means of solvingthe navigation problems.

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INPUTDEVICES

OUTPUTDEVICES

ANALOGUECOMPUTINGELEMENTS

The input devices are radio sensors such as VOR,DME, Omega, ADF, Doppler, Loran, Decca, ILS, and

non-radio sensors such as the Air Data Unit andInertial Navigation System.

The output of such sensors will be electrical

analogues of the quantities being monitored.

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Thus aircraft analogue computers are purpose builtto solve one particular problem and as such usuallyform an integral part of a particular equipment.

This lack of flexibility, together with limited

accuracy and susceptibility to noise and drift, hasled to the introduction of digital computers, madepossible by integrated circuits.

Even so, the analogue computer, or ratheranalogue computing circuits, are still extensivelyused because as stated above, the sensors produceanalogue signals.

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Consider an aircraft approaching a DME beacon.The distance to go is given as an electricalanalogue signal at the output of the aircraft's DME

equipment.

By using an analogue computer, this signal can beused to provide an indication to the pilot of his

ground speed.

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DME O/PDISTANCE

TO GO

GROUNDSPEED

INDICATOR

ANALOGUECOMPUTING

DISTANCE

TIME

TIMING

As the input signal represents distance, a sampleof change in distance divided by the lapsed timewill provide ground speed.

A suitable block diagram to carry out thiscalculation is shown in Figure

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In the digital computer there are basically twotypes of input, namely

Instructions Data

from various radio and non-radio sensors, whichwill be referred to collectively as information.

Information must of course, be coded into a form,which the rest of the computer can understand,such as digital form.

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Coded information is passed to the memory inwhich it is stored until needed by the other units.

The memory is divided into a large number of cells,each of which can store a word representing apiece of information.

Each cell has a unique address, through whichaccess to the information contained within that cellcan be obtained.

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There are usually two types of memory,long term and temporary stores .

The latter, often termed registers, will beused to hold intermediate results incalculations and data, which is to beprocessed next in the calculating sequence

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The arithmetic unit performs the actual arithmeticoperations called for by instructions. It can becompared with a calculator

The results of the calculations must be displayed ina suitable form easily interpreted by the pilot

This is the function of the output unit, which reads

from the store.

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The control unit directs the overall functioning ofthe computer according to the program ofinstructions in store

This program is known as software as opposed tothe actual circuitry, which is termed hardware.

Although control is drawn as a separate unit in thefunctional block diagram, the control hardware,which comprises timing circuits and electronicswitches, is spread throughout the computer

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Information is read into the appropriate address ofthe store under the control of the software.

In aircraft navigation applications, incoming datafrom sensors updates the contents of the store at arate dependent upon the timing of the computercontrol.

The control acts on information held in store in theappropriate sequence.

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The basic task will be to transfer data from store tothe arithmetic unit, to carry out the necessarycalculations using registers to store theintermediate results, then writing the final resultinto the store

The final control function will be to transfer datafrom store to the output as a result of built ininstructions, or on specific instructions from thepilot.

This process of input - store - calculate - store -output is carried out sequentially in accordancewith software requirements.

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CPU

CONTROL &ARITHMETIC

UNIT

I/P

O/P

MEMORYINPUT/OUTPUT

UNIT

CONTROL BUS

DATA BUS

CLOCK

ADDRESS

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The data bus transfers data between memory, CPU

and I/O units, under the control of signals sentthrough the control bus.

For example, if data is to be transferred (sent) from

the CPU to a memory location, the control unitwithin the CPU places an output instruction on theCPU, and write instruction on the memory unit.

When the data arrives at the memory, it must bewritten into the memory at a given address.

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The address is already present, having been sent

by the CPU along the address bus.

The address bus is one-way only. The control bususually has one set of wires for input sensing lines,

and one set for output controls

Data buses are usually bi-directional; that is, datais either transferred, or fetched along the same set

of wires.

The control unit usually decides in which directiondata will travel.

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This unit provides the interface between thecomputer and the computer peripherals.

A computer peripheral is any unit, which isattached to, but is not part of, the computer - e.g.visual display units, printers, etc.

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A simple computing system may have only oneinput and one output.

In such cases, an analogue-to-digital converter

(ADC) may suffice for the input, and a digital-to-analogue converter (DAC) for the output.

Alternatively, complex-computing systems can

literally service thousands of peripherals

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Figure illustrates a simple I/O unit. The I/O unitcan be described as a fan-out (and fan-in) device.

INPUT/OUTPUTUNIT

COMPUTER DATA BUS (8 BITS)

ADDRESS BUS

PERIPHERAL 1 PERIPHERAL 2 PERIPHERAL 3

PORT 1 PORT 2 PORT 3

CONTROL BUS

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The computer's 8-bit bi-directional data bus can beconnected to port 1, 2 or 3.

The port chosen is dependent upon the address, onthe address bus.

The system illustrated allows three peripherals tocommunicate with the computer.

Only one peripheral at a time can send data to thecomputer, or receive data from the computer.

However, this is not a problem, because thecomputer works very much faster than theperipheral, and hence, it appears that the computerservices all three peripherals simultaneously.

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Peripherals can have either serial or paralleloutputs.

Also, as stated previously, peripherals work at amuch slower speed than that of the computer.

The I/O system must, therefore, be capable of'conditioning' the data received from theperipherals to a form, which is readily digestible bythe computer, and vice versa

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The memory unit is used for the storage of binarycoded information.

Information consists of instructions and data

where:

Instructions are the coded pieces of information thatdirect the activities of the CPU.

Data is the information that is processed by the CPU.

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The memory hardware contains a large

number of cells or locations.

Each location may store a single binary digitor a group of binary digits.

The cells are grouped so that a completebinary word is always accessed.

Word length varies typically from 4-bits up to64-bits depending upon machine size.

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Each location in the memory is identified by aunique address, which then allows access tothe word.

Consequently, to obtain information from thememory, the correct address must be placedonto the address bus

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There are fundamentally two types of

memory primary memory secondary memory

Primary memory is essential; no computercan operate without this.

Secondary memory is necessary tosupplement, or back, the primary memory onlarge computing systems; hence, it is oftencalled backing memory

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There are two types of semi-conductorprimary memories:

ROM (Read Only Memory) RAM (Random Access memory).

Both types employ solid state circuitry, andare packaged in IC form.

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DATA BUS

ROM RAM

MEMORY ADDRESS REGISTER& CHIP SELECT DECODER

ADDRESS BUS

TOINPUT/OUTPUT

DEVICE

TOINPUT/OUTPUT

DEVICE

FROMCPU

TOCPU

NOTE: CONTROL BUSOMITTED FOR SIMPLICITY

Figure shows how these primary memories areconnected to a simple computer bus

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The RAM-type memory will allow data to bewritten into it, as well as read from it.

With very few exceptions, RAM lose theircontents when the power is removed and arethus known as Volatile memory devices.

All computers use RAM to store data andprograms written into it either fromkeyboard, or external sources such asmagnetic tape/disk devices.

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RAMs are often described in terms of thenumber of bits, i.e. 1s and 0s, of data thatthey hold, or in terms of the number of datawords, i.e. groups of bits, they can hold.

Thus a 16384 bit ram can hold 16384 1s and0s.

This data could be arranged as 16384 1-bitwords, 4096 4-bit words or 2084 8-bitwords.

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Semiconductor memories vary in size, e.g.4K, 64K, 128K, etc.

Hence we are using K defined as:K =2 10 = 1024

Thus a 16K memory has a storage capacity of16 X 1024 = 16384 words

A 128K memory of 1310672 words and soon.

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There are two main members of the RAM family: Static RAM. Dynamic RAM.

The essential difference between them is the wayin which bits are stored in the RAM chips.

In a static RAM, the bits of data are written in theRAM just once and then left until the data iseither read or changed.

In a dynamic RAM, the bits of data are repeatedlyrewritten in the RAM to ensure that the data isnot forgotten.

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Flip-Flops are the basic memory cells in a staticRAM.

Each flip-flop is based on either two bipolartransistors or two Metal Oxide Semiconductors

Field-Effect Transistors (MOSFETS).As many of these memory cells are needed asthere are bits to be stored.

Thus, in a 16K-bit static memory there are16384 flip-flops, i.e. 32768 transistors.

All these transistors are accommodated on asingle silicon chip approximately 4mm 2 .

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Figure shows a basic memory cell in a staticRAM

TR1 TR2

CELL SELECT LINE

+5V

LOGIC 1OUTPUT/INPUT

LOGIC 0OUTPUT/INPUT

16K MEMORY= 16,384 FLIP-FLOPS

= 32,768 TRANSISTORS

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The 7489 TTL Ram package has 64 memorycells, each cell is capable of holding a singlebit of data.

The cells are organised into locations, andeach location is capable of holding a 4-bitword.

Thus the 7489 is capable of storing 4-sixteen4-bit words, i.e. four memory cells are usedat each location

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Figure shows the memory organisation of the7489 static RAM.

1 3 4 5 6 7 82

16 14 13 12 11 10 915

A

ME WE D1 S1 D2 S2

B C D D4 S4 D3

S3

Vee

Vcc

D A T A

I N P U T 1

D A T A

I N P U T 2

D A T A

I N P U T 3

D A T A

I N P U T 4

S E N S E

O U T P U T 1

S E N S E

O U T P U T 2

S E N S E

O U T P U T 3

S E N S E

O U T P U T 4

ADDRESSB,C & D

A D D R E S S

A M E M

W R I T E

ENABLES

4 BITDATA OUT

FOUR MEMORY CELLS

1 0 1 10 1 1 00 0 1 10 1 0 0

16 LOCATIONSEACH HOLDING

FOUR BITS

0

1

2

3

4

5

67

8

9

10

11

12

13

14

15

READ/WRITESIGNALS

4 BITDATA IN

1 1 0 1

4 BITADDRESS

1 1 10

1 1 10 1 1 10

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Each location is identified by a unique 4-bitaddress so that data can only be written orread from that location.

The number of words stored in the memorydetermines the length of the address word.I.E. 16 = 2 4 .

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The problem with RAM is that its memory isvolatile, i.e. it loses all its data when thepower supply is removed.

A non-volatile memory is a permanentmemory that never forgets its data.

One type of non-volatile memory is the Read

Only Memory (ROM).

A ROM has a pattern of 0s and 1s imprintedin its memory by the manufacturer.

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It is not possible to write new data into aROM, which is why it is called a Read-OnlyMemory.

The organisation of data in a ROM is similar

to that of a RAM.

Thus a 256-bit ROM might be organised as a256 X 4-bit memory, and so on.

The ROM may be regarded as the ReferenceLibrary of a computer.

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This type of memory is used extensively inairborne digital systems, although integratedcircuits are being developed with mostmodern aircraft systems.

This system works by a Ferro-magneticmaterial will become magnetized if placed inthe proximity to an electric current.

Each bit in the magnetic core memory is aferrite ring in which a magnetic field can beinduced by a current flowing in a wire

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Figure shows typical ferrite ring for storing asingle bit.

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Although the wire carrying the current iswound round the ring, the same effect isobtained if the wire passes through the ring.

This is a more convenient way to set the

magnetic state of each ring when a plane ofcores is built.

The advantage of this type of memory is thatwhen the power is removed it holds its state,i.e. it is a non-volatile memory.

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A matrix of cores containing 16 bits of informationis shown in Figure

Y1 Y2 Y3 Y4

X1

X2

X3

X4

X1 & Y1 CURRENTMAGNETIZES THE

CORE

CURRENT ISINSUFFICIENT

TO MAGNETIZE

CORE WITH ONLYONE CURRENT

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The user can program a PROM after purchase.Each memory bit element in the PROMcontains a nichrome or silicon link that actsas a fuse.

The user can selectively 'burn out' or 'blow'these fuses by applying pulses of current tothe appropriate pins of the IC.A memory element with a non-ruptured fusestores a 1 and a ruptured fuse stores a 0.The programming is irreversible, so it mustbe right first time.

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Figure shows the circuit for a PROM

+5V +5VSENSE(HIGH)

SENSE(LOW)

0

FUSELINK

1

NO FUSELINK

LOGIC0

LOGIC1

0V

ADDRESSLINE

TR1

TR2

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PROMs are capable of high operating speeds,

but consume a relatively large amount ofpower.

However, since they are non-volatile, they

can be switched off when not being accessed.

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These memory devices can be programmed,erased and then reprogrammed by the useras often as required.

In some devices, the information can beerased by flooding them with ultraviolet light,whilst in others, voltages are applied to theappropriate pins of the device.

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This memory device combines the non-volatility of the ROM with the electricallyalterable characteristic of the RAM .

It is, therefore, considered as a non-volatileRAM.

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The use of semi-conductor memory elementshas been made possible by the use of largescale integrated circuits (LSI) which providereliability, ease of application and good

storage capacity per unit volume.

A disadvantage of such memories is that apower supply is needed to hold the

information stored

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Batteries may be provided to prevent loss ofmemory in case of power failure. This is notnecessary for memories made up of ferritecores

Typically, 15 elements are needed for oneword, therefore for a 2,000 word RAM wewould need 30,000 elements arranged in a

three dimensional matrix.

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Figure shows a 9-word store, each wordconsisting of 3-bits.

W1 W2 W3W4 W5 W6W7 W8 W9

Y1

Y2

Y3

Y-DRIVEAND

DECODE

X-DRIVEAND

DECODE

MEMORYADDRESSREGISTER

REMAINDER OF COMPUTER

MEMORYCONTENT

REGISTER

SENSEAMPX1 X2 X3

W O R D

L E N G

T H

SENSE WIRE

LSB

MSB

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If one wishes to read the word at address W5,the memory address register sends, by way ofthe X and Y drive and decode circuits, a 1-bitalong wires X2 and Y2.

The contents of W5 are then read sequentiallyfrom least significant bit to most significantbit by the sense wire, which is connected toeach of the 27 elements.

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As part of a general-purpose computer ROMsmay be used to store information which is

unchanged over most, if not all, of theoperational life of the equipment.

If all the hardware of a computer is wired inusing ROMs then we no longer have ageneral-purpose computer and hence havelost the inherent flexibility.

However, saving in circuitry results, and somedegree of flexibility can be regained byhaving interchangeable ROMs mounted onprinted circuit boards (PCB) which can beremoved from and fitted to the computer.

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The CPU is the heart of any computingsystem.

It executes the individual machineinstructions, which make up a program.

The CPU is formed from the following

interconnected units: ALU (Arithmetic Logic Unit) Registers Control Unit

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MEMORY(REGISTERS)

ARITHMETICUNIT

COMPU

TER

HI

GHWA Y

MEMORY

INPUTOUTPUT

UNIT

CPU

CONTROL

CLOCK

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This is where the mathematics and logicfunctions are implemented.

It is not essential for the ALU to subtract,divide, or multiply, as these functions areeasily achieved by using addition inconjunction with 2's complement arithmetic.

However, more powerful processors includesophisticated arithmetic hardware capable ofdivision, multiplication, fixed and floatingpoint arithmetic etc. Large processors alsoemploy parallel operation for high speed

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These are temporary storage units within theCPU.

Some registers have dedicated uses, such asthe program counter register and theinstruction register.

Other registers may be used for storing eitherdata or program information.

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Figure illustrates the principal registers withinthe CPU.

MEMORY

MEMORYADDRESSREGISTER

INPUTOUTPUT

ADDRESSDECODE

PROGRAMCOUNTERREGISTER

INSTRUCTIONDECODE

REGISTER

CONTROLUNIT

TIMING

STATUS FLAGREGISTER

TEMPORARY REGISTER

ACCUMULATORREGISTER

IN

TERNAL

HIGHWA

Y

PORT 1

PORT 2

PORT 3

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The instructions that comprise a program arestored in the computer's memory.

Consequently, the computer must be able tosequentially access each instruction.

The address of the first instruction is loaded

into the program register, whereupon theinstruction is fetched and loaded into anotherregister, appropriately called the instructiondecode register

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Whilst the CPU is implementing the fetched

instruction (e.g. Add, Shift, etc), the programcounter register is incremented by 1 toindicate the address of the next instruction tobe executed.

This system, therefore, provides sequentialexecution of a program, provided that theprogram is written and stored sequentially inthe memory.

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As stated above, the program counterregister locates the address at which the nextinstruction is to be found.

The instruction itself is then transferred frommemory into the instruction decode register.As the name implies, this register alsoincorporates a decoder.

The output from the decoder places thenecessary logic demands onto the ALU - i.e.shift, add, etc.

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This register is really part of the ALU, and it isthe main register used for calculations.

Consequently, it always stores one of theoperands, which is to be operated on by theALU.

The other operand may be stored in anytemporary register.

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This register is a set of bi-stables whichoperate independently of each other.

The bi-stables independently monitor theaccumulator to detect such occurrences as anegative result of a calculation, a zero result,an overflow, etc.

When such an occurrence arises, the outputof the respective bi-stable is set (logic 1).

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It is this register that gives a computer itsdecision-making capability.

For example, if the result of a calculation in anavigational computer is zero, the program

could instruct the autopilot to hold itspresent course.

Alternatively, if the zero flag was not set, the

computer would then decide to takecorrective action

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There are many other registers within a CPU,some of which are general-purpose registers.

These can be used to store operands or

intermediate data within the CPU, thuseliminating the need to pass intermediateresults back and forth between memory andaccumulator.

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This unit is responsible for the overall actionof the computer.

Because it is responsible for timingoperations it includes a clock (normallycrystal controlled), so that instructions anddata can be transferred between units understrict timing control (synchronous operation).

The crystal and the clock generator mayeither be contained within the CPU, orsupplied as separate components.

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Microprocessor can be defined as the centralprocessing part of a computer containedwithin an IC (Integrated Circuit).

MICROPROCESSOR(CPU) COMPUTER HIGHWAY

ROM

RAM

OUTPUT

INPUT

INPUT/OUTPUTPORTS

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The microprocessor is small, lightweight, andrelatively cheap when compared to any CPU.

But it is also relatively slow, capable ofprocessing only hundreds of instructions per

second, compared to a large CPU which canprocess thousands of instructions persecond, or a very fast CPU which can processmillions of instructions per second (mips)

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However, many computing applications cantolerate the relative speed disadvantage ofthe microprocessor.

Microprocessors are typically available in 4, 8and 16-bit word lengths.

Microprocessors for machine control (lathes,

robots, petrol pumps, etc) often incorporateADC and DAC on the same chip, plus a smallamount ROM and RAM.

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Some microprocessors incorporate all theelements of a total computing system: I/O,ROM, RAM and CPU.

Manufacturers designate these as single chipmicrocomputers.

Obviously, their computing power is

somewhat limited, because there is a limitedamount of space available in just one IC.

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A Flight Management System (FMS) is acomputer-based flight control system and iscapable of four main functions:

Automatic Flight ControlPerformance ManagementNavigation and GuidanceStatus and Warning Displays

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The FMC receives input data from four sub-system computers:

Flight Control Computer (FCC).Thrust Management Computer (TMC).Digital Air Data Computer (DADC).Engine Indicating & Crew Alerting System (EICAS).

The communication between these computers istypically ARINC 429 data format.

Other parallel and serial data inputs are receivedfrom flight deck controls, navigation aids andvarious airframe and engine sensors.

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The FMC contains a large non-volatile

memory that stores performance andnavigation data along with the necessaryoperating programs.

Portions of the non-volatile memory are usedto store information concerning:

Airports.Standard Flight Routes.Nav Aid Data.

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Since this information changes, the FMSincorporates a Data Loader.

The data loader is either a tape or disk drivethat can be plugged into the FMC.

This data is updated every 28 days.

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NAV DATA

BASE

OPERATIONPROGRAM

STORAGE

FMC

INITIAL AIRLINEBASE & 28 DAYUPDATES

RAW DATA FORCOMPUTATIONS

ROLLCHANNEL

PITCHCHANNEL

MODETARGET

REQUESTS

DISPLAYS

AILERONCONTROL

ELEVATORCONTROL

THRUSTLEVER

CONTROL

REQUESTEDROUTELATERALVERTICAL

MEMORY STORAGE16 BIT WORDS

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Variable parameters for a specific flight areentered into the FMS by either data loader, orControl Display Unit (CDU).

This data will set the required performancefor least-cost or least-time en-routeconfiguration.

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The CDU provides a means for the crew tocommunicate with the FMC.

It contains pushbutton key controllers and adisplay screen.

The keys are of two types: Alphanumeric keys Dedicated keys

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Alphanumeric keys,

This can be used to enter departure and destinationpoints and also Waypoint if not already stored ontape; they will also be used if the flight plan needsto be changed during the flight.

Dedicated keys, These are used for specific functions usually

connected with display. For example, by using theappropriate key the pilot can call up flight plan,Waypoint data, flight progress, present position,etc.

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When, for example, a departure point isentered using alphanumeric keys, theinformation is often held in a temporaryregister and displayed to the pilot; this isknown as a scratchpad display.

Once the pilot has checked the information iscorrect, he can enter the data into thecomputer store by pressing the appropriate

dedicated key typically labelled "Load" or"Enter".

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During a normal flight, the FMS sendsnavigation data to the EFIS, which can thendisplay a route map on the EHSI.

If the flight plan is altered by the flight crewen-route, then the EHSI map will changeautomatically.

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REGISTERSSEQUENCING

&ADDRESSING

SENSORS:

VOR/DME - OMEGADOPPLER - COMPASS

ETC

MAGNETIC TAPECASSETTE/CARTRIDGE

PUSH BUTTONCONTROLLER

ALPHANUMERICDEDICATED

MAGNETIC CARD READER

TO CONTROL

FROM CONTROLTO

STORE

FROM CONTROL

AD

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The analogue signals must be converted into

digital signals before being fed to thecomputer memory.

ADCs, which may be an integral part of the

sensor equipment achieve this, oralternatively a converter unit may beinstalled, which carries out all necessaryanalogue to digital conversion.

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Many different kinds of output device are used,including traditional devices such as relativebearing indicators and steering indicators.

With these, suitably designed digital to analogueconverters must be used. Similar outputs couldbe fed to an autopilot.

Digital read out can be obtained by use of hybrid(digital and analogue) servo systems, whichposition an output counter drum or alternativelyby use of 7 segment indicators.

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A ROM, which has the wired in program toconvert from binary code to the appropriate

drive, drives the segments, which may belight emitting diodes (LED) or liquid crystals(LCD).

Cathode ray tubes (CRT) are beingincreasingly used as output devices both fordisplay of alphanumeric information and, lesscommonly, electronic maps.

CRTs are essentially analogue devices and assuch require DACs, which will provide thenecessary fairly, complicated drives.

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Moving map displays may also be used as a

means of presenting navigation informationto the pilot.

The map itself may be an actual chart fittedon rollers, or alternatively projected film.

Closed loop servos, which drive the map, are

fed from the computer via DACs.

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ACCESS TIME The time interval required to communicatewith the memory, or storage unit of a digitalcomputer

Or the time interval between the instant atwhich the arithmetic unit calls for informationfrom the memory and the instant at whichthis information is delivered.

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ADDRESS A name or number that designates thelocation of information in a storage ormemory device.

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DATA PROCESSING The handling, storage and analysis ofinformation in a sequence of systematic andlogical operations by a computer.

DECODER A circuit network which convert encodedinformation back to normal format is known

as a Decoder

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INSTRUCTIONA machine word or set of characters inmachine language that directs a computer totake a certain action.

Part of the instruction specifies the operationto be performed, and another part specifiesthe address.

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LANGUAGE

A defined group of representative charactersor symbols combined with specific rulesnecessary for their interpretation.

The rules enable the translation of thecharacters into forms (such as digits) whichare meaningful to a machine.

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MAGNETIC CORE A form of storage in which information isrepresented by the direction of magnetisation

of a core.

Advantage of this kind of memory store isthat will retain its contents even if electrical

power is removed (Non-volatile).

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PROGRAMME A plan to get a solution to some specificproblem.

A precise sequence of coded instructions or aroutine for solving a problem with acomputer.

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WORD OR BYTE) An ordered set of characters which has atleast one meaning and is stored, transferred,or operated upon by the computer circuits as

a unit.