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TheDigitalEncoder
Unlikeamultiplexerthatselectsoneindividualdatainputlineandthensends
thatdatatoasingleoutputlineorswitch,aDigitalEncodermorecommonly
calledaBinaryEncodertakesALLitsdatainputsoneatatimeandthen
convertsthemintoasingleencodedoutput.Sowecansaythatabinaryencoder,
isamulti-inputcombinationallogiccircuitthatconvertsthelogiclevel"1"
dataatitsinputsintoanequivalentbinarycodeatitsoutput.
Generally,digitalencodersproduceoutputsof2-bit,3-bitor4-bitcodes
dependinguponthenumberofdatainputlines.An"n-bit"binaryencoder
has2inputlinesandn-bitoutputlineswithcommontypesthatinclude4-to-2,8-
to-3and16-to-4lineconfigurations.Theoutputlinesofadigitalencoder
generatethebinaryequivalentoftheinputlinewhosevalueisequalto"1"and
areavailabletoencodeeitheradecimalorhexadecimalinputpatterntotypically
abinaryorB.C.D.outputcode.
4-to-2BitBinaryEncoder
Oneofthemaindisadvantagesofstandarddigitalencodersisthattheycan
generatethewrongoutputcodewhenthereismorethanoneinputpresentatlogic
level"1".Forexample,ifwemakeinputsDandDHIGHatlogic"1"bothatthe
sametime,theresultingoutputisneitherat"01"orat"10"butwillbeat"11"
whichisanoutputbinarynumberthatisdifferenttotheactualinputpresent.
Also,anoutputcodeofalllogic"0"scanbegeneratedwhenallofitsinputsare
at"0"ORwheninputDisequaltoone.
Onesimplewaytoovercomethisproblemisto"Prioritise"thelevelofeachinput
pinandiftherewasmorethanoneinputatlogiclevel"1"theactualoutputcode
wouldonlycorrespondtotheinputwiththehighestdesignatedpriority.Thenthis
typeofdigitalencoderisknowncommonlyasaPriorityEncoderorP-
encoderforshort.
PriorityEncoder
ThePriorityEncodersolvestheproblemsmentionedabovebyallocatinga
priorityleveltoeachinput.Thepriorityencodersoutputcorrespondstothe
currentlyactiveinputwhichhasthehighestpriority.Sowhenaninputwitha
n
12
0
higherpriorityispresent,allotherinputswithalowerprioritywillbe
ignored.Thepriorityencodercomesinmanydifferentformswithanexampleofan8-inputpriorityencoderalongwithitstruthtableshownbelow.
8-to-3BitPriorityEncoder
PriorityencodersareavailableinstandardICformandtheTTL74LS148isan8-
to-3bitpriorityencoderwhichhaseightactiveLOW(logic"0")inputsand
providesa3-bitcodeofthehighestrankedinputatitsoutput.Priorityencoders
outputthehighestorderinputfirstforexample,ifinputlines"D2","D3"and
"D5"areappliedsimultaneouslytheoutputcodewouldbeforinput"D5"("101")as
thishasthehighestorderoutofthe3inputs.Onceinput"D5"hadbeenremoved
thenexthighestoutputcodewouldbeforinput"D3"("011"),andsoon.
Thetruthtablefora8-to-3bitpriorityencoderisgivenas:
DigitalInputsBinaryOutput
DDDDDDDDQQQ
00000001000
0000001X001
000001XX010
00001XXX011
0001XXXX100
001XXXXX101
01XXXXXX110
1XXXXXXX111
Fromthistruthtable,theBooleanexpressionfortheencoderabovewith
inputsDtoDandoutputsQ,Q,Qisgivenas:
OutputQ
76543210210
07012
0
OutputQ
OutputQ
ThenthefinalBooleanexpressionforthepriorityencoderincludingthezero
inputsisdefinedas:
Inpracticethesezeroinputswouldbeignoredallowingtheimplementationofthe
finalBooleanexpressionfortheoutputsofthe8-to-3priorityencoderaboveto
beconstructedusingindividualORgatesasfollows.
DigitalEncoderusingLogicGates
1
2
EncoderApplications
KeyboardEncoder
Priorityencoderscanbeusedtoreducethenumberofwiresneededinaparticular
circuitsorapplicationthathavemultipleinputs.Forexample,assumethata
microcomputerneedstoreadthe104keysofastandardQWERTYkeyboardwhereonly
onekeywouldbepressedeither"HIGH"or"LOW"atanyonetime.Onewaywouldbe
toconnectall104wiresfromthekeysdirectlytothecomputerbutthiswouldbe
impracticalforasmallhomePC,butanotherbetterwaywouldbetouseapriority
encoder.
The104individualbuttonsorkeyscouldbeencodedintoastandardASCIIcodeof
only7-bits(0to127decimal)torepresenteachkeyorcharacterofthekeyboard
andtheninputtedasamuchsmaller7-bitB.C.Dcodedirectlytothecomputer.
Keypadencoderssuchasthe74C92320-keyencoderareavailabletodojustthat.
PositionalEncoders
Anothermorecommonapplicationisinmagneticpositionalcontrolasusedonships
navigationorforroboticarmpositioningetc.Hereforexample,theangularor
rotarypositionofacompassisconvertedintoadigitalcodebya74LS1488-to-3
linepriorityencoderandinputtedtothesystemscomputertoprovidenavigational
dataandanexampleofasimple8positionto3-bitoutputcompassencoderis
shownbelow.Magnetsandreedswitchescouldbeusedateachcompasspointto
indicatetheneedlesangularposition.
PriorityEncoderNavigation
CompassDirectionBinaryOutputQQQ
North000
North-East001
East010
South-East011
South100
South-West101
West110
North-West111
InterruptRequests
OtherapplicationsespeciallyforPriorityEncodersmayincludedetecting
interruptsinmicroprocessorapplications.Herethemicroprocessorusesinterrupts
toallowperipheraldevicessuchasthediskdrive,scanner,mouse,orprinter
etc,tocommunicatewithit,butthemicroprocessorcanonly"talk"toone
peripheraldeviceatatime.Theprocessoruses"InterruptRequests"or"IRQ"
signalstoassignprioritytothedevicestoensurethatthemostimportant
peripheraldeviceisservicedfirst.Theorderofimportanceofthedeviceswill
dependupontheirconnectiontothepriorityencoder.
IRQNumberTypicalUseDescription
IRQ0SystemtimerInternalSystemTimer.
IRQ1KeyboardKeyboardController.
IRQ3COM2&COM4SecondandFourthSerialPort.
IRQ4COM1&COM3FirstandThirdSerialPort.
IRQ5SoundSoundCard.
IRQ6FloppydiskFloppyDiskController.
IRQ7ParallelportParallelPrinter.
IRQ12MousePS/2Mouse.
IRQ14PrimaryIDEPrimaryHardDiskController.
IRQ15SecondaryIDESecondaryHardDiskController.
Becauseimplementingsuchasystemusingpriorityencoderssuchasthestandard
74LS148priorityencoderICinvolvesadditionallogiccircuits,purposebuilt
integratedcircuitssuchasthe8259ProgrammablePriorityInterruptControlleris
available.
DigitalEncoderSummary
Thentosummarise,theDigitalEncoderisacombinationalcircuitthatgenerates
aspecificcodeatitsoutputssuchasbinaryorBCDinresponsetooneormore
activeinputs.Therearetwomaintypesofdigitalencoder.TheBinary
EncoderandthePriorityEncoder.
TheBinaryEncoderconvertsoneof2inputsintoann-bitoutput.Thenabinary
encoderhasfeweroutputbitsthantheinputcode.Binaryencodersareusefulfor
compressingdataandcanbeconstructedfromsimpleANDorORgates.Oneofthe
maindisadvantagesofastandardbinaryencoderisthatitwouldproduceanerror
012
n
atitsoutputsifmorethanoneinputwereactiveatthesametime.Toovercomethisproblempriorityencodersweredeveloped.
ThePriorityEncoderisanothertypeofcombinationalcircuitsimilartoa
binaryencoder,exceptthatitgeneratesanoutputcodebasedonthehighest
prioritisedinput.Priorityencodersareusedextensivelyindigitalandcomputer
systemsasmicroprocessorinterruptcontrollerswheretheydetectthehighest
priorityinput.
Inthenexttutorialaboutcombinationallogicdevices,wewilllookat
complementaryfunctionoftheencodercalledaDecoderwhichconvertann-
bitinputcodetooneofits2outputlines.n
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