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TheMultiplexer
Adataselector,morecommonlycalledaMultiplexer,shortenedto"Mux"or"MPX",arecombinationallogicswitching
devicesthatoperatelikeaveryfastactingmultiplepositionrotaryswitch.Theyconnectorcontrol,multipleinputlines
called"channels"consistingofeither2,4,8or16individualinputs,oneatatimetoanoutput.
Thenthejobofa"multiplexer"istoallowmultiplesignalstoshareasinglecommonoutput.Forexample,asingle8-
channelmultiplexerwouldconnectoneofitseightinputstothesingledataoutput.Multiplexersareusedasonemethod
ofreducingthenumberoflogicgatesrequiredinacircuitorwhenasingledatalineisrequiredtocarrytwoormore
differentdigitalsignals.
DigitalMultiplexersareconstructedfromindividualanalogueswitchesencasedinasingleICpackageasopposed
tothe"mechanical"typeselectorssuchasnormalconventionalswitchesandrelays.Generally,multiplexershavean
evennumberofdatainputs,usuallyanevenpoweroftwo,n,anumberof"control"inputsthatcorrespondwiththe
numberofdatainputsandaccordingtothebinaryconditionofthesecontrolinputs,theappropriatedatainputis
connecteddirectlytotheoutput.AnexampleofaMultiplexerconfigurationisshownbelow.
4-to-1ChannelMultiplexer
AddressingInput
Selectedba
00A
01B
10C
11D
TheBooleanexpressionforthis4-to-1MultiplexerabovewithinputsAtoDanddataselectlinesa,bisgivenas:
Q=abA+abB+abC+abD
InthisexampleatanyoneinstantintimeonlyONEofthefouranalogueswitchesisclosed,connectingonlyoneofthe
inputlinesAtoDtothesingleoutputatQ.Astowhichswitchiscloseddependsupontheaddressinginputcodeon
lines"a"and"b",soforthisexampletoselectinputBtotheoutputatQ,thebinaryinputaddresswouldneedtobe"a"=
logic"1"and"b"=logic"0".
Thenwecanshowtheselectionofthedatathroughthemultiplexerasafunctionofthedataselectbitsasshown.
2
Addingmorecontroladdresslineswillallowthemultiplexertocontrolmoreinputsbuteachcontrollineconfigurationwill
connectonlyONEinputtotheoutput.
ThentheimplementationofthisBooleanexpressionaboveusingindividuallogicgateswouldrequiretheuseofseven
individualgatesconsistingofAND,ORandNOTgatesasshown.
4ChannelMultiplexerusingLogicGates
Thesymbolusedinlogicdiagramstoidentifyamultiplexerisasfollows.
MultiplexerSymbol
Multiplexersarenotlimitedtojustswitchinganumberofdifferentinputlinesorchannelstoonecommonsingleoutput.
Therearealsotypesthatcanswitchtheirinputstomultipleoutputsandhavearrangementsor4to2,8to3oreven16to
4etcconfigurationsandanexampleofasimpleDualchannel4inputmultiplexer(4to2)isgivenbelow:
4-to-2ChannelMultiplexer
Hereinthisexamplethe4inputchannelsareswitchedto2individualoutputlinesbutlargerarrangementsarealso
possible.Thissimple4to2configurationcouldbeusedforexample,toswitchaudiosignalsforstereopre-amplifiersor
mixers.
AdjustableAmplifierGain
Aswellassendingparalleldatainaserialformatdownasingletransmissionlineorconnection,anotherpossibleuse
ofmulti-channelmultiplexersisindigitalaudioapplicationsasmixersorwherethegainofananalogueamplifiercanbe
controlleddigitally,forexample.
DigitallyAdjustableAmplifierGain
Here,thevoltagegainoftheinvertingamplifierisdependentupontheratiobetweentheinputresistor,Rinandits
feedbackresistor,RfasdeterminedintheOp-amptutorials.Asingle4-channel(Quad)SPSTswitchconfiguredasa4-
to-1channelmultiplexerisconnectedinserieswiththeresistorstoselectanyfeedbackresistortovarythevalueofRf.
Thecombinationoftheseresistorswilldeterminetheoverallgainoftheamplifier,(Av).Thenthegainoftheamplifiercan
beadjusteddigitallybysimplyselectingtheappropriateresistorcombination.
Digitalmultiplexersaresometimesalsoreferredtoas"DataSelectors"astheyselectthedatatobesenttotheoutput
lineandarecommonlyusedincommunicationsorhighspeednetworkswitchingcircuitssuchasLAN′sandEthernet
applications.SomemultiplexerIC′shaveasingleinvertingbuffer(NOTGate)connectedtotheoutputtogiveapositive
logicoutput(logic"1",HIGH)ononeterminalandacomplimentarynegativelogicoutput(logic"0",LOW)onanother
differentterminal.
ItispossibletomakesimplemultiplexercircuitsfromstandardANDandORgatesaswehaveseenabove,but
commonlymultiplexers/dataselectorsareavailableasstandardi.c.packagessuchasthecommonTTL74LS1518-
inputto1linemultiplexerortheTTL74LS153Dual4-inputto1linemultiplexer.Multiplexercircuitswithmuchhigher
numberofinputscanbeobtainedbycascadingtogethertwoormoresmallerdevices.
TheMultiplexerisaveryusefulcombinationaldevicethathasitsusesinmanydifferentapplicationssuchassignal
routing,datacommunicationsanddatabuscontrol.Whenusedwithademultiplexer,paralleldatacanbetransmittedin
serialformviaasingledatalinksuchasafibre-opticcableortelephoneline.Theycanalsobeusedtoswitcheither
analogue,digitalorvideosignals,withtheswitchingcurrentinanaloguepowercircuitslimitedtobelow10mAto20mA
perchannelinordertoreduceheatdissipation.
Inthenexttutorialaboutcombinationallogicdevices,wewilllookatthereverseoftheMultiplexercalled
theDemultiplexerwhichtakesasingleinputlineandconnectsittomultipleoutputlines.
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