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GORESET
TheLogic"OR"Gate
Definition
ALogicORGateorInclusive-ORgateisatypeofdigitallogicgatethathasanoutputwhichisnormallyatlogiclevel"0"
andonlygoes"HIGH"toalogiclevel"1"whenoneormoreofitsinputsareatlogiclevel"1".Theoutput,Qofa"LogicOR
Gate"onlyreturns"LOW"againwhenALLofitsinputsareatalogiclevel"0".InotherwordsforalogicORgate,any
"HIGH"inputwillgivea"HIGH",logiclevel"1"output.
ThelogicorBooleanexpressiongivenforalogicORgateisthatforLogicalAdditionwhichisdenotedbyaplussign,
(+)givingustheBooleanexpressionof:A+B=Q.
Thenwecandefinetheoperationofa2-inputlogicORgateasbeing:
"IfeitherAorBistrue,thenQistrue"
2-inputTransistorORGate
Asimple2-inputlogicORgatecanbeconstructedusingRTLResistor-transistorswitchesconnectedtogetherasshown
belowwiththeinputsconnecteddirectlytothetransistorbases.Eithertransistormustbesaturated"ON"foranoutput
atQ.
LogicORGatesareavailableusingdigitalcircuitstoproducethedesiredlogicalfunctionandisgivenasymbolwhose
shaperepresentsthelogicaloperationoftheORgate.
TheDigitalLogic"OR"Gate
2-inputORGate
SymbolTruthTable
BAQ
000
011
2-inputORGate101
111
BooleanExpressionQ=A+BReadasAORBgivesQ
3-inputORGate
SymbolTruthTable
3-inputORGate
CBAQ
0000
0011
0101
0111
1001
1011
1101
1111
BooleanExpressionQ=A+B+CReadasAORBORCgivesQ
LiketheANDgate,theORfunctioncanhaveanynumberofindividualinputs.However,commercialavailableORgates
areavailablein2,3,or4inputstypes.Additionalinputswillrequiregatestobecascadedtogetherforexample.
Multi-inputORGate
TheBooleanExpressionforthis6-inputORgatewillthereforebe:Q=(A+B)+(C+D)+(E+F)
Ifthenumberofinputsrequiredisanoddnumberofinputsany"unused"inputscanbeheldLOWbyconnectingthem
directlytogroundusingsuitable"Pull-down"resistors.
CommonlyavailableORgateIC''sinclude:
TTLLogicTypes
74LS32Quad2-input
CMOSLogicTypes
CD4071Quad2-input
CD4075Triple3-input
CD4072Dual4-input
Quad2-inputORGate7432
InthenexttutorialaboutDigitalLogicGates,wewilllookatthedigitallogicNOTGatefunctionasusedinbothTTLand
CMOSlogiccircuitsaswellasitsBooleanAlgebradefinitionandtruthtable.
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