英语翻译
Digitaldevices
Themostbasicdigitaldevicesarecalledgates.Gatesoriginallygottheirnamefromtheirfunctionofallowingorretarding(“gating”)theflowofdigitalinformation.Eachgaterepresentsalogiccircuitdiagramwitharepresentativesymbol.Figure8-1showssymbolsforthecommongates.BothsetsofsymbolsillustratedaredefinedinIEEE/ANSIstandard91-1984.
Weuseahighvoltagelevel(H)torepresentlogic1andalowvoltage(L)torepresentlogic0.TheoutputofanANDgateis1onlyifbothinputsare1.Thetruthtableisshowninfigure8-2.Agateiscalledacombinationcircuitbecauseitsoutputdependsonlyonthecurrentinputcombination.
ANDgate,ORgateandNOTgate(orinverter)arethethreemostimportantkindsofgates.Anydigitalfunctioncanberealizedusingjustthesekindsofgates.
Aflip-flopisadevicethatstoreseither0or1.Thestateofflip-flopisthevaluethatitcurrentlystores.Thestoredvaluecanbechangedonlyatcertaintimedeterminedbya“clock”input,andthenewvaluemayfurtherdependontheflip-flop’scurrentstateandits“control”inputs.
Adigitalcircuitthatcontainsflip-flopiscalledasequentialcircuit,becauseitsoutputatanytimedependsnotonlyonitscurrentinputbutalsoonthepastsequenceofinputsthathavebeenappliedtoit.Inotherwords,asequentialcircuithasmemoryofpastevents.
ANSI:TheAmericanNationalStandardsInstitute美国国家标准化组织
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