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1、RS协议英文介绍样本 S RS协议英文介绍本文档所提供的信息仅供参考之用，不能作为科学依据，请勿模仿。 文档如有不当之处，请联系本人或网站删除。 RS-422and RS-485application noteChapter1:Overview IntroductionThe purposeof this application noteis todescribe themain elementsof an RS-422and RS-485system.This application note attemptsto coverenough technicaldetails so that t
2、he system designerwill haveconsidered allthe importantaspects inhis data system design.Since bothRS-422and RS-485are datatransmission systemsthat usebalanced differentialsignals,it is appropriate todiscuss bothsystems in the sameapplication note.Throughout this application notethe genericterms of RS
3、-422and RS-485will be used torepresent theEIA/TIA-422and EIA/TIA-485Standards.Data TransmissionSignals UnbalancedLine DriversEach signalthat transmitsin an RS-232unbalanced datatransmission systemappears on the interfaceconnector asa voltagewith referenceto a signal ground.For example,the transmitte
4、ddata(TD)from aDTE deviceappears onpin2with respect to pin7(signal ground)on aDB-25connector.This voltagewill benegative if the line is idleand alternatebetween thatnegative leveland apositive levelwhen data is sentwith amagnitude of5to15volts.The RS-232receiver typicallyoperates within the voltage
5、range of+3to+12and-3to-12volts as shown in Figure.Balanced LineDrivers In a balanced differential systemthe voltageproduced by the driverappears acrossa pair of signallines thattransmit onlyone signal.Figure showsa schematicsymbol for a balanced line driver and thevoltages thatexist.A balancedline d
6、riverwill producea voltagefrom2to6volts acrossits A and Boutput terminalsand willhave asignal ground(C)connection.Although properconnection to the signal ground isimportant,it isnt used by a balancedline receiver indetermining thelogic state of the data line.A balancedline driver can also have aninp
7、ut signalcalled an“Enable”signal.The purposeof thissignal is to connectthe driverto its output terminals,A and B.If the“Enable”signal isOFF,one canconsider the driver asdisconnected from the transmission line.An RS-485driver musthave the“Enable”control signal.An RS-422driver mayhave thissignal,but i
8、t is not always required.The disconnectedordisabledcondition of the linedriver usuallyis referredto asthe“tristate1”condition of the driver.1The term“tristate”es from the factthat thereisathird outputstateof an RS-485driver,in additionto theoutput statesof“1”and“0.”Balanced LineReceivers Abalanceddi
9、fferential line receiversenses thevoltage stateof the transmission lineacross twosignal inputlines,A and B.It willalsohave asignal ground(C)that is necessary inmaking the proper interfaceconnection.Figure isa schematicsymbol fora balanceddifferentialline receiver.Figure alsoshows thevoltages th本文档所提
10、供的信息仅供参考之用，不能作为科学依据，请勿模仿。 文档如有不当之处，请联系本人或网站删除。 at areimportant to the balancedlinereceiver.If thedifferential input voltage Vabis greaterthan+200mV the receiver willhave aspecific logicstate onits outputterminal.If theinputvoltage is reversedto less than-200mV the receiver willcreate theopposite log
11、icstate onitsoutputterminal.The inputvoltages that a balancedlinereceivermust senseare shownin Figure.The200mV to6V rangeis requiredto allowfor attenuationon the transmission line.EIA Standard RS-422Data TransmissionThe EIA StandardRS-422-A entitled“Electrical Characteristicsof BalancedVoltage Digit
12、alInterface Circuits”defines thecharacteristics ofRS-422interface circuits.Figure isa typicalRS-422four-wire interface.Notice thatfive conductorsare used.Each generatoror drivercan driveup toten (10)receivers.The twosignaling statesof the line aredefined asfollows:a.When the“A”terminal of the driver
13、is negativewith respectto the“B”terminal,the line is in a binary1(MARK orOFF)state.b.When the“A”terminal of the driveris positivewith respectto the“B”terminal,the line is in a binary0(SPACE orON)state.Figure shows the conditionof thevoltage of the balancedline foran RS-232to RS-422converter whenthe
14、lineis in the“idle”condition orOFF state.It alsoshowstherelationship of the“A”and“B”terminals of an RS-422system and the“-”and“+”terminal markingsused onmany typesof equipment.The“A”terminal isequivalent to the“-”designation,and the“B”terminal equivalentto the“+”designation.The samerelationship show
15、nin Figurealso appliesfor RS-485systems.RS-422can withstand a mon mode voltage(Vcm)of7volts.Common modevoltageisdefined asthe meanvoltage of the Aand Bterminals withrespecttosignalground.EIAStandardRS-485Data TransmissionThe RS-485Standard permitsabalancedtransmission lio beshared in a party line or
16、multidrop mode.As manyas32driver/receiver pairscan sharea multidropwork.Many characteristicsof the drivers andreceivers arethe sameas RS-422.The rangeof themon modevoltage Vcmthat thedriverandreceiver cantolerate isexpanded to+12to-7volts.Since thedrivercan be disconnectedor tristatedfromthe line,it mustwithstand thismon modevoltage rangewhile in the tristatecondition.Some RS-422drivers,even withtristate capability,will notwithstand thefull Vcmvoltagerangeof+1
