National Instruments PCI-8430 - Page 154

RS-232DTEversusDCE
IntheRS-232specification,DTE(DataTerminalEquipment)andDCE
(DataCommunicationsEquipment)refertothetypesofequipmenton
eitherendofaserialconnection.Ingeneral,DTEandDCEreferto
computerequipmentandmodems,respectively.BecausetheRS-232
specificationmainlyinvolvesconnectingaDTEdirectlytoaDCEand
viceversa,thepinoutsaredefinedsothatcablingissimple.Thatis,a
cableconnectedacomputertoamodembywiringpin1topin1,pin2to
pin2,andsoon.Thismethodisknownasstraight-throughcabling.
Thefollowingfigureshowsstraight-throughcablinginaDTE-to-DCE
interface.
Straight-ThroughCablinginaDTE-to-DCEInterface
Straight-throughcablingisstillthestandardmethodtoconnectamodem
toyourPC.However,becausemanyapplicationsuseserial
communicationtoconnecttwoormoreDTEswithoutmodems,the
cablingbecomesmorecomplicated.IftwoDTEsarewiredtogetherusing
astraight-throughcable,onetransmitterisconnectedtotheother
transmitter,andonereceiverisconnectedtotheotherreceiver.Inthis
setup,notransmissionscanoccur.Thus,theseapplicationsmustusea
cablingschemethatconnectsthetransmitterononedevicetothe
receiverontheotherdeviceandviceversa.Thismethodisknownas
null-modemcabling,becauseitreplacesthetwomodemsthattraditional
RS-232applicationswouldrequirebetweenthetwoDTEs.To
communicatefromoneDTEserialporttoanother,useanull-modem
cable.
Thefollowingfigureshowsnull-modemcablinginaDTE-to-DTE
interface.
Null-ModemCablinginaDTE-to-DTEInterface
PCI,PXI,USB(oneportonly),ENET,ExpressCard,andPCMCIARS-