National Instruments PCI-8430 - Page 149

DuplexArchitectures
Duplexreferstothemeansofbandwidthusageinaserialsystem.The
twocommonmeansofbi-directionalserialcommunicationarefullduplex
andhalfduplex.Half-duplexcommunicationinvolvesatransmitteranda
receiverconnectedtoeachendofthesamewireorpairofwires.
Becausethesametransmissionlinebothsendsandreceivesdata,
devicescannotsenddatainbothdirectionsatthesametime.First,one
devicetransmitsoverthewire(s)tothereceiveroftheseconddevice.
Whenthefirstdevicefinishestransmitting,bothdevicesswitchthe
connectionsfromtheirtransmittertotheirreceiver,orviceversa.The
devicethatwasreceivingdatacanthentransmitovertheline.
Infull-duplexcommunication,thedevicesuseaseparatewire(orpairof
wires)forsimultaneoustransmissionineachdirection.Thus,thedevices
donotswitchbetweentransmittingandreceiving.
Inadifferentialserialbus(suchasRS-422orRS-485),ahalf-duplex
systemtransmitsandreceivesoverthesametwistedpairofwires.Thus,
half-duplexcommunicationisoftenreferredtoastwo-wire
communication.Likewise,full-duplexcommunicationisoftenreferredto
asfour-wirecommunication,becausethefull-duplexsystemusesa
separatepairofwiresforcommunicationineachdirection.