National Instruments PCI-8430 - Page 146

RS-422
AsspecifiedintheEIA/RS-422-AStandard,ElectricalCharacteristicsof
BalancedVoltageDigitalInterfaceCircuits,RS-422definesaserial
interfacemuchlikeRS-232.However,RS-422usesbalanced(or
differential)transmissionlines.Balancedtransmissionlinesusetwo
transmissionlinesforeachsignal.Thestateofeachsignalis
represented,notbyavoltagelevelononelinereferencedtogroundasin
RS-232,butratherbytherelativevoltageofthetwolinestoeachother.
Forexample,theTXsignaliscarriedontwowires,wireAandwireB.A
logical1isrepresentedbythevoltageonlineAbeinggreaterthanthe
voltageonlineB.Alogical0isrepresentedbythevoltageonlineAbeing
lessthanthevoltageonlineB.Differentialvoltagetransmissioncreatesa
signalthatismoreimmunetonoiseaswellasvoltagelossdueto
transmissionlineeffects.Thus,youcanuseRS-422forlongerdistances
(upto4,000ft)andgreatertransmissionspeeds(upto10Mbytes/s)than
RS-232.