National Instruments PCI-8430 - Page 185

Whyisn'ttheAuto232transceivermodeonmy
USB-232working?
TheAuto232modeofoperationallowsyoutoconnecttoeitheraRS-232
DTEorDCEdeviceusingeitheranull-modemorstraight-throughcable.
TheUSB-232portwillautomaticallyswitchapproximatelyevery250ms
betweenDTEandDCEoperatingmodesuntilatleastoneRS-232
receiverline(RX,CTS,DSR,RI,orDCD)hasavalidvoltage.Auto232is
availableontheUSB-232/2andUSB-232/4,andisnotavailableonthe
USB-232(singleport).
Notalldevicesandapplicationscantakeadvantageofthisfeature:
Somedevicescangeneratevalidvoltagesonreceivelinesforboth
modes,eventhoughthedeviceitselfisonlyDTEorDCE.Thus,
thesedevicesmaynotoperatecorrectlywhenusingAuto232
mode.
SomeapplicationsimmediatelystartcommunicatingafteraCOM
portisopen,thusnotallowingenoughtimefortheproper
operatingmodetobeselected.
SomecablingschemesshortRI,DCD,andDSRtogether.While
thisdoesnotpresentaprobleminDTEmode,thesesignalsare
drivenwheninDCEmode.Thus,byshortingthesignalstogether,
theoutputswillbedouble(ortriple)drivenwhichmaycausesignal
noiseorotherartifacts.Useastraight-throughcableinstead.
IfproblemswithAuto232cannotberesolved,settheUSB-232tothe
correctmode(DTEorDCE)foryourdeviceandapplication.Bydefault,
theUSB-232portsaresettoDTEmode.