National Instruments PCI-8430 - Page 101

Two-WireMode:DTRControlledwithEcho
Youcanusethismodeinhalf-duplexsystemswhereyouneedtocontrol
thetransmitterprogrammatically.IntheDTR-with-echomode,the
transmitteristri-statedwhentheDTRsignaloftheUART(Universal
AsynchronousReceiver/Transmitter)isunasserted.Totransmit,your
applicationfirstmustenablethetransmitterbyassertingDTR.Afterthe
dataisfullytransmitted,yourapplicationunassertsDTRtodisablethe
transmitter.Becausethereceiverisalwaysenabledinthismode,you
receivepacketsnotonlyfromotherdevices,butalsoyourtransmitter.
Thus,yourreceiverechoesalldatayoutransmit.