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Appendix A Serial Port Information
PCMCIA Serial for Windows Me/9x A-6 ni.com
Figure A-4. 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 serial port to another, use a
null-modem cable.
Figure A-5 shows null-modem cabling in a DTE-to-DTE interface.
Figure A-5. Null-Modem Cabling in a DTE-to-DTE Interface
Rx D
Tx D
Pin 3
Pin 2
DTE
Pin 3
Pin 2
DCE
Pin 3
Pin 2
DCE
Pin 3
Pin 2
DTE
Rx D
Tx D
Rx D
Tx D
Rx D
Tx D
Pin 2
Pin 3Pin 3
Pin 2
DTE DTE