Appendix B
Serial Port Information
PCMCIA Serial for Windows 95
B-6
©
National Instruments Corporation
Figure B-4 shows straight-through cabling in a DTE-to-DCE interface.
Figure B-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. You should use a null-modem cable to communicate from one
serial port to another.
Figure B-5 shows null-modem cabling in a DTE-to-DCE interface.
Figure B-5.
Null-Modem Cabling in a DTE-to-DCE Interface
Pin 2
Rx D
Tx D
Rx D
Tx D
Pin 2
Pin 2
Pin 3
Pin 3
Pin 3
Pin 3
Pin 2
DTE
DCE
DCE
DTE
Rx D
Tx D
Rx D
Tx D
Pin 2
Pin 3
Pin 3
Pin 2
DTE
DTE
