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8 External Connections
RS-232-C devices are classified by their function as either Data Terminal Equipment
(DTE) or Data Communication Equipment (DCE). Generally, data terminal equipment is
defined as the communication source and data communication equipment is defined as the device
that provides a communication channel between two DTE-type devices.
Telephone
line
DCE
DCE
DTE
DTE
RS-232-C
RS-232-C
Modem
Modem
Terminal
Terminal
Figure 16 --- Use of DTEs and DCEs in a communications link
DTE- and DCE-type devices have complementary pinouts to allow terminals and
modems to be connected directly using a one-to-one cable as shown in Figure 16. In many
applications, DCEs are unnecessary, and in these cases a cable called a "null modem cable" or
"modem eliminator cable" is used to directly connect two DTE-type devices. A typical null
modem cable is also shown in Figure 16.
RxD
TxD
RTS
CTS
DTR
DSR
DCD
GND
(3)
(2)
(4)
(5)
(20)
(6)
(8)
(22)
(7)
RI
TxD
RxD
CTS
RTS
DSR
DTR
DCD
GND
(3)
(2)
(4)
(5)
(20)
(6)
(8)
(22)
(7)
RI
RxD
TxD
RTS
CTS
DTR
DSR
DCD
GND
(3)
(2)
(4)
(5)
(20)
(6)
(8)
(22)
(7)
RI
DCD
GND
(8)
(22)
(7)
Typical DTE-to-DCE cable
RI
DCD
GND
RxD
TxD
RTS
CTS
DTR
DSR
(3)
(2)
(4)
(5)
(20)
(6)
(8)
(22)
(7)
RI
DCD
GND
(8)
(22)
(7)
RxD
TxD
RTS
CTS
DTR
DSR
(3)
(2)
(4)
(5)
(20)
(6)
Typical DTE-to-DTE null modem cable
Figure 17 --- Cabling requirements for RS-232-C devices
(cables using 25-pin connectors shown)
Quatech QSC-100 User's Manual 24