7 External ConnectionsRS-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