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7. External Connections
EIA/TIA-232-E 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
EIA/TIA-232-E
EIA/TIA-232-E
Modem
Modem
Terminal
Terminal
Figure 11 --- 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 12. 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 12.
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 12 --- Cabling requirements for EIA/TIA-232-E devices
(cables using 25-pin connectors shown)
DS-100 User’s Manual 6-1