RS232 Terminology
DTE
DCE
Data Terminal Equipment
Data Communication Equipment
Computer or terminal
Modem
Summary of main RS232 signals
DB-25
DB-9
Signal
Name
1
7
Direction
5
FG
SG
Frame Ground
Signal Ground
-
2
3
2
3
TD
RD
Transmit data
Receive data
DTE → DCE
DCE → DTE
4
5
7
8
RTS
CTS
Request to send
Clear to send
DTE → DCE
DCE → DTE
6
20
6
4
DSR
DTR
Data set ready
Data terminal ready
DCE → DTE
DTE → DCE
8
9
1
9
DCD
RI
Data carrier detect
Ring indicator
DCE → DTE
DCE → DTE
The numbers in the DB-25 column are the pins numbers for a DB-25 connector.
The numbers in the DB-9 are typical pin numbers for a DB-9 connector. Sometimes
DB-9 connectors are used but only pins 2, 3 and 7 are
For a normal connections (computer to a modem), the signal on both ends pass directly
through. That is, TD (pin 2) is connect to pin2 at both ends.
A null modem connection can be used to connect two computers together. Since each
computer interprets the signals from its own point of view, the connectors on either end
are slightly different. The TD signal at one end is connected to pin 2 of the connector,
while the same signal line is connected to pin 3 (RD) at the other end. At one end the RD
signal is connected to pin 3 but at the other end is is connected to pin 2 (TD). This makes
since the transmit line for a computer at one end is viewed as the receive line by the computer at the other end.
There are two other differences in a null modem connection. First, the DTR from one
end is connected to DSR and DCD at the other end. Second, the RTS at one end is connected to CTS at the other end.