The Free On-line Dictionary of Computing (18 March 2015):
(Formerly "RS-232") The most common
asynchronous serial line standard. EIA-232 is the EIA
equivalent of ITU-T V.24, and V.28.
EIA-232 specifies the gender and pin use of connectors, but
not their physical type. RS-423 specifies the electrical
signals. 25-way D-type connectors are common but often only
three wires are connected - one ground (pin 7) and one for
data in each direction. The other pins are primarily related
to hardware handshaking between sender and receiver and to
carrier detection on modems, inoperative circuits, busy
The standard classifies equipment as either Data
Communications Equipment (DCE) or Data Terminal Equipment
(DTE). DTE receives data on pin 3 and transmits on pin 2
(TD). A DCE EIA-232 interface has a female connector. DCE
receives data from DTE on pin 2 (TD) and sends that data out
the analog line. Data received from the analog line is sent
by the DCE on pin 3(RD).
Originally DCE was a modem and DTE was a computer or terminal.
The terminal or computer was connected (via EIA-232) to two
modems, which were connected via a telephone line.
The above arrangement allows a computer or terminal to be
connected to a modem with a straight-through (2-2, 3-3) cable.
It is common, however, to find equipment with the wrong sex
connector or with pins two and three reversed, requiring the
insertion of a cable or adaptor wired as a gender mender or
null modem. Such an adaptor is also required when
connecting a computer directly to a terminal or to another
computer without the use of modems.