(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 conditions etc.
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.