IBM 5170 Technical Reference page 369

Communications
Information-processing equipment used for communication is
called data terminal equipment (DTE.) Equipment used to
connect the DTE to the communication line is called data
communication equipment (DCE.)
An adapter connects the data terminal equipment to the data
communication line as shown in the following figure:
Data Data
Communications
Terminal Communications Line
Equipment Equipment
--~~
::2:: ­
Voice Line
EIA/CCITT
Cable Conforming to EIA
Adapter
or CCITT Standards
The EIA/CCITT adapter allows the DTE to be connected to the
DCE using EIA or CCITT standardized connections. An external
modem is shown in the figure; however, other types of DCE also
can be connected to the DTE using EIA or CCITT standardized
connections.
EIA standards are labeled RS-x (recommended standards-x), and
CCITT standards are labeled V.x or X.x, where x is the number
of the standard.
The EIA RS-232 interface standard defines the connector type,
pin numbers, line names, and signal levels used to connect data
terminal equipment to data communications equipment for the
purpose of transmitting and receiving data. Since the RS-232
standard was developed, it has been revised three times. The
~
three revised standards are RS-232A, RS-232B, and the presently
used RS-232C.
The CCITT V.24 interface standard is equivalent to the RS-232C
standard; therefore, the descriptions of the EIA standards also
apply to the CCITT standards.
Communications 8-3