IBM 5150 Technical Reference page 252

Information processing equipment used for communications is
called data terminal equipment (DTE). Equipment used to connect
the DTE to the communications line is called data communications
equipment (DCE).
An adapter is used to connect the data terminal equipment to the
data communications line as shown in the following illustration:
Data
Data
Communications
Terminal
Communications
Line
Equipment
Equipment
Z
Voice Line
EIA/CCITT
Cable Conforming to EIA Modem
Adapter
or CCITT Standards Clock
The EIAI CCITT adapter allows data terminal equipment to be
connected to data communications equipment using EIA or CCITT
standardized connections. An external modem is shown in this
example; however, other types of data communications equipment
can also be connected to data terminal equipment 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 the RS-232A, the 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