IBM AT 5170 Technical Reference page 417

Hardware
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
~~---
~
Voice Line
EIA/CCITT
Cable Conforming to EIA
Adapter
or CCITT Standards
The EIA/CCITT adapter allows the data terminal equipment to
be connected to the data communications equipment using EIA or
CCITT standardized connections. An external modem is shown
in the figure; however, other types of data communications
equipment also can be connected to the 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 RS-232A, RS-232B, and the presently
used RS-232C.
Communications 8-3