In popular use, RS-232 refers to a serial interface that complies with much of
the standard TIA-232-F: Interface between Data Terminal Equipment. The name
RS-232 dates to an earlier edition of the standard.
The standard??™s current publisher is the Telecommunications Industry Association
(TIA). Previous versions were a product of the Electronics Industries Association
(EIA). A similar standard is encompassed by V.24 and V.28 from the
Chapter 4
44
International Telecommunication Union (ITU) and ISO 2110 from the International
Organization for Standardization (ISO).
The standard defines the names and functions of signals, electrical characteristics
of the signals, and mechanical specifications, including pin assignments.
Earlier versions didn??™t include all of these items. The addition of new material,
such as recommended connectors, documented what had become standard
through popular use.
The standard designates 25 lines in the interface, but RS-232 ports rarely support
more than the nine signals in Table 4-1. The additional signals are
intended for use with synchronous modems, secondary transmission channels,
and selecting a transmission speed on a dual-rate modem. Some applications
require only three lines (or even two, if the link is one way).
Much of the RS-232 terminology reflects its origin as a standard for communications
between a computer terminal and an external modem.
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