"A practical guide to RS-232 interfacing" - читать интересную книгу автора (Hughes L.E.)

A Practical Guide to RS-232 Interfacing

by Lawrence E. Hughes. Mycroft Labs, Inc.
P.O. Box 6045, Tallahassee, FL 32301

The following information is intended to collect together in
one place, and explain in relatively simple terms, enough of
the details of the RS-232 standard to allow a technician to
construct and/or debug interfaces between any two "RS-232
Compatible" devices. A more detailed coverage of the subject
may be found in the book "Technical Aspects of Data
Communication" by John E. McNamara (1977, Digital Press).

This guide is necessary due to the casual way that vendors
implement "RS-232" interfaces, sometimes omitting required sig-
nals, requiring optional ones, or worse, implementing signals
incorrectly. Due to this, and a lack of readily available
information about the real EIA standard, there is often consid-
erable confusion involved in trying to interface two RS-232
devices.


BACKGROUND

RS-232-C is the most recent version of the EIA (Electronics
Industry Association) standard for low speed serial data commu-
nication. It defines a number of parameters concerning voltage
levels, loading characteristics and timing relationships. The
actual connectors which are almost universally used (DB-25P and
DB-25S, sometimes called "EIA connectors") are recommended, but
not mandatory. Typical practice requires mounting the female
(DB-25S) connector on the chassis of communication equipment,
and male (DB-25P) connectors on the cable connecting two such
devices.

There are two main classes of RS-232 devices, namely DTE (Data
Terminal Equipment), such as terminals, and DCE (Data
Communication Equipment), such as modems. Typically, one only
interfaces a DTE to a DCE, as opposed to one DTE to another
DTE, or one DCE to another DCE, although there are ways to do
the later two by building non-standard cables. Rarely if ever
are more than two devices involved in a given interface
(multidrop is not supported). A serial port on a computer may
be implemented as either DTE or DCE, depending on what type of
device it is intended to support.

RS-232 is intended for relatively short (50 feet or less),
relatively low speed (19,200 bits per second or less) serial
(as opposed to parallel) communications. Both asynchronous and