Rs-232C - Vector ZCB User Manual

Vector ZCB Single Board canputer
1.5.4 RS-232C theory
This manual cannot describe the RS-232C protocol in detail. For a full
description, obtain a copy of the RS-232C EIA STANI:lrillD document, published
by Electonic Industries Association, Engineering Department, 2001 Eye
Street, N.W., Washington, D.C. 20006. Alternately, i f you have access to
Datapro or Auerbach reports on communications, they contain thorough
articles describing the protocol and its implications. The following
information, however, will be of :i.rrn't'Ediate relevance in this manual:
An
RS-232C signal can either
be
POSITIVE (+12 Vdc) or NEGATIVE (-12 Vdc).
Positive is ON or SPACING, Negative is OFF or MARKING. (These terms are
industry wide conventions that date back to the days of key telegraphy.)
RS-232C line drivers typically invert these signals when they are converted
to
and
fran TI'L signals. Hence, RS-232C POSITIVE corresponds to TTL low
(about 0 Vdc) and RS-232C NEGATIVE corresponds to TI'L high (about 5 Vdc).
An
RS-232C
cable consists of 25 lines. An RS-232C transmit or receive
data line carries a serial sequence of POSITIVE and NEGATIVE pulses that
correspond with the characters you want to transmit or receive. There is
also associated formating and parity information attached to. the information
by the canmunication device such as an 8251. In addition to the transmit
and receive data lines, there are ground lines, (lines 1 and 7), and there
are handshaking lines that are used by canmunication, terminal, and canputer
equipment to inform each other of their status (lines 4, 5, 6, 8, 20, 22,
and a few others that are rarely used). The full RS-232C protocol also
specifies a set of rarely used "secondary" lines which have the same
definitions as some of the primary lines, but carry an independent set of
signals. Altogether there are 25 RS-232C lines defined, but most
applications use only a few of them.
In the real world, very few devices require "full RS-232C" protocol. In
fact, very few devices even require all of the handshaking lines mentioned
above. Many require one or even none. Further, many devices use
handshaking lines differently than defined by RS-232C, violating the
protocol. In short, it is confusing at this time to say that a given device
requires "full RS-232C." You must specify exactly what signals it sends and
expects to receive on each line.
It is important to understand that most of the RS-232C lines are
directional, that is, the protocol specifies which direction the signal
travels on each line, relative to the ends of the cable. Therefore, the
protocol specifies that at one end of an RS-232C cable there must be a
device of the type called "Data Communications Equipment", or "DCE" for
short, and at the other end there must be a device of the type "Data
Terminal Equiprent, or "DI'E" for short. The direction of the signal on a
given line can be determined once you decide which end of your' cable has
which kind of device.
The terms Data Communication Equipment and Data Terminal Equipment derive
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Rev. l-B 6/11/80