This
manual
cannot
describe the
RS-232C protocol in detail.
For a full
description, obtain a copy of the RS-232C E~
STANDARD document,
published
by Electonic
Industries
Association,
Engineering
Department,
2001
Eye
Street, N.W., Washington, D.C. 20006.
Alternately,
if you
have
access
to
Data
Pro or Auerbach
reports
on communications,
they
contain
thorough
articles
describing
the protocol
and
its implications.
The
following
information, however, will be of bronediate relevence 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.
RS-232C line drivers
typically invert
these
signals
when
they are converted
to and
from TTL
signals.
Hence,
RS-232C
POSITIVE corresponds to
TIL
low (about
a
Vdc) arid
RS-232C NffiATIVE corres~nds
to TTL high
(about 5 Vdc).
(TTL is the kind of
signal used within the computer.)
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 communication 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
communication, terminal, and computer
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
speci
f
ies
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 ~rld,
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
Equi?ffient", or
"DCE
1'
for
short,
and
at the other
end
there
must
be a device
of the
type
"Data
Terminal Equifffient,or "OTE" 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.
Summary of Contents for Bitstreamer II
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