User`s guide
Vector
ZCB
Single
Board
canputer
1.5.4
RS-232C
theo;y
This
manual
cannot
describe
the
RS-232C
protocol
in
detail.
For
a
full
description,
obtain
a
copy
of
the
RS-232C
EIA
S'I'ANDA.RD
document,
published
by
Electonic
Industries
Association,
Engineering
Department,
2001
Eye
Street,
N.W.,
Washington,
D.C. 20006.
Alternately,
if
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
inn'oodiate
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
TTL
signals.
Hence, RS-232C POSITIVE
corresponds
to
TTL
low
(abaJt
0 Vdc)
and
RS-232C
NEX?A.TIVE
corresponds
to
Tl'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
carmunication
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,
tetminal,
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
sane
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.
'fherefore,
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
"I1J.'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
Carmunication
Equiprrent
and
Data
Terminal
Equiprrent
derive
1-4
Rev.
1-8
6/11/80