Transmit (XMT) and receive (RCV),
for
any given device,
is
usually relative to that
device. That
is
, data goes
out
of
a device on XMT and comes in on
RCV.
To connect
two
RS-232-C devices
when
given the
common
names
of
the
signals, you should
connect
XMT
to
RCV
(in one direction) and
RCV
to
XMT (in the
other
direction). If
one
of
the devices
is
wired
as
a data set and the other
as
a data terminal, then you
should
connect
DTR
to
DTR,
DSR
to
DSR
,
RTS
to
RTS,
and
so
on. If,
on
the
other
hand,
they
are each wired
as
data terminals, you should be careful
how
things are
connected.
The Signal Ground
connection
must always be made. RS-232-C requires that the
ground potential
of
the
two
devices be equal. That
is,
their
grounds must be con-
nected together. Devices for
which
this requirement
cannot
be met cannot be con-
nected via a RS-232-C interface.
Data terminal ready
(DTR)
is
used by
RS-232
-C
to
allow
the terminal
to
signal its
readiness
to
send
or
receive data. This
is
a signal
to
automatic answering modems
that
they
have permission
to
answer the ringing
of
the telephone line.
Data
set
ready
(DSR)
is
used by the data set
to
signal its readiness
to
send
or
receive
data. This indicates that communications are established.
Request
to
send
(RTS)
is
used by the data terminal
to
tell the data set
it
wishes
to
send data. Some modems (Bell 202
for
example) require this line
to
switch
directions.
Clear
to
send
(CTS)
allows the data
set
to
signal its readiness
to
pass
data from the
data terminal.
The carrier detect (CRX) line allows the data
set
to
tell the data terminal that the
communication
link
is
established. This often differs little from
DSR
, except that
DSR
usually refers to "telephone
off
the
hook
" (answered), whereas
CRX
means
something like " I hear the
modem
at the
other
end and
we
can talk
now
."
When
CRX
goes OFF, data set ready OFF usually follows a
few
seconds later, indicating
that the other end
has
"hung up."
In normal operation, DTR,
DSR
, and
CRX
are all
ON.
For full-duplex operation
RTS
and
CTS
are also both
ON
.
However
, it
is
often unnecessary
to
have all these lines
ON
-either
one
or
the other devices on the
RS-232
-C connection does not have
all the lines,
or
it
is
all right
to
ignore them (one
of
the
properties
of
the RS-232-C
standard
is
that not all
of
it needs
to
be implemented -it's perfectly all right
to
leave parts out). To operate the ATARI 830 Acoustic
Modem
,
for
instance, none
of
the
control
lines need
to
be used. In fact, the ATARI 830 Acoustic
Modem
ignores
DTR and
RTS,
and it turns
DSR
,
CTS
, and
CRX
on and
off
together (with carrier).
Note
that the
communication
link
shown in Figure
1-1
is
not
defined by RS-232-C.
In particular, this
link
seldom
has
more than
the
"e
quivalent
"
of
XMT and
RCV-
that
is,
only
data lines and
no
control.
However
,
as
often
as
not this
link
is
a full-
duplex
link,
so
data can go both ways simultaneously. ASCII characters are the
most
common
data sent,
so
the data sent each
way
can be either
"co
ntrol" data
or
"data" data.
What
is
RS-232-C?
5
