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Chapter
2
A
loss
of
carrier"
disconnect
can
result
other
than
from
being turned
UFF
by one
of
the
modems.
If
you drop
the
receiver
onto
the
tele-
phone
cradle,
or
shout
into the handset, the noise
generated
may
be
strong
enough
to
MASK
the
carrier
so
that the
DCM
212 thinks
carri-
er
is
turned
OFF. Although
the
low
speed
carrier
is
less
susceptible
to noise
than
the
high speed
carrier,
there
is
always the
possibility
of
data
errors.
Condition 2
—
loss
of
data terminal ready
(DTR)
Data
terminal
ready
(DTR)
is
a protocol
signal that
the
DCM
212
receives
from
your
computer
or
terminal.
It
must be
true (asserted)
before
any
modem
operation
is
allowed.
If
you
get to
this
point (on-
line
mode),
it
was
asserted.
DTR
is
defaulted
ON
in
the
Modem.
This
means
that
if
your
com-
puter does not have
a
DTR
control
wire,
the
DCM
212
will
make
sure
this
signal
is
set
up
such
that
you
may
use the
Modem. Of
course,
if
your
cable
connection
is
incorrect
and
the
DTR
pin
at
the
Modem
is
shorted
out,
you
will
never
be
able
to get
the
Modem
to
talk.
On
computers
with
DTR
control,
you
can
stop
any
function
by
forc-
ing
the
signal to
disassert.
Beware
of
the accidental
DTR
switch
OFF!
For example,
if
you
de-
cide to
change a
terminal
feature (for
example, turn
ON
a
printer port)
while
you
are
communicating
with
a
time-sharing
service,
you
could
switch
your
terminal software
around
to
its
utilities
menu
and, suddenly,
the
DCM
212 could drop
its
telephone connection!
Some
terminal (and
software) systems
turn
OFF DTR
when
the
system
is
doing
utilities
work.
Note:
If
DIP
Switch
8
is
set
to
the
ON
position, the
DCM
212
will
assume
that
DTR
is
always
present.
Condition 3
—
reception
of
a
long space
A
long space
is
a
start bit
that takes
anywhere from
a half-second
to
one
full
second
to
end.
It
is
not a
character,
although
some
systems
consider
it
to
be
the
null
(0)
word
with a
framing
error
(it
lasted
too
long before
the stop
bit
happened
along).
25
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