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HP 262SA - Page 147

HP 262SA
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Data Communications
Table
7-6.
Control Characters Used
in
Multipoint Data Transmission{Continued)
ASCII
CONTROL
CODE
CHARACTER
(HEX)
DESCRIPTION
TTD
02 05
Temporary
Text Delay.
This
character
is
sent
to inform
the
receiving
station
that
the
sender
is
temporarily
out
of
text
but
that
there
is more to follow.
The
receiver
must
respond
with
a NAK for
the
sender
to continue.
This
sequence will
continue
until
the
sender
has
more
data
to send.
The
first
TTD will be
sent
immediately.
SubsequentTTDs
will be
sent
two seconds
after
receipt
of
the
NAK.
Transmission
control
characters.
These
characters
are
used
to initialize, synchronize,
and
terminate
data
without
affecting
data
integrity.
SYN
16
Synchronous Idle.
This
eharacter
is
used
to
establish
and
maintain
character
timing
between
sending
and
receiving
stations.
At
the
beginning
of
each
transmis-
sion a
minimum
of
three
SYN
characters
are
required.
During
transmission
two SYN
characters
are
inserted
at
one second
intervals.
SYN
characters
should
also be
inserted
at
one second
intervals
into
all
data
sent
to
the
terminal,
although
the
terminal
will only
initiate
error
recovery
if
it
does
not
receive a
SYN
character
within
three
seconds.
PAD
7F
or
FF
PAD.
This
character
is
used
to
ensure
that
the
last
character
of
every
transmission
has
time
to be prop-
erly
received
before
the
receiving
station
begins
transmitting.
All
transmissions
must
be
terminated
with
a
trailing
PAD. (Note
that
accuracy
of
the
PAD
character
cannot
be
guaranteed.)
In
addition,
a
trail-
ing
PAD
may
be used
after
an
EaT
when
it
is
used
in
a
POLL
or
SELECT
sequence.
In
this
case,
if
the
PAD
is
issued,
it
must
be a
7F
or
FF
(hex),
although
FF
(hex) is
preferred.
If
the
trailing
PAD
character
is
not
used,
the
communications
interface
will
wait
at
least
40
msec before
continuing
to allow all
data
to be
properly
received.
This
may
significantly
slow
communications.
DLE
EaT
10 4
Disconnect. When
this
sequence is received by
the
ter-
minal
instead
of
a
normal
response
or
text
block,
the
terminal
will
attempt
to disconnect
the
modem
at-
tached
to
the
communication
line.
This
sequence
should only be used on switched lines.
7-35

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