Data Communications
TEXTTERMIN
ATI
ON.When
the
terminal
is receiving
text
(Text-In Mode)
it
will acceptonly
the
block protocol control
character
ETB (octal 27), ETX (octal 3), or ENQ (octal
5)
as
a
text
block terminator. Table 7-6 lists
the
characters used for control ofblock
data
transfers.
ETB Indicates
the
end ofa block with one or more blocks to follow.
ETX Indicates
the
end of
the
current
block
and
the
end of
the
text
transfer.
ENQ
Indicates
that
the
current
block
has
been aborted.
The
terminal
will respond
to
the
ENQ
with
a NAK to request
the
retransmission
of
the
aborted
text
block.
When
the
terminal
issending
data
(Text-OutMode),
it
will
terminate
text
blocks
with
either
an
ETB
or
an
ETX character.
All characters
sent
orreceivedbetween
the
STX
character
and
the
terminating
character
must
not
be more
than
40 milliseconds
apart
for asynchronous operation. Synchronous operation
requires SYN or DLE SYN characters to be
sent
as
fill
characters
if
no
text
characters
are
ready for transmission. SYN insertion
must
also be performed
at
one second
intervals
within
text
blocks.
The
terminal
may
send
an
STX ENQ as a TemporaryText Delay (TTD) notification
instead
of
the
next
block of
data.
This
indicates
that
there
ismore
text
tocome
but
that
it
is
not
ready
tobe
transmitted.
ATTD should be answered with a NAK to
request
the
transmission
of
the
text
block, or
an
EOT to
reset
the
terminal
to control mode.
Table 7-6. Control Characters Used
in
Multipoint
Data
Transmission
ASCII
CONTROL CODE
CHARACTER
(HEX)
DESCRIPTION
Data
link
control
characters.
These
characters
are
used
to
frame
messages
and
ack-
nowledgements for
both
transmitted
and
received
text
blocks.They
are
also used to control all
communications
in
an
orderly fashion.
DLE
10
Data
Link Escape. This is
the
first
character
in
two
byte control characters. The DLE
character
is usually
treated
as
data
when used alone.
ACKO
(DLE
0)
10 30
Acknowledge
O.
These control
characters
are
sent
by
the
terminal
after
being selected to tell
the
computer
that
the
terminal
is ready to accept a
text
block. They
are
also
sent
by
the
receiving
station
(computer or
terminal) after even
text
blocks (2, 4, etc.) to tell
the
sending station (terminal or computer)
that
the
block
was received properly (see ACKl).
The
alternating
ACKO/ACKI
sequence is initialized to
ACKO
following
select sequence or to ACKI
after
a poll sequence.
7-32
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