Data
Communications
Table
7-6.
Control
Characters
Used in Multipoint Data Transmission{Continued)
ASCII
CONTROL
CODE
CHARACTER
(HEX)
DESCRIPTION
ACKI (DLE
1)
10 31
Acknowledge
1.
These control
characters
are
sent
by
the
receiving
station
(computer or
terminal)
after
odd
text
blocks (1, 3, 5, etc.) to tell
the
sending
station
(terminal
or computer)
that
the
block
was
received
properly (see
ACKO).
WACK
(DLE;)
103B
WaitBefore'I'ransmit.These
characters
are
sent
by
the
receiving
station
to indicate
that
the
last
block
was
properly received
but
that
the
receiving
station
re-
quests
that
the
sender
wait
before
sending
the
next
block.
The
sending
station
should
then
send
an
ENQ.
The
receiving
station
will
then
return
an
ACKO/l
if
it
is
ready
to receive
data
ora
WACK
in
order
to continue
waiting. When
the
terminal
needs to
send
a WACK,
it
will send
the
first
WACK
immediately.
Subsequent
WACKs will be
sent
two seconds
after
receipt of
the
ENQ.
NAK 15
Negative Acknowledge.
This
character
is
returned
in
response to a
text
block to indicate
that
the
block
was
rejected because of a
bad
block check,
parity
error,
framing
error
(async only),
or
character
overrun.
When received by
the
terminal
after
it
has
sent
a
text
block,
the
terminal
will
retransmit
the
block.
ENQ
05
Enquiry. This
character
is
always
used
to
terminate
a
POLL or SELECTsequence (except for group
and
line
selects).
It
is also used by
the
sending station to
request
a
retransmission
of
the
acknowledgement for
the
pre-
vious
text
block. When used
as
a block
terminator,
ENQ
indicates
that
the
computer
has
aborted
the
block (forward
abort
or
TTD).
The
terminal
will
respond
with
a NAK to acknowledge
the
abort
com-
mand. The
terminal
will
not
terminate
a block
in
this
manner,
although
it
will send STX
ENQ
as
a TTD.
STX
02
Start
ofText.
This
character
must
be
the
first
character
in
every
text
block.
It
tells
the
receiving
station
to
begin accumulating a block check character.
The
STX
character
is
not
included
in
the
block check.
7-33
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