Data Communications
BREAK
KEY
OPERATIONS.
The.
key allows
the
user
to tell
the
host
computer
program
that
he
wants
to
abort
the
current
operation.
When
the
terminal
is
in
Text-In mode
and
the.
key
is
pressed,
the
terminal
sends
an
RVI
«DLE»
instead
of
an
ACKO
or
ACK1
after
the
current
text
block is received.
The
host
program
must
then
respond to
the
RVI
in
an
appropriate manner.
NOTE:
In
some cases
the
terminal
does
not
geta chanceto
transmit
the
RVI
and
youwill
have
to press
the.
key a second time. Before doing so, however, you should
wait
a
reasonable period of
time
(approximately 3 seconds) because
all
of
the
data
in
the
input
datacomm buffers when
the.
key was pressed
must
be
transferred
to
the
display before
the
ttbreak" is visibly evident.
When
the
terminal
is
in
Control mode
and
the.
key ispressed,
the
terminal
clears all
data
in
the
datacomm
output
buffers (the
data
is lost)
and
then
sends
<STX>
GID DID
<CAN>
<ETX>
in
response to
the
next
poll from
the
host. The host
program
must
then
respond
in
an
appropriate manner.
When
the
terminal
is
in
Text-Out mode
the.
key is acknowledged
at
a
later
time.
Monitor Mode
Monitor Mode allows a
terminal
to monitor
the
data
transfers
between
the
computer
and
other
multipoint
terminals
on
the
same communication line. This is a useful technique when
developing communications programs or
testing
multipoint networks.
The monitor
must
be placed
in
the
line between
the
computer
and
the
other
terminals
in
order
to monitor
both
sides of
the
communication exchanges.
Figure
7-21 shows a sample communi-
cation line
using
a
terminal
in
Monitor Mode. (Note
that
the
monitor
cannot
detect
data
sent
from
terminal
AA to
the
computer.)
Note
that
the
monitor will
not
respond to poll or select sequences addressed to
it
while
in
Monitor Mode.
Monitor Mode
is
enabled
and
disabled using
the
MON
I
TOR
MODE
function key (press
.,
then
service
keys_,
and
then
MONITOR
MODEIIII).
When enabled,
an
asterisk
appears
in
the
uMONITOR MODE" label display. Note
that
Monitor Mode
cannot
be enabled while
the
datacomm portion of
the
terminal
is active.
While
in
Monitor Mode,
data
communications between
the
computer
and
udownstream"
terminals
will be displayed on
the
monitor.
Data
from
terminals
will be framed by
ttl
ess
than"
and
ttgreater
than"
symbols
«data»
and
will include control
and
Block Check
Characters
regardless of
parity
errors, framing errors, or block check
errors
(see figure 7-22).
7-44
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