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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 get a chance to transmit the RVI and you will 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 is pressed, 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 ttless 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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