Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 81)

Command Initiation, Execution, and
Monitoring
Chapter 7
7-9
In this example, a storage bit, called the “transition” bit, is manipulated to
control the sending of the command. This bit is LATCHED whenever a
transition of input 11111 is detected, UNLATCHED only when the DONE
bit is set ON. A “compare” bit, 01111 in this example, is used to
manipulate the transition bit. In rung 2, the compare bit is controlled to
match the ON/OFF status of the input. Because the input and the compare
bit are programmed to have matching states, both ON or both OFF, the
conditions of RUNG 1 can be TRUE only when the input has just
changed from ON to OFF or from OFF to ON. Thus rung 1 conditions set
up a “one-shot,” TRUE only long enough to LATCH the transition bit.
Note that these rung conditions are FALSE as soon as the processor scans
rung 2. The order of these rungs is important for this reason.
With the transition bit LATCHED, the START bit, in turn, is LATCHED
in rung 3. This initiates the command. In normal operation, the DONE bit
UNLATCHES the START bit in rung 4 and then UNLATCHES the
transition bit in rung 5. In faulted operation, however, rungs 3 and 4
repeatedly re-try the command in much the same manner as in the
example of Figure 7.3.
Timed
The START bit may be LATCHED periodically to send a command at a
user-determined time interval. Figure 7.5 shows example rungs for this
type of command initiation.