Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 86)

Command Initiation, Execution, and
Monitoring
Chapter 7
7-14
The OFF-DELAY timer is useful in this application because it is
continually reset when its rung conditions go TRUE. This means that the
timed bit, 03015, remains on for as long as any FAULT bit is changing
state during programmed re-tries. This keeps the output indicator on until
after the DONE bit indicates command completion.
NOTE: Using the rungs of Figure 7.7, the indicator goes ON
automatically at power-up, or whenever the mode select switch on the
processor is changed from the PROGRAM LOAD (or PROG) mode to
any other mode. However, the indicator only remains on initially for the
preset interval; after this time, the indicator is valid for fault conditions.
Rungs 1 and 2 examine all 8 FAULT bits of each type. Should fewer than
8 command rungs be programmed at a station processor, fewer bits need
be examined. Then, should command rungs be added subsequently, the
appropriate bits could be addressed in rungs 1 and 2. Conversely, if more
than 8 command rungs were programmed at a station, additional rungs
would be needed to examine both REMOTE and LOCAL FAULT bits for
the additional commands. Status bits controlled by these additional rungs
could then be examined in branches of rung 3, parallel to those shown.
Of course, other methods can be used to monitor REMOTE and LOCAL
FAULT bits. Such factors as availability of output terminals, memory
space, and type of application dictate the specifics of FAULT bit
monitoring and program response.
Use of FAULT bits in start-up and troubleshooting procedures is described
in chapter 9.
In addition to its REMOTE/LOCAL FAULT bit control, a 1771-KA2
module also provides an automatic timer for monitoring command
completion. While it functions automatically during module operation,
the timer uses a preset value entered in the user program. This feature
enables the module to monitor command execution time without using
timer (TON) instructions in the user program.
Figure 7.8 shows the significance of the timeout preset interval. From the
time the START bit is set ON, the module must set either a DONE bit or a
REMOTE or LOCAL FAULT bit within the timeout preset interval.
Should the module detect no DONE or FAULT bit response within the
preset interval, some type of fault is assumed. As a response to this type
of timeout situation, the module sets the LOCAL FAULT bit ON and
Timeout Preset Value