Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 83)

Command Initiation, Execution, and
Monitoring
Chapter 7
7-11
executed, but also point to the general type of fault condition that
prevented command completion.
The user program must monitor the REMOTE and LOCAL FAULT bits
for each command. The recommendations of this section describe two
methods for monitoring FAULT bits and using these bits to signal a fault
condition.
Diagnostic FAULT Rungs
The purpose of monitoring REMOTE/LOCAL FAULT bits is to control
one or more output indicators to signal a fault condition. Fault indicators
controlled for this purpose may be as simple as a warning light or an
annunciator, or as complex as a line printer or CRT terminal used to
display a fault message. By controlling the fault indicator device, the user
program can alert user personnel to the nature and location of a fault
condition.
To monitor the REMOTE and LOCAL FAULT bits, a programmer must
understand their timing relationship to the corresponding START bit.
Figure 7.2 summarizes this relationship.
The section titled “Controlling the Start Bit” showed how the FAULT bit
can be programmed to UNLATCH the START bit in a fault situation and
thus provide automatic re-tries through the program. When used in this
manner, however, a FAULT bit will be rapidly cycled ON and OFF if a
fault is detected. Because the fault bit can be constantly changing state at
a rapid rate, the program must use some method of detecting this transient
state of any FAULT bit and of controlling the output device based on this
state.
Figure 7.6 shows a simple method for the control of a fault indicator.
Here, either the REMOTE or LOCAL FAULT bit can LATCH the output
indicator ON. The indicator remains ON until the DONE bit is energized.
This then UNLATCHES the output indicator in the second rung of
Figure 7.6. This example allows for the transience of the FAULT bits,
since the first rung need only be TRUE once for the output indicator to be
latched.