Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 208)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ40
When the scanner encounters a communication fault, it tries twice to
complete the transfer. It sets the error bit after the second unsuccessful
try.
When the scanner and/or processor detects a block transfer error, the
transfer is halted. Transfers from the module are prevented until:
Your program clears the instruction’s control word (clears the error,
Figure A.17)
You locate and correct the error
Figure A.17
Resetting the Control Word after a Block Transfer Error
CTRL WORD
03
MOV
MOV FROM A TO R
A : STORAGE WORD
0000000000000000
R : CTRL WORD
0000000000000000
13
Detecting Faults
Block transfer error detection and resulting processor shutdown are safety
features of Allen–Bradley programmable controllers. We recommend that
you adapt such safety features to your application. However, you may
want your program to reset block transfer instructions whenever an error
is detected. Block transfer errors can occur intermittently due to electrical
noise in the environment, and may not be critical to system operation.
This allows your system to continue operation, and allows you to observe
the frequency and location of such errors.
The processor can record where faults are occurring in the I/O chassis,
and the frequency of occurrence. To observe this information you must
create the following files. Refer to section titled “Entering the Getting
Started Program,” step 7 (P. 1-26), for the procedure.