Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 237)

ASCII Module
For PLCĆ3 Proessor
Appendix B
B-15
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 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 B.8
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.