Allen-Bradley 1756-CNB/E, User Manual

Page: 84 / 186

Publication 1756-UM523F-EN-P - December 2006
84 Configure and Program the Controller
Look for Array Shift Instructions
These instructions might corrupt data during a switchover:
•
BSL
•
BSR
•
FFU
Because these instructions shift data within an array, an interruption
by a higher priority task and a subsequent switchover leaves the data
with an incomplete shift.
•
If a higher priority task interrupts one of these instructions, the
partially shifted array values are sent to the secondary controller.
•
If a switchover occurs before the instruction completes its
execution, data remains only partially shifted.
•
The secondary controller starts its execution at the beginning of
the program. When it reaches the instruction, it shifts the data
again.
Look for Scan-Dependent Logic
A rung that must read the output of another rung during the same
scan might miss a scan during a switchover.
Scan-Dependent Logic
A. The CTU instruction counts each scan.
B. The EQU instruction uses the count of each scan
(scan_count.ACC).
43065
A.
B.
C.
D.