Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 162)

Publication 1747-UM010B-EN-P - September 2003
9-20 Programming Techniques
You should place the instruction’s control element data file within the
same 62 words of data. If this is not done, the 1747-BSN module could
transfer the control values responsible for control of the file at a
different point than when it transfers the file. A time lag between
when the file is updated and when the associated control element is
updated in the secondary processor. As a result, the following may
occur during switchover:
•
a file operation that was running may be off by one word,
•
a word could be operated on by the file twice, or
•
a word could not be operated on at all.
FIFO Instructions
The type of data you are transferring, as well as how you transfer the
data to the secondary processor, determines the behavior of the FIFO
instructions.
If you are transferring only selected areas of the data files which do
not include data for FIFO and bit instructions, the instructions are
executed normally.
However, if you are transferring all of the data table files over the
HSSL, you may encounter problems. For example, if you are shifting
four bits of BCD data through several words bit by bit, the data table
contains invalid data until each shift is complete. Should the 1747-BSN
module transfer one or more of these values to the backup system
and the primary system fails, the data table of the backup system
contains intermediate values for an indefinite amount of time. The
result is illegal BCD values.
Block Transfer Instructions
It is impossible to guarantee that both processors are executing a
given BTW or BTR at the same time. This is because the remote I/O
scans are not synchronized and the block transfer data is not
transferred instantaneously by both SLC 500 processors. The BTR files
in the secondary processor always reflect information contained in the
primary processor, except with a time delay.
To avoid the possibility of block transfer data changing during the
scan of the program, you can buffer data at the beginning of the
ladder program to ensure continuity.