Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 159)

Publication 1747-UM010B-EN-P - September 2003
Programming Techniques 9-17
The total number of data blocks is 128. In each program scan, the
system transfers all 128 data blocks. Total transfer time is
approximately 700 ms.
Other transfer methods
Data transfer methods 1 and 2 were designed to be used as sample
methods. In a real application, the user would develop other methods
of transferring data. These alternatives would optimize the data
transfer throughput without causing an excessive loss of time due to
the M-file transfers.
Accounting for Instructions
That Could Cause Problems
During Switchover Some instructions may operate unpredictably when a switchover
occurs if you fail to observe certain programming considerations. For
example, you must consider transferring the control element and data
elements for instructions that are also being executed in the secondary
processor. You also must consider transferring data files which contain
more than one word.
The following instructions are discussed in this section:
•
timer
•
counter
•
logical, arithmetic, compare and move
•
diagnostic, sequencing, file
•
file copy and fill
•
FIFO
•
block transfer
•
message
In the remainder of this chapter, we describe the behavior of these
instructions in the SLC 500 backup system.
IMPORTANT
The primary and secondary systems do not
necessarily execute the same instructions at the same
time (the systems’ program scans are not
synchronized). The SLC 500 processor executes block
transfers to the program scan asynchronously.
Therefore, the processor interrupts the program scan
asynchronously to access block transfer write (BTW)
and block transfer read (BTR) files.