Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 100)

Programming Techniques
Chapter 7
7-3
This manual provides two solutions to programming your PLC-5 backup system
to transfer data table values. These methods are:
transferring data one block at a time
transferring data multiple blocks at a time
We describe the operation of these programming methods on the following
pages. To view sample programs that use these methods, refer to appendix A,
“Sample Programs.”
Method 1 — Transferring Data One Block at a Time
In the first programming method, transferring data one block at a time, both
the primary and the secondary processors execute and scan identical programs.
This method requires that you execute BTR and BTW instructions on alternate
program scans. You are able to do this because the primary 1785-BCM module
has the ability to identify and increment the block ID by one. (For more
information about the block ID, refer to Chapter 5, “Operating Your PLC-5
Backup System.”)
Using this method, your application program can use the block ID returned in
the block transfer read (BTR) instruction to control the data file of the block
transfer write (BTW) instruction control file.
This method of programming results in a slower overall throughput of data
transfer, but gives tighter control of the transfer, allowing automatic
re-transmission of a block if an error in the transfer should occur.
A program that uses this method contains:
A BTW and BTR instruction in each PLC-5 processor. These instructions
should be of a free-running nature and be the same in both the primary and
secondary processors.
A chained series of File Copies (COP).
Program operation varies depending on if the system is primary or secondary.
The primary 1785-BCM module executes a 64 word BTW instruction and a 2
word BTR instruction; the secondary 1785-BCM module executes a 2 word
BTW instruction and a 64 word BTR instruction.
Programming Methods to
Transfer Data Table Values