Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 104)

Programming Techniques
Chapter 7
7-7
Program Operation in the Primary System
The primary processor’s program executes the 3 BTW instructions to the
primary 1785-BCM module. After the 3 blocks have been transferred, the BTR
shows that the next block to be sent is 1. In this way, once the 1785-BCM
module confirms that the number of the last block sent (3) equals the size of the
segment, the module requests a transfer of new data.
If an error occurs in the transmission of a block from the primary controller to
its 1785-BCM module, the module sets the backplane error bit (bit 1 is local,
bit 9 is remote) and continues to receive all of the remaining blocks.
Program Operation in the Secondary System
The secondary processor’s application program enables 3 free-running BTR
instructions. The program then uses the block IDs of each instruction and COPs
(file copy) to move the data to its memory at the proper starting address.
In addition, the secondary 1785-BCM module compares the segment length
specified in write block to the segment length sent by the primary 1785-BCM
module. If these segment lengths are different, the secondary 1785-BCM
module turns on the programming error bit (bit 3 – local, bit 11 – remote). If a
block transfer error occurs between the secondary processor and its 1785-BCM
module, the module turns on the backplane error bit (bit 1 – local, bit 9 –
remote) and continues sending all valid data blocks.
Figure 7.3 shows how a program that uses the multiple-blocks-at-a-time method
works.