Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 78)

Operating Your PLC-5 Backup System
Chapter 5
5-14
To provide status information, or to pass additional block transfer values, you
must provide support ladder programming. Refer to Chapter 7, “Programming
Techniques” and Appendix A “Sample Programs” for information on
developing ladder programs for the backup system.
Primary (Transmitter) Program
Upon initialization of the backup system, or when the secondary PLC-5 is not in
operation, the primary 1785-BCM Module responds to the BTR instruction that
the block number is one (block ID = 1). This assures the correct initiation of the
block transfers, principally when the secondary PLC-5 is placed in RUN mode.
After execution of the first BTR instruction, the 1785-BCM module is ready to
execute the necessary sequence of instructions to transfer the data table words
from the primary system to the secondary.
Important: Remember that, in block transfer instructions, you must specify the
LENGTH field as 0 (zero). This allows the 1785-BCM module to determine
automatically the block size depending on if the module is in the primary or
secondary system. In the primary system, the 1785-BCM module will return 2
words in the BTR instruction, while the processor will write 64 words to the
1785-BCM module.
Each word received by the primary 1785-BCM module is stored in its buffer. If
the primary module receives the data block (of up to 64 words) without errors, it
automatically sends the block to the secondary 1785-BCM module over the
HSSL. Each block is checked for errors, and if no errors are pending, the
primary module transmits the data after the current data block, one block at a
time.