Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 67)

Operating Your PLC-5 Backup System
Chapter 5
5-3
Sequence of Transfer
The sequence of transfer from the primary to the secondary system is as
follows:
1. The data from the primary processor’s data table is sent to the primary
1785-BCM module by means of a Block Transfer Write (BTW)
instruction.
2. The data is sent to the secondary 1785-BCM module over the HSSL.
3. The secondary processor reads the data from the secondary 1785-BCM
module by means of a Block Transfer Read (BTR) instruction.
Method of Transfer
Data table information is transferred from the primary processor to the
secondary processor in a segment that is composed of 1-64 sequentially ordered
blocks. Each block has 64 words. The first 2 words of each block are reserved
for a system status word and block identification (block ID) word; the
remaining 62 are reserved for data.
The content of the block transfer instruction’s first two words depends on the
type of instruction (BTW or BTR), and also on the application program run in
the primary and secondary processor. Table 5.A briefly describes block transfer
instructions in the PLC-5 backup system. (For detailed information about
words in the block transfer instructions, refer to the “Block Transfer Read
Instruction” and “Block Transfer Write Instruction” sections of this chapter.)
Table 5.A
Block Transfer Instructions in the PLC-5 Backup System
Instruction Word Function of word is to:
Primary System Secondary System
1 – System Status inform the processor about the status of the primary and secondary system
Block-Transfer Read 2 – Block Identification
(block I/D) indicate the number of the next block
that should be transferred to the
module indicate the number of the block being
transferred from the module
Block-Transfer Write
1 – Number of Blocks in Segment indicate the number of data blocks
within a segment being transferred
from the processor to the primary
1785-BCM module (1–64). Must be
same as secondary system. indicate the number of data blocks
within a segment being received by the
secondary 1785-BCM module (1–64).
Must be same as primary system.
2 – Block Identification
(block I/D) indicate the number of the block that is
being transferred from the primary
processor to the primary 1785-BCM
module reserved for future use