Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 72)

Operating Your PLC-5 Backup System
Chapter 5
5-8
When the primary 1785-BCM module receives the last block (block ID =
number of blocks in segment) through a BTW instruction, the module returns a
block number equal to 1 in the next BTR instruction. This re-initializes the data
transmission process.
The 1785-BCM module also specifies a block ID equal to 1 under the following
conditions:
power-up
programming
error
switchover
In the secondary system, word 2 of the BTR instruction indicates the block
number of the data just read, so that the data can be moved to the proper address
in the data table. If the Fast Data-Transfer mode is enabled, as soon as the
secondary module receives a block, the module sets the data valid bit and
releases this block for reading.
If the Fast Data-Transfer mode is disabled, the module returns the data valid bit
as a 1 (one) only when the secondary module receives the complete data
segment. This means that the data valid bit remains on until the last block is
read from the secondary 1785-BCM module. If no blocks are available to be
read, the 1785-BCM module returns the block ID as 0 (zero) and the file
contains only two words (status and block ID).
Block Transfer Write (BTW) Instruction
The local processor sends information to the 1785-BCM module using the BTW
instruction. In the backup system, the block transfer write (BTW) instruction
uses the first two words to indicate:
the number of blocks in the segment being sent
the block identification number (block ID)
The BTW in a primary system transfers these two words, plus 62 words of data;
the BTW in the secondary system transfers only these two words of data.
Figure 5.5 shows the words of the BTW instruction.