Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 80)

Operating Your PLC-5 Backup System
Chapter 5
5-16
The primary 1785-BCM module sends several data blocks that make up a
segment sent to the secondary 1785-BCM module. Until the secondary
1785-BCM module correctly receives all blocks of the segment, the data valid
bit returned to its local processor through the BTR instruction will not turn on.
This bit will turn on only when the complete segment is correctly received and
the number of blocks in the segment match that sent by the primary 1785-BCM
module. Transfer from the 1785-BCM and the secondary processor can now
begin.
During transfer of the information from the 1785-BCM module to the secondary
processor, the 1785-BCM module sets the block ID of the first block equal to
one and increments it on each successive block.
In the BTR data, the block ID word always contains the number of the last
block received from the buffer of the 1785-BCM module. If the buffer is
empty, or if the 1785-BCM module cannot send a new segment, the block ID
equals 0 (zero) and only the first two words are received. Figure 5.10 shows
how the secondary program operates.
Figure 5.10
Execution of the Secondary Program
BTW
BTR
Segment
Length
64 words
Secondary Processor Secondary 1785-BCM Module
2 words
1. Processor executes BTW to tell
1785-BCM module the length of
segment to be read.
2. Processor detects a data block
in the secondary 1785-BCM
module with a block ID of 1.
3. Processor executes BTR
instructions to receive data
blocks from the secondary
1785-BCM module.
Buffer
From primary
1785-BCM
11062I
Important: It is up to you to:
determine which parts of the data table must be shared by the processors
develop a program that sequences all blocks to and from the 1785-BCM
modules
account for the asynchronous nature of queuing and moving of BTW and
BTR data in the PLC-5 processor.