Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 79)

Operating Your PLC-5 Backup System
Chapter 5
5-15
Figure 5.9
Execution of Primary Program
BTR
BTW
Block ID = 1
64 words
Primary Processor Primary 1785-BCM module
Buffer
1. At program initialization,
1785-BCM module specifies
block ID of 1 so that BTW are
initiated in correct sequence.
2. Primary processor sends data to
primary 1785-BCM module.
The module stores this data in its
buffer.
3. When primary 1785-BCM
module receives data block
without errors, it sends data to
secondary 1785-BCM module
over the HSSL.
2 words
To secondary 1785-BCM
11061I
Secondary (Receiver) Program
In the secondary program, the BTR instruction is used to obtain the system
status, the block ID word, and its respective data. The BTW instruction informs
the secondary 1785-BCM module of the size of the segment to be received, that
is, the number of blocks which must be accepted before turning on the data
valid bit.
The number of blocks which must be accepted before turning on the data valid
bit depends on the switch setting for Fast Data-Transfer mode and word 1 of the
BTW segment length.
The secondary PLC-5 processor will read data from the secondary 1785-BCM
module according to the Fast Data Transfer mode setting.
Fast Data-Transfer Mode Enabled
The secondary PLC-5 processor continuously executes BTR instructions to
obtain the system status word and data from the secondary 1785-BCM module.
With the Fast Data Transfer mode, as soon as the secondary 1785-BCM module
correctly receives a data block, the data valid bit returned to its local processor
through the BTR instruction will immediately turn on and the block is released
for reading.
Fast Data Transfer Mode Disabled
The secondary PLC-5 processor continuously executes BTR instructions to
obtain the system status word and data from the secondary 1785-BCM module.